The Web Surfers Guide to Diagnostic Equipment

Tools to help you fix your computer-equipped Corvette

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by Hib Halverson
copyright 1997 by Shark Communications

Corvette is one of most sophisticated cars GM makes. Depending on model year and options, computers may control the engine, transmission, shock absorbers, climate control, sound system, supplemental inflatable restraints, anti-theft devices and door locks. They also enhance braking and traction on poor road surfaces.

All this has made servicing our cars more complex and expensive. However, the complexity and cost would be many times what it is, if it were not for diagnostic equipment. Visit a Chevrolet dealer and you will be amazed at the line-up of high-technology devices service technicians have to address computer-related problems. The proliferation of diagnostic aids have made service easier and cheaper because finding the problem is a less time consuming process. But, it wasn’t always that way.

I remember the early years of on-board computers. Back then, service technicians at dealers and independents, where all whining the same sob story, "I hate these %^&* computers! Heck, you think I was fixing a !@#$ rocket ship, not a Corvette."

If dealer techs were crying the blues, imagine what it was like for the do-it-yourselfer (DIY) working on computers in the early ’80s. Back then, diagnostic equipment specific to servicing on-board computers was a rarity at Chevrolet dealers and generally not available to DIY enthusiasts, at all.

Thankfully, that sad state of affairs doesn’t exist, today. We may choose from a variety of diagnostic products limited only by budget and knowledge of use. There are four general types: 1) reasonably-priced, basic devices that do not process information from on-board computers, 2) "scan testers" that cost more, but offer a higher level of diagnostic aid because they process information 3) software-based scan testers or scan tester enhancements that work on a personal computer, 4) premium scan testers and other, up-level equipment.

The mother’s milk of automotive on-board computer troubleshooting is the diagnostic trouble code (DTC). Any Corvette computer, or powertrain control module (PCM), has some level of self-diagnostic capability such that, if it detects a problem; it stores data about that problem in its memory. This data is encoded in strings of either two or four digits. All ’80-’95 cars send two-digit DTCs, ‘96-up cars send four-digit codes and ’94s and ’95s may send both, depending on the type of equipment being used to read them.

Almost all Corvettes with computers have a diagnostic link connector (DLC) which is kind of an electronic gateway to the PCM and a Corvette’s other on-board computers. The DLC can be used to send commands to the PCM telling it to transmit codes. All 1982-’93 Corvettes have 12-pin DLCs. Base-engine ’81s have 5-pin DLCs; however, ’81s with L82s lack a DLC because they do not have computer controls. Corvettes built for sale in California during model year 1980 (MY80) have a peculiar system (see sidebar) without a DLC.

All ’81-’93 computer cars send DTCs if one connects the DLC’s PCM "diagnostic" or "test" pin to its ground pin, turns on the key, but doesn’t start the engine, and watches the "check engine" or malfunction indicator light (MIL) flash. Service manuals contain information that decodes the flashes.

The ’94-’97s have a 16-pin DLC. MY94/’95 LT5 PCMs, because they lack OBD-II capability, will transmit codes via MIL flashes; however, ’94-’97 LT1/4 and LS1 PCMs have: OBD-II capability, no PCM diagnostic terminal and do not transmit codes via the MIL.

All 81-’97 Corvette PCMs also transmit DTCs, along with other information related to the on-board computer system, over "serial data" lines which have a pin or, in later years, pins in the DLC. To read this serial data you need a scan tester or other computer device that connects to the DLC and is intended for automotive diagnostic use.

Got all this? There will be a test, later…just kidding.

Just the Basics, Please

The least expensive tools covered in this article are "code access keys." These fork-like, metal stampings join pins-A and -B of a 12-pin DLC. They are marketed by MascoTech, Mac Tools, Wells Manufacturing and others. They are better than a jumper wire because they solidly link the pins. A device that accomplishes the same task in a more foolproof manner is the "Code Scanner" from Sunpro. It makes the pin-A to-B connection by locking to the connector body. An added attraction of Sunpro’s device is that, on ’89-’93s, it can command display of antilock brake system (ABS) codes.

On 1989 or later Corvettes, the DLC accesses DTCs from on-board computers other than the PCM. In fact, by 1991, there were seven different readouts available, each flashing codes via either lights on the driver information center (DIC) or on ’91-’96 speedometer displays. Each of these diagnostic modes can be entered by grounding a particular pin of the DLC and, in some cases, making DIC key panel entries. Knowing which pin to ground, which key to press and how to decode light flashes or read the speedometer takes experience or time sifting through Service Manuals. Plus, you need to have the right code access key or must make a jumper wire.

While a detailed discussion of on-board computers other than the PCM is beyond the scope of this article, readers working on MY89-’96 cars and who want to read the memories of these other computers, as well as those of ’89-’93 PCMs, ought to consider purchasing the Performance Choice "Code Analyzer." This piece of equipment can ground most of the ’89-’96 diagnostic pins. The user connects the Analyzer to the DLC, selects the proper pin to ground via a rotary switch and a two-position, model year switch, turns on the key with the engine off and codes are read with the appropriate flashing light or the ’91-’96 speedometer. The Code Analyzer comes with a manual covering all diagnosis modes used on ’81-’93 cars. Performance Choice extends support back to MY81 with an optional adaptor and extends partial support up to MY96 with a different adapter and a manual supplement. Performance Choice’s Code Analyzer is a great DIY product and the only non-serial, diagnostic device that can read PCM codes on all Corvettes from MY81-’93, ZR-1s to MY95 and all non-PCM codes from MY89-’96 cars.

If there is a confusing aspect of the Performance Choice Code Analyzer, it’s the final few words of the last sentence above. If you are considering this device for use with a 1994, ’95 or ‘96 car, understand that the "User’s Manual," included with the so-called "1994-95 OBD-II Supplement," states in several places that the device will enable display of PCM codes from ’94-’96 Corvettes. Reality is that this update does not support OBD-II, at all. In fact, the Code Analyzer with the Supplement will not enable display of any  codes from ’94-up PCMs except those of LT5s. With LT1/4 engines, after the 1993 model year, it only enables display of codes from the chassis or body computers. To access PCM codes after 1993, a scan tester is required.

If you are troubleshooting the ignition system of a computer Corvette, there may be diagnosis procedures that instruct you to test spark output. This is done with a spark gap tester, a specialized type of spark plug with a ground clip that you clamp to a good engine ground. They are available from a number of sources. You connect the plug wire and crank the engine. If the spark can jump the calibrated plug gap, then you know the ignition is capable of spark. KAL Equip, the service-trade arm of the Actron Corporation which makes Sunpro products for the consumer market, markets an adjustable spark gap tester (P/N 4617) that functions with all types of ignition systems use on Corvettes, even non-computer cars.

Even though computers do the "thinking", old-fashioned vacuum operates some of the sensors and control devices, such as EGR valves, emissions canister purge valves, MAP sensors, dashpots, delay valves and so forth. Troubleshooting may require a temporary source of vacuum, typically a hand-operated vacuum pump, available from a variety of tool manufacturers. I use the Neward Enterprises’ Mityvac Silverline pump (p/n 4000) because of its ease of operation, rugged construction and detailed manual. Other hand pumps are available from Sunpro and Mac Tools.

A very common device in on-board computer service is the digital multimeter (DMM) which measures voltage, resistance, current, engine rpm, dwell angle and duty cycle. They come in a variety of flavors from cheap, swap meet junk up to high-end stuff. Several steps up from the swap meet level and a great value is the Sunpro CP7678. It’s user-friendly, reasonably-priced and has functions many DIY’s need.

Some diagnostic procedures require a DMM with a minimum of 10 megohms input impedance for accurate measurement. If you need that feature, a more up-scale unit will be required such as the Mac Tools ET18DMM or the Performance Choice 17048. Both the Mac and Performance Choice units are standouts. The Mac is built by Fluke, the famed test instrument manufacturer and is built to service-trade standards.

Another handy item is the Sunpro Sensor Tester Plus. Computer Corvettes have many sensors that send data to the PCM. The Sensor Tester Plus is a stand-alone device that easily checks many of them. It has the required 10meg. impedance for oxygen and knock sensor testing, comes with a detailed manual and is an excellent companion to Sunpro’s non-10meg-ohm DMM.

A volt-ohm-milliammeter (VOM) is similar to a DMM, but has an analog display. VOMs are valuable in the solution of intermittent problems (or just "intermittents") some of which exist so briefly that most digital units won’t indicate because they can’t react quickly enough. However, a brief voltage spike will cause a VOM needle to jump. You may not get an accurate reading, but you will know something is changing. That alone can be an important diagnostic clue.

Our favorite VOM is the Simpson 260 which is well-suited for Corvette service use because of its accuracy and durability. Less expensive VOMs are available from sources such as Radio Shack. If you’re on a budget and don’t need the milliammeter function; Sunpro sells a low-cost, analog, volt-ohmmeter (p/n CP7610).

Performance Choice makes a number of specific, wiring harness patches that allow easy DMM or VOM of testing oxygen sensors (O2S), manifold absolute pressure (MAP) sensors, manifold air temperature (MAT) sensors, throttle position sensors (TPS), knock sensors (KS), coolant temperature sensors and idle air control (IAC) motors. These patches can be used with any DMM, however certain tests require a unit with 10meg. input impedance. The patches are available individually or in "kits" according to model year and we love the "kit by model year" approach to packaging this stuff. It makes trying to select the right test harnesses as simple as looking in the Mid-America catalog for your model year and ordering.

Often overlooked, because it’s been around for half-a-century or more, is the test light or continuity tester. Though discount stores sell test lights, we prefer better-quality units from Mac Tools (p/n ET125C) or Sunpro (p/n CP7845). Both have a replaceable lamp, an impact-resistant housing, a spring-protected cable and a heavy-duty ground clip. Some diagnostic tasks may find a conventional test light inadequate. Many PCM circuits run at 5 volts and may not illuminate a 12-volt test light. The solution, in that case, is Mac Tools’ "Mac Light" circuit tester (p/n ET120). This unit works on as little as 3.5 volts and uses LEDs to indicate a ground or a hot source.

It is likely that the Corvette in question has electronic fuel injection. Diagnostic work may involve fuel pressure readings and injector testing. Fuel pressure gauges specific to EFI are available from Kent-Moore and other sources. K-M markets Chevrolet dealer service tools and has a gauge for ’85-’97 port fuel injection engines (p/n J34730-1A) and another for ’82-’84 Cross-Fire (dual throttle-body) injection engines (p/n J29658-D). You may also need a device that momentarily energizes an injector while you read fuel pressure or voltage drop. Injector testers are available from Kent-Moore (p/n J39021) and other sources.

I use Kent-Moore’s Port Fuel Injection Diagnostic Kit (p/n J34730-E). It contains the port-fuel pressure gauge, the Injector Tester along with other items needed to diagnose EFI on all 85-up Corvettes.

Corvette PCMs take cooling system temperature into account when setting the fuel and spark schedules used to run the engine. Cooling systems on Corvettes built since the mid-’80s not only run at higher temperatures but sometimes don’t have the extra capacity we saw in ’60s and ’70s models. Consequently, they are often running closer to their design limits. That in itself is not a problem; however, these two situations make it important for your car’s cooling system to be operating efficiently. If you suspect a cooling system problem, you may want a diagnostic device that can directly read coolant temperature and run a system pressure check. I have experience with two different brands of this equipment. Both can assist in diagnosing problems with thermostats, temperature sensors and electric fan operation. Additionally, information gained with either of these tools can help pinpoint coolant leaks, cracks in the block or heads and failed head gaskets.

The first is Neward Industries’ Cooling System Retrofit Kit (p/n 4510) intended for use with its Mityvac hand-operated pump, discussed earlier. Adding this kit to a Mityvac allows you to pressure test a cooling system via an adapter that connects to the pressure cap mount. If you don’t have a Mityvac, but do have a source of air pressure and want cooling test capability, KAL Equip has its Cooling System Analyzer (p/n 2967) that performs the same tests.

In addition to adapters for pressure testing, both the Mityvac and KAL kits include a dial thermometer one can use to measure cooling system temperature at the cap mount. Both adapters allow temperature measurement while the cooling system is under pressure via a test "port" into which you can insert the thermometer probe without pressure loss.

Ever had an engine noise you can’t track down? For example, ’85-’93 Corvettes sometimes suffer from an anomaly called "fuel-line hammer." The noise sounds kind of like a valve lifter that needs adjustment, but actually comes from fuel pressure fluctuations caused by the opening and closing of the fuel injectors. The fluctuation sets up a resonance in the fuel rails and flexible fuel lines near the engine that, mainly at idle, is audible from inside the car.

The point, here, is you may need some kind of listening device to avoid misdiagnosis of fuel line hammer or to help you track down other engine noises. In the old days, there were industrial-strength versions of a doctor’s stethoscope; however, today, we have electronics. KAL Equip makes a nifty, Electronic Stethoscope (p/n 2563) that is perfect for listening to injectors, fuel rails, valve covers and other noisy engine parts for diagnostic purposes.

Another traditional tool you may want in a modern diagnostic situation is a timing light. Spark schedules of most 1980-or later Corvette engines are computer-controlled, but 1980-’91 engines, except LT5, used a distributor and required an initial advance adjustment. To check that initial advance or to read the spark advance curve without a scan tester; you need a timing light.

With ’92-up engines, a timing light is unnecessary because timing is not adjustable and there are no marks. A weird exception to this rule is the LT5 which, in spite of timing that can’t be adjusted, has timing marks…go figure.

So-called "dial-back" timing lights, through an internal microprocessor, allow the user to determine a given spark advance by 1) turning a dial until the timing mark on the balancer aligns with the TDC mark on the front cover’s timing scale, then 2) reading the value to which the dial points. Standard timing lights, ie: no dial-back, can check initial advance, but for them to read advance requires the balancer to either be degreed or fitted with a timing tape. Many good timing lights are available; however, be careful about shopping strictly for price. Some inexpensive lights have trouble triggering accurately above 4000 rpm. If you are going to check advance curves, you may need a light with stable triggering above that rpm.

There are two units we looked at for this story. Both performed well in our evaluation. The first was a non-dial-back light from MSD (p/n 8990) that is popular with many racers and has been in our shop inventory for several years. We also tried a KAL Equip dial-back light (p/n 4165) that is unique in that it uses a digital display rather than analog and it reads engine speed as well as spark timing. Both of these units: have all-metal cases, have metal inductive pick-ups that won’t melt if inadvertently touched to an exhaust manifold, are capable of stable triggering up to 8000 rpm and are 12-volt powered via a set of battery clips.

Scan Testers

A scan tester, "scanner" or, more properly, a "hand-held diagnostic computer" is a microprocessor device a little larger than a portable cassette recorder. It’s powered by the car battery and connects to the DLC. It "scans" engine computer operational data and displays it on a small screen. Scan testers are operated by a simple keyboard. Software is usually in plug-in cartridges covering a specific manufacturer’s vehicles. While simple PCM diagnostic procedures with any ‘81-’93 Corvette are possible without a scan tester; advanced service work, high-performance development or any work with ’94-up PCMs requires one of those pieces of equipment.

The most basic scanner operation is reading trouble codes. To do this, connect the tester, turn on the ignition key, select the tester’s DTC function and codes appear on the screen. Another basic feature is a "clear codes" function. This eliminates temporarily removing the PCM fuse or disconnecting the battery to erase DTCs once repairs are complete.

The most valuable scan tester ability is "snapshot memory". This helps solve intermittents by saving a block of PCM data present at the approximate time the intermittent occurred. The user "plays back" this data, looking at what the computer was doing during the period of time covered by the snapshot. Often, this play back contains valuable clues we DIY service techs can use to fix our Corvettes.

All scanners have a list mode that displays PCM data in real time. All read at least two, user-selectable data parameters and some read more. Even when your computer-equipped Corvette is running fine, you’d be surprised at the stuff you’ll learn just by having someone drive while you scan in the list mode.

A price leader in the scan tester market is MPSI’s "Pro-Link 9000." The Pro-Link is half the price of other scanners making it very attractive to those on a budget. MPSI achieves this through emphasis of basic features and sale by direct mail. The unit has an LCD display that shows four lines of data. MPSI’s GM software cartridge covers ’81-’95 powertrain, body and chassis computers.

The OTC "Monitor 4000 Enhanced" is quite popular amongst independent repair shops and has many of the primary scan tester functions listed earlier. It costs more than the MPSI, but includes software for Corvette chassis and body computers along with that for, ah…foreign makes including Ford and Chrysler. OTC software is divided into two ranges. One spans MY81-93 the other spans MY89-96 and includes OBD-II capability.

We prefer the TECH-1A scan tester. In service with Chevrolet dealers since 1985 and made available to DIYs in 1990, the "T-1" was designed and is manufactured by the Vetronix Corporation of Santa Barbara, California to meet a GM specification. About 80,000 T-1 and approximately 600,000 T-1 software cartridges are in use around the world. With ’81-’95 GM powertrain software, cables, adapters and manuals it costs about $1500. I have seen it street-priced as low as $1300. It can be purchased from Mac Tools or NAPA stores.

A plethora of software is available for the T-1. In addition to the standard GM powertrain cartridge needed for engine diagnosis, there are cartridges for chassis and body computers, an "all-in-one" powertrain cartridge that supports vehicles from all three Detroit manufacturers along with other specialized software.

TECH-1s offer basic scanner operations along with bidirectional abilities that give the tester temporary control of PCM functions for diagnostic purposes. A few of these operations are: injector balance tests on ’94 or later LT1s, LT4s and ’90-’95 LT5s, tests of the LT5’s secondary fuel system and, with a variety of Corvettes, control of idle speed, EGR valve, AIR pump operation and reset of "fuel trim" values.

There are T-1 options, such as 12-volt-powered "TECH-1 Printer" which connects direct to the tester and prints the data list or snapshots on an adding machine tape. There are software cartridges for chassis and body computers.

Another cool option is the "Functional Test Director" (FTD) cartridges. GM designed features into the PCMs of ’86-’91 Corvettes that allow a T-1 with an FTD to run automated tests on a variety of engine computer functions. At the end of the sequence, you’re presented with results from each test. If there is a serious problem, testing is terminated and the scanner alerts you of the difficulty.

All TECH-1s can be upgraded. The original T-1 can be upgraded to the T-1A level. Either unit can be upgraded with the OBD-II capability necessary to service 1996 or later Corvettes by adding Vetronix’s "OBD-II Application Kit" which consists of an interface adapter, a specific OBD-II software cartridge and an instruction manual.

The TECH-1 has been in production for 12 years, an unheard-of lifespan for a computer device. It is a credit to Vetronix’s outstanding hardware and software design that the same basic tester supports all MY81-’97 GM vehicles with computers. We asked Vetronix spokesperson Jason Alexander about the TECH-1A’s future and he told us, "There are a huge number of T-1s out there and the unit continues to sell well. Vetronix will continue to develop TECH-1 software on an annual basis for at least the next five years and probably longer."

Of the scan testers discussed, only TECH-1 is GM-validated, uses GM-validated software, is bidirectional and has the FTD option. Only the T-1 and the OTC Monitor 4000 Enhanced support the OBD-II features of ’94-’96 Corvettes.

Right at the time this article was posted on the Web, the Actron Manufacturing had entered the scan tester market this fall with an entry-level unit aimed at DIYs. Actron spokesperson, Bruce Heath, told us just before post time that this new piece of equipment, the Sunpro CP9110, has a four-line display and will work in the data list and snapshot modes. It supports ’84-’95 Corvette PCMs and will support the ’82 car if VIN data is entered as if it were from an ’84. The Sunpro is intended to be list-priced at about $475.00 for the tester and a GM software cartridge. Undoubtedly, street prices maybe lower. A future, extra cost option maybe OBD-II support. At the time this buyer’s guide was posted, Actron was only selling the unit direct, however, Bruce Heath told us that within 4-6 weeks, it will be on the shelves at mass-market retailiers that carry Actron’s SunPro brand.

Your Corvette can Talk to your PC

If you own a PC with an Intel 286 or better processor, you have additional Corvette diagnostic options. For automotive service, a laptop is desirable as it’s tough to drag a desktop machine around on a road test. However, if a desktop is all you have; get an extension cord and haul it out to the garage. At least, you can run the program while the car idles in the shop.

Rinda Technologies’ Diacom is a software-based scanner that reads powertrain data and has basic scan test functions. Though it was intended to run under MS-DOS, it is compatible with what Bill Gates’ considers his gift to mankind, Microsoft Windows 95. However, Diacom is not compatible with Windows versions 3.xx and earlier. The current release version of Diacom, 2.83, fully-supports ’82-’93 Corvettes and supports non-OBD-II functions of ’94s and ’95s. With an optional, five-pin DLC adapter, support is extended back to computer ’81s.

Diacom has some advantages over hardware devices. Because a computer screen has more area, it displays more data in list mode. Also, during our evaluation, I discovered that when connected to certain PCMs, Diacom reads specific data parameters that scan testers do not. While some of these additional parameters may be of little value in service work, they could be of interest to one doing vehicle development. Another advantage is that, in snapshot mode, Diacom samples data at faster rate than some scan testers. If the intermittent you are solving is of short duration, Diacom might detect it whereas some of the hardware devices may not.

The uplevel, Diacom-Plus sells for $579 has all standard Diacom features and adds: 1) more sophisticated snapshot ability with additional memory and trigger options and 2) a line-graph mode that displays three, user-selectable data parameters. If you want to perform any engine management system development work, Diacom-Plus is the way to go and in some cases may be a better choice than a scan tester due to the ease in which it stores data on the PC’s hard drive.

Both Diacoms have limitations: 1) They do not support ’96 or later Corvettes; however Rinda Technologies is developing an upgrade that will and it may be available by the time you read this story. 3) Due to differences in how the PCM transmits engine speed data over the serial data link, Diacom will not display engine speeds over 6375 rpm when connected to a ’90-’95 LT5. 2) They do not support chassis or body computers. 3) They are not compatible with a few older, 386-based laptops using oddball chip-sets. Generally, these machines were price leaders. If you have questions on compatibility, resolve them with Rinda Technologies, and 4) the cable that connects the laptop to the DLC is too short for road test use. A "keyboard extension cable," available at most computer stores, is required.

Bottom line? If you already own a laptop PC and you’re working on a ’81-’95 Corvette; for all the stuff it does and its $299 price tag; "standard" Diacom is an outstanding value for the DIY service technician.

For someone with both a PC and a TECH-1, a useful program is a scan tester enhancement called "Techview." Distributed by Vetronix through Mac Tools, it allows a PC to act as a display for a TECH-1. It is available in two versions, one that runs under DOS and another for Windows 3.x, NT and 95. In its list mode, Techview shows up to 42 data parameters simultaneously and in color, if you have a color monitor. Techview also has three graphic modes: bar-chart, line-chart and overlap line-chart. It saves snapshots to a PC’s disk drive which makes it valuable to those wanting to store data for future reference.

Where Techview really shines is in its bar chart mode. Four user-selectable engine parameters are displayed in real-time, vertical, color-bar form for a revealing picture of how different engine management data interact.

Both the OTC and the MPSI testers also transmit data for display or storage on a PC. For the Monitor 4000 Enhanced, OTC has a program called "E-Z Event Plus" that functions like Vetronix Techview. The MPSI Pro-Link, requires no additional software but transmits only in the list mode.

The Top of the Line

We promised you diagnostic equipment for your Corvette from mild-to-wild. Well…now you get wild. We talked earlier of the lowly test light. Would you like a fully-computerized test light? If so, Mac Tools quenches your thirst for high-tech with their "Logic Probe" (ET130). This unit uses a microprocessor-controlled, triple-LED set-up to test continuity of DC circuits running at as little as one volt. It also tests the AC or pulsed-DC output of a variety devices such as distributor pick-up coils, crankshaft and camshaft position sensors or ABS speed sensors. The Logic Probe can even test some electrical functions of fuel injectors while the engine is running.

Mac also sells the ZR-1 of digital multimeters. Manufactured to Mac’s specifications by Tektronix, this piece of equipment (p/n ET332) has outstanding accuracy for an automotive DMM. It’s rugged, has a high-impact plastic case and is surrounded by a rubber boot will take any punishment you can dish out. It has all typical DMM functions along with the ability to test diodes, measure AC voltage, capacitance, injector pulse width and temperature.

During our evaluation of the ET332 we were impressed with some of its unique, high-tech automotive features. It’s the only DMM we’ve seen that senses voltage spikes you normally could only measure with a VOM. This "peak hold" feature senses spikes as short as one millisecond. It’s, also, the only unit we’ve seen with a digitally-generated, "pseudo-analog" bar graph that reads simultaneously with the numeric display. Some clever electronics allow the Mac ET322 to measure injector pulse width. It has a data recording function which, like a scan tester’s snap shot mode, assists in solving intermittents. Controls are intuitive and to save the battery, it even shuts itself off after 15 minutes of inactivity. If you want the finest DMM available to advanced DIYs, the Mac Tools ET332 is your baby.

More high-end, "technogeekery" is Vetronix’s second-generation scan tester, the "Mastertech," which could be described as: TECH-1…only more so. It uses T-1 software, offers all T-1 functions, uses the convenient and familiar T-1 interface and, with simple installation of a optional "daughterboard," is OBD-II compatible. Like other Vetronix equipment, it’s built to tough, dealer service standards and is available from Mac Tools.

Use Mastertech in its "enhanced" mode and you have a truly amazing, hand-held diagnostic computer. I loved its 3"x3" display that shows either: 16-lines of user selectable data in list or snapshot modes, six data parameters in bar-graph mode or two in line-graph mode.

Another Mastertech option is the "Enhanced Diagnostic Lead Set" that includes some optional test leads, an interface box and two instruction manuals. It adds an oscilloscope mode that can be used to graphically represent the electrical operation of sensors, actuators, injectors and ignition systems. While use of an oscilloscope would be considered an advanced diagnostic technique, the Vetronix Mastertech offers adequate waveform display ability at a fraction of the cost of a dedicated oscilloscope. For those inexperienced in using waveform display as a diagnostic tool, Vetronix adds a very intuitive, on-line help program called the "Waveform Assistant" that has a library of example wave forms. For instance, suppose you want to see how the passenger side O2S is working on a ZR-1, but you don’t know what an ideal O2S waveform should look like. The Mastertech’s Waveform Assistant is always there to show you.

After comparing their performances in a diagnostic road test situation, I preferred the Mastertech over a color, 486DX2 laptop running Diacom. Only get 16 lines of data can be displayed at a time vs. 30, but in a road test situation; Mastertech is easier to set-up and use and you still can select on-the-fly which 16 data parameters you want to display. Another advantage is that the Vetronix Mastertech has software for all on-board computers in all MY81-’97 Corvettes including those with OBD-II.

If you’re wallet runeth over, you’re starting your high-end, diagnostic tool box from scratch and you’re looking at Diacom-Plus and a whiz-bang Pentium laptop to run it on; consider the Mastertech instead. It’s a more capable, less complex and less expensive package. The Vetronix Mastertech is leading-edge stuff and one of the most advanced scan testers on the market at this writing.

The newest piece of diagnostic equipment of interest to Corvette enthusiasts goes with the newest Corvette, the C5. The GM Tech 2 "Hand-Held Diagnostic Service Tool", was introduced in February of 1996. The result of a two-year development, it’s manufactured by the Hewlett-Packard Company under a contract with the GM Service Technology Group (STG). By the fall of last year, most Chevrolet dealers had at least one T2 and, at the start of the 1997 model year, it was declared an "essential tool" for all GM dealers.

STG chose not to make a Tech 2 available for us to test, saying that only experienced service personnel can operate the unit. Additionally, we contacted Hewlett-Packard with a similar request, but they did not reply at all. Nevertheless, we bent our rule requiring equipment covered in this buyer’s guide be evaluated by our staff because T2 is one of the diagnostic devices for Corvettes. Like the TECH-1A, it will eventually be available to DIYs. Perhaps, at a later date, we can do an in-depth analysis of it.

For now, suffice to say that manufacturer specifications indicate that the new GM tester surpasses the Vetronix TECH-1A in diagnostic power but falls short of Vetronix’s Mastertech, our benchmark for high-end scan testers. The Tech 2 displays 11 vehicle data parameters simultaneously, more than the T1’s two, but less than the Mastertech’s 16. The T2 uses a 32-bit, 16-Mhz processor compared to the Mastertech’s 16-bit/10-Hhz and TECH 1A’s 8-bit, 2-Mhz chips. T2 memory is 10 Megabytes compared to the T1’s optional, 1 Mb. The new GM unit and the Mastertech are of similar size, shape and weight. Like Mastertech, T2 has a larger display screen and an enhanced keyboard with programmable keys. Unlike the Mastertech, the Tech 2 was designed under some stringent cost constraints. To meet those goals, hardware shortcuts were taken which make the Tech 2, in spite of a speedy central processor, not as strong a tester in the service-diagnostic environment as it could be. In fact, we hear reports from the field that GM’s dealer network is not unified in their acceptance of the Tech 2. Perhaps that will change as time goes on.

While the Tech 2’s software may eventually backdate to the 1992 model year, the unit will not accept T1/Mastertech software cartridges because GM and H-P adopted the "PC Card" system for software storage. During the development of the Tech 2, H-P had difficulty with the learning curve of automotive diagnostic software. Interestingly, GM turned to the Vetronix Corporation, the manufacturer of the TECH-1A and a competitive bidder on the Tech 2 contract, for assistance. For the last couple of years, Vetronix has had a dozen people working in Detroit developing T2 software.

Beginning with the 1997 model year, dealer-specific, recalibration software is only available for the Tech 2. Vetronix will continue to develop new T1/Mastertech software for aftermarket sale until at least 2001 and probably longer but without the recalibration feature. For pre-’92 Corvettes, the TECH-1A remains the scan tester of choice for GM dealers and discriminating DIYs.

STG tells us that the Tech 2 sells to GM dealers for $1885.00, a price that is only a few of hundred dollars more than the TECH-1A. It is possible that the price to go up when the unit is eventually made available to third-party users. The time at which GM and H-P will release the Tech 2 for sale to we DIYs has not been determined.

Information, Please

All this diagnostic equipment is useless if you have no service data. The advent of computer-controlled engines has made service manuals indispensable. The best stuff comes from GM’s Service Technology Group (STG). To prepare this article, we used the 1995 and 1996 Chevrolet Corvette Service Manuals. Non-factory service manuals, such as those available from Haynes for some Corvettes, are desirable for their lower prices and coverage of a span of model years. However, to get the price down, non-factory manuals are condensed and because of that, occasionally, data is omitted. Nevertheless, when confronted by cosmic price tags for factory manuals, Haynes books are an attractive value.

Books on general engine computer subjects are also available, such as the excellent, How to Repair & Modify Chevrolet Fuel Injection, published by Motorbooks International or Automotive Computer Codes and Electronic Engine Management Systems from Haynes’ Techbook series.

GM Service Technologies Group (STG) distributes training publications to the general public through MascoTech. Fundamentals of Computer Command Control (CCC), and, if you use a TECH-1, the Tech-1 Handbook are outstanding references. Additionally, if you are working on an older computer car and don’t have a factory manual, STG’s 1981-1986 Computer Command Control Performance Diagnosis is a great resource. If you are interested in learning more about oscilloscopes in automotive service, STG publishes an outstanding book, Engine Performance Testing, that contains a large quantity of instructional material on that subject.

Lastly, both Wells Manufacturing and Actron, discussed in the first section of this article, have video tapes available detailing the use of their diagnostic tools.

Actron Manufacturing Co.
(Sunpro and KAL)
9999 Walford Av.
Cleveland, OH 44102
216 651 9200

Chevrolet Service Manuals
Helm, Inc.

29784 Little Mack
Roseville, MI 48066-2298
800 345 2233

MascoTech (GM-STG publications)
1972 Brown Rd.
Auburn Hills, MI 48326-1701

Mid America Designs
(Performance Choice)
Box 1368
Effingham, IL 62401

Pris Tech, inc. (Mityvac)
6952 Fairgrounds Pkwy.
San Antonio TX 78238
800 648 9822

Rinda Technologies (Diacom)
5112 N. Elston Av.
Chicago, IL 60630-2429

Vetronix Corporation
2030 Alameda Padre Sierra
Santa Barbara, CA 93103


Autotronic Controls Corp.
1490 Henry Brennan Dr.
El Paso TX 79936
915 857 5200

Haynes North America, Inc.
861 Lawrence Dr.
Newbury Park, CA 91320

Mac Tools
4635 Hilton Corporate Dr.
Columbus, OH 43232-0940
800 848 6500

Micro Processor Systems, Inc. (MPSI)
6405 19 Mile Rd.
Sterling Heights, MI 48314
800 639 6774

Motorbooks International
Box 2
Osceola, WI 54020

OTC division of SPX Corp.
655 Eisenhower Dr.
Owatonna, MN 55060-1171

Simpson Electric Co.
853 Dundee Av.
Elgin, IL 60120-3090

Wells Manufacturing Co.
Box 70
Fond du Lac, WI 54936-0070


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Bastardized and Orphaned
The Strange Case of the ’80 California Cars

After catalytic converter was introduced in 1975, emissions control development at GM turned to perfecting the early, rudimentary iterations of the digital engine controls we now know well.

Then, as now, the "People’s Republik of Kalifornia" was the trend-setter in emissions regulation. Smog laws in California for 1980 were more restrictive than those of the other 49 states. These more restrictive standards would be enforced nationwide in 1981. To maintain acceptable drivability while meeting California emissions law and to prove the concept of digital engine controls, GM decided one of the early tests of their budding technology in a production application would be on Corvettes sold in California that year.

Those cars all carried a 5.0-liter engine (LG4) fitted with a system called "Computer Controlled Catalytic Converter" or "C-4". Even this "prehistoric" version of digital control had a positive effect on the emissions/drivability/performance equation. Horsepower of these computer, five-liters was only 10 shy of the larger, but non-computer, L48 5.7L engine that was the base powerplant in the other 49 states. Emissions were less than and drivability, when the system was working properly, was comparable to the L48.

No 5.7L engine was available in California that year, the LG4 earned the buyer a $50 credit and all 3221 of these cars had automatic transmissions. An ’80 C-4 car is identified by the letter H as the fifth character of its VIN and an engine block code of ZCA.. Other indications are an oxygen sensor screwed into the driver side exhaust manifold and a special electronically-controlled carburetor. That year was the only time the C-4 system appeared on a Corvette. For 1981, all cars had 5.7s equipped with a refined system carrying the Chevy marketing weenies’ new name: "Computer Command Control."

Although the C-4 system helped with emissions and drivability, it had a nasty downside. Since on-board diagnostics were still in their infancy; the system had deficiencies that made fault diagnosis very difficult.

The speed at which things happened in the system was rather slow. We figure, back then, engineers used sun dials to benchmark PCM processor speed. Ok…it wasn’t quite that bad, but in some cases, a problem that would set a DTC would have to exist as long as five minutes before it would be "memorized." This slowness made many problems into intermittents that, in later years, would be "hard codes". Once a DTC finally set in memory; there was another big problem.

The ’80 PCMs had "volatile" memory. That is, if the ignition was turned off; the memory was erased. If a DTC was set, anyone servicing the car had to read codes before the ignition was turned off. Interestingly, Chevrolet left provision to make the memory permanent by simply putting battery voltage to an orange wire that runs from the ECM’s "S" terminal. The basics of this procedure are discussed in the 1980 Corvette Service Manual; however, illustrations showing the location of the wires are incorrect. On Corvettes, the unconnected orange wire is near the PCM and the battery which are both in the well behind the driver seat. Connect this unused orange wire to the positive battery post and the PCM memory becomes permanent. This bizarre situation existed due to Chevrolet’s fear that 1) a permanent memory would drain the battery and 2) that it, perhaps, forgot to test the current drain of the memory to see if that fear was justified.

I suggest, if owners of '80 C-4 cars start seeing check engine lights, that they make the orange wire connection during the diagnosis period. If your battery condition is an issue; buy a trickle charger. You are far better off with the permanent memory when trying to solve a problem.

The biggest bastardization of all, however, was the PCMs in these cars did not transmit the serial data that a scan tester reads, so as far as a tester goes; ’80 C-4 cars are orphans. Intermittents, of which there are undoubtedly many because of the system’s slowness, are far more difficult to solve. To work on intermittents occurring with these cars you will need a top-drawer DMM that has a data recording feature, such as the Mac ET332, some of the harness patches Mid America sells or SunPro’s sensor testers and the troubleshooting charts in the 1980 Corvette Service Manual…oh, yeah; plenty of patience, too.

Thankfully, there is at least a way to read the PCM memory on an ’80 C-4 car. They transmit trouble code information when the PCM’s "diagnostic ground" or "trouble code" test lead is grounded. This is a white and black wire wire tipped with a green connector that can be found near the PCM. The best way to do this is run a jumper from the green connector to the negative battery post. The engine must be running to transmit codes and, like most Corvettes, they are displayed via flashes of the "check engine" light.

Specific service information on ’80 C-4 cars is difficult to find. The section on C-4 in the ’80 Corvette Service Manual is a reprint of material used in the Caprice, Malibu, Monza and Camaro books indicating that the decision to use this system on Corvette may have been made after manuals for MY80 went to press. Although service procedures are listed in print; there are no diagrams as to component placement or wiring harness routing on Corvettes. However, for what information that does exist; the factory manual appears to be the only place you will find it. Other, non-factory manuals for 1980 vehicles mention little about C-4 and nothing about it on Corvettes.

I don’t envy people working on ’80 C-4 cars, especially those trying to track down intermittents. However, the C-4 system was a major milestone in the history of computer engine controls on production automobiles. Besides allowing Corvettes to meet California emissions standards, this early PCM "experience" provided valuable knowledge that made the systems used in later model years far better.


Related Article: Digital Assistants

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