Diagnosing The GM TPI

If this light comes on, it’s time to plug into the ALDL port for a check up. If you don’t have access to a Diacom unit, place a jumper such as a paper clip between the upper right two terminals. You can also buy a cheap code scanner at an auto parts store that does the same thing. This will flash the “check engine” light in respect to the code it’s giving. For example, a code 12 will flash once, pause, and then flash twice. While this is helpful, it will not give you anywhere near the data the Diacom provides.

Start by installing the software into your computer—any MS-DOS based equipment with at least 256K of memory will do (even the most archaic computer has this). The advantage of a laptop is the ease of working in the car. However, even a desktop home computer will suffice with the aid of an extension cord.

Plug the cable into the parallel printer adapter and you’re ready to get into the brain of GM’s ECM.

The ALDL connector is located on the lower portion of the dash on the Firebird, just above the right knee. Our Diacom package came with two harnesses to fit all ’82-and-up GMs along with the special plug for TPI-specific applications.

After you have the connector plugged in and the software on, be sure to have your vehicle identification number handy, as the computer will ask you to punch it in. Once this is done, the software will then either ask you if the correct car is displayed or give you the option of clicking on the correct car should two or more arise.

This was a shocker! We idled the car for five minutes and as soon as we turned the car off, the trouble code light blinked immediately. Hitting the trouble code key revealed six codes.

We figured the easiest code to tackle first would be the code 51 (faulty or improperly installed ECM prom) by removing and replacing the ECM prom. We removed the computer from under the dash, loosened two bolts, and removed the plate. Inside there are two clips (arrows) that you can push to remove the prom. Once removed, we simply blew air on the connectors and firmly placed it back into place. Be careful, too much force can break the circuits on the board.

After linking the Diacom back to the ECM, we were down to five codes, only this time the code we were trying to eliminate didn’t disappear. At this point, we called on the expertise of Myron Cottrell at TPI Specialties. Myron wasn’t the least bit surprised by this situation, he said that since we weren’t getting an accurate reading, the codes may have arisen from either a bad ground or a small voltage spike when we turned the car off.

We then disconnected the ECM fuse and the ground cable from the battery to erase any codes that were stored in the ECM. Call it overkill, but we did this for 20 minutes, even though a minute would have sufficed. Hey, we wanted to make sure there were no codes stored whatsoever! After running the car for another five minutes, it looked as if we were on the right track—no codes! At this point it was safe to say that the battery’s ground cable may not have been as secure as we thought.

Of course it was getting too easy. After a few minutes went by we received a code 33 (Mass Air Flow sensor high). When talking to Myron for the second time, he asked if the car was driveable. Apparently when a code 33 reveals itself, these cars normally go into a limp mode, locking the car at 22-degrees of timing, which causes terrible driveability. Interesting, but that wasn’t the case with us. CC

Our project Fire-Turd has run well and never really skipped a beat (we constantly cross our fingers!). Through the use of modern technology, the Diacom identified several codes and helped us eliminate all but one. This could be caused by a marginally failing device, or a fault somewhere in the wiring harness itself. At this point you should consult a factory service manual, which contains step-by-step diagnostics for isolating the fault causing the code. Usually all that’s required is a multimeter, test light, and jumper wires. In our particular case, Myron believes it’s just a bad connection somewhere, and told us to replace the Burn off and MAF Power relays first, then to look into replacing the Mass Air Flow sensor. After all, spending $20 on a couple of sensors is a lot cheaper than handing over $300 for a new meter! We’ll keep playing with the ground and change the relays, but the car runs fine and idles great, and the fan works, so we’re ready to head back over to the nitrous refill station.

It’s great to open the hood of your car and see nothing but the motor staring back at you. Move up a few generations on any vehicle, though, and you’ve got miles of wiring snaking through every nook and cranny. If your carbureted car doesn’t start, chances are you can pinpoint the source of the problem pretty quickly by checking fuel delivery, spark, and so on. But what happens when you turn the key in a late-model car like our ’88 Firebird and nothing happens? Fergettaboutit. Unlike pre-computer cars, these vehicles rely heavily on what the Electronic Control Module (ECM) tells the engine and the rest of the car to do. Based on information gathered by the ECM from sensors on the engine, the ECM sends signals back to the injectors and ignition to maximize performance in any given driving condition. Unless you have a lot of prior experience in diagnosing TPI cars, it’s not hard to suddenly realize why so many shops charge a high premium for such repairs. However there is an alternative to parts swapping: diagnostic equipment like the Diacom Test Software from Rinda Technologies. Just install the software in your laptop computer and plug it into the Assembly Line Diagnostic Link (ALDL) port with a supplied 12-way connector plug. The Diacom Test Software displays a multitude of engine and performance data and gives you access to any trouble codes stored within the ECM.

Since our Firebird has more than proved that it’s a reliable ride, we really didn’t anticipate any surprise codes to pop out. For starters, it passed smog. More importantly, it hasn’t left us stranded (yet) despite all the nitrous floggings we’ve hit it with, and that we still haven’t done an oil change since we procured the car. The only problem that we have encountered is the radiator puking into the overflow and the electric fans not operating. Of course, we assumed the temperature gauge worked—but we were wrong. Other than that, we’ve cruised worry-free all over L.A. in 90-plus-degree weather and stop and go traffic. Now that we’ve made the car look sexy with its fresh coat of paint, new interior, and killer wheels and seats, it was time to plug into the ALDL port for a check up. So when we plugged in the Diacom software we were pretty surprised to see six trouble codes stored in the ECM. Follow along to see what this computer thing is all about.