Every now and then I get a vehicle I wish I never started on. This time it was a 97' Chevy 1500 K series. No start or communication with PCM.
Should be an easy in-and-out job, or at least that's what I thought it was going to be. The truck was from one of the many Spanish-speaking shops that use my services. I don't speak any Spanish (wish I did), but the guys at this shop know I'll be patient with the language barrier, and somehow communicate what needs to be done.
It all started with a rebuilt transmission they installed. After it was installed they had a service bulletin to do; which required moving a couple of wires on the PCM. The directions they had were very good. In fact, it had detailed photos along with the instructions to make the update. They tried to do it, only to find out one of the wires didn't match the drawings. (There was a year difference between the directions and the actual year of the truck.) Instead of going any farther, they sent me the truck to see if I could help them out with the update. When the PCM was plugged back in with all the wires back in their connectors, the truck wouldn't start. Now, instead of driving it over, they had to tow it.
The truck showed up the next day, not a pretty sight to say the least. It had over 250,000 miles on the odometer, with bent and twisted body parts. The driver's door was sagging to the point it would catch the fender every time you opened the door. Not exactly a fine ride, but as a work truck … it'll do. The shop owner explained the problem to me, and told me he needed it back as soon as possible.
You know, it's not that "Murphy" hangs around my shop, but he sure wanted me to earn my keep on this one. Some day I'm going to have a little "communication" with Murphy himself. He really knows how to make things tougher than they need to be.
I hooked up the scanner to see what was going on. Although it had perfect communication with the ABS, the Airbag system, and class 2, it could not connect to the PCM. No communications flashed on the scanner's screen. My first thoughts were the wires were not back in correctly, or one had pulled out when they re-connected the computer. That's where the fun began. Every single wire was not only in the correct spots, but all the powers and grounds were there. I've seen these problems before, and it almost always turns out to be a corrupted, or lost, data line. Now of course, there are a few other possibilities, but the data line is usually where I go first.
My method of checking for this kind of problem is to cut the data line between the computer and the ALDL 16 pin connector. Then run a bypass line from the PCM directly to the ALDL, and check the data line again. This time that didn't work… still nothing. I re-checked the powers and grounds. They were as good as the first time I had checked them. Now things are getting a little serious. Time to check it with the scope. After verifying the voltages and ground signals (again) with the scope; I looked for a scope reading for the actual data line. Oh, oh… flat line, it's a bad PCM. I haven't a clue what they could have done to the computer from just changing a wire or two, but it sure was dead. I called the shop owner to let him know.
"I'll bring another one tomorrow morning," he tells me.
The next day a "salvage" computer was dropped off at my shop. Without hesitation I popped it into the truck, turned the key on, and ran it through security setup. (Thinking I'm done, and there wasn't any reason to check any farther.) But, it still wouldn't start. Ok, what's the deal? Did I miss something? Now I'll have to re-check everything I just did yesterday, again. I checked the powers and grounds, and all the other wires and connections this time. But there still wasn't any communication with the computer. Could it be another bad PCM? It sure seemed that way. I'll have to call the shop owner and let him know what I found out. This time he decided to go to one of the discount parts stores, (where he gets his parts from), and pick one up.
Here I go again… another computer and another failed attempt at communication. Not that I wanted to spend the rest of the day doing all the tests over, but there had to be something I was missing or a really good reason for this catastrophe. Before calling the shop owner again I wanted to make absolutely sure there were no mistakes in my results. I just so happened to have an extra set of connectors off an old harness from a car I salvaged out.
I made up a set of bench test plugs. The connectors are clearly marked with the color and the numbered connection pins. To get the PCM to talk on the bench, all I needed were the positive and ground leads and of course that very important data line installed into the test plugs. This way, I could bench test the PCM without interfering with the wiring in the truck. A simple scope reading could tell me what was going on. No communication should be a thing of the past now.
I wasn't a bit surprised… the weirdness continued. This PCM doesn't even turn on, completely dead. I re-checked my pin positions several times, I was absolutely sure I had them right. It can only mean one thing… another bad computer.
Would you believe this went on for 3 more times? Yes, yes, it did. Each time he would bring me another computer, I would check it with my little bench plug set up. Each time it did the same thing. I couldn't be sure but there's a pretty good chance he wasn't communicating his needs to the part store. When the part store checks the flash program in the PCM they are NOT using the same leads that the actual truck uses. Only one positive lead and the data line are the same. Their pin configuration for reprogramming at the store is almost completely different from the trucks!
Finally on the 6th try I got what I wanted… COMMUNICATION! WHOO HOO! I can't remember a job that I went over the same test procedures so many times just to get the results I expected. There's that old saying; "Doing the same thing over and over again and expecting different results is the definition of crazy." Oh yea, I was totally feeling the "crazy" on this job.
There was a point where I started doubting myself, but I stuck to the test results and double-checked my work each and every time to make sure I had it right. Even though the shop owner was getting concerned that I may not solve his problem; he made the commitment to stick with me. We've done a lot of work for each other over the years, and he was confident that I'd find the problem.
I'm glad he let me stay at it… he put a lot of trust in me to get the job done. Even though we had a language barrier between the two of us, the main language barrier was actually the computer not communicating. I could barely understand what the shop owner was telling me, but I'm sure that PCM couldn't understand either Spanish or English. For me, this one took a lot of composure to get it done. After seeing so many bad computers in a row, (I still think it was the same PCM from the part store) you can imagine how frustrated I was getting.
In every different direction there was a communication problem. From the shop owner, the part store reflashing the PCM and the bench test that I used. I may not speak Spanish, but I can speak a little computer. I guess in some way, I really am bilingual.
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