Most of the time when a shop calls me in for a diagnostic job, there’s a pretty cut and dry fix. Every once in a while, a job comes along that becomes a challenge and all too often makes me wonder why I do what I do. But that’s a story for another time.
After performing the test with the scan tool, all the modules reported for duty except the PCM. I did visually check to see if the PCM was there (don’t laugh, I’ve seen stranger things) and that the connector was plugged in, which it was. The next step is to see if the PCM is getting the proper powers and grounds to make it functional. Remember, when working on a network problem it is essential that a good wiring schematic is available. When looking at the schematic of the PCM, I looked for the power feeds going to it. Using a wire schematic feature I highlighted the two power circuits in question. The PCM is located beneath the brake booster, by the bottom of the firewall, which is not an easy place to access.
The next step is to check the ground circuits. These circuits are Terminals 3 and 25 along with Terminals 77 and 103. All four terminals checked out fine and passed the headlight test. So, I have good power and grounds at the PCM. It’s time to check to see if the network circuits are shorted or open. This particular vehicle uses a Standard Corporate Protocol (SCP) network. Every module on this network communicates with each other except the Restraint Control Module (RCM). The RCM is on another network called the ISO 9141 or International Standards Organization network. Both networks are read at the Diagnostic Link Connector (DLC) by the scan tool.
A couple of days later, I come back to program the PCM. I hook up my scan tool and guess what? No communication with the PCM. This is every mobile techs nightmare. I misdiagnosed this vehicle. I promptly rechecked my previous diagnosis and came up with the exact same results. This PCM has got to be faulty. By all circumstances this PCM should be alive. Now what? I plug the PCM connector back into the PCM and start to reread my wiring schematics to see if I am missing anything. I also did a quick search on Identifix and iATN to see if anybody else has had the good fortune to come across the same problem as I have. No luck there either.
Replacing the IAC motor and installing the original PCM fixed this car. Luckily the shop was able to return the PCM because it wasn’t programmed. Having these painful learning experiences can sometimes help make a good tech a better tech.
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