I looked it up at work. I was right. The PCM controls the field circuit to the alternator. The PCM has a 'voltage sense' input that it uses to determine the voltage in the charging system. It has a preset value built in and when the voltage goes under that value, it grounds the field wire to the alternator until the voltage rises above that value, at which time it removes the ground. This happens very fast to maintain a constant voltage under a wide range of loads on the electrical system. If the PCM stops functioning, the charging system will fail. This could be intermittent, like what you're describing. This could ALSO be the ONLY part of the PCM that has a problem so don't assume that just because everything else is working that the PCM is OK because that may not be the case. A way to check this is to disconnect the small wire connector from the alternator. With the vehicle running, and a test light connected to the POS+ battery post, probe the wire(I think it's a red wire or a black wire) and see if the light lights up. If so, a 'ground' is present, meaning the PCM IS grounding the field circuit. See if you can get your hands on a wiring diagram to be sure exactly WHICH wire it is. If NO ground is present, the PCM is NOT regulating the voltage and should be replaced. Hope this helps. Let me know if I can help more.
Friday, November 7th, 2008 AT 4:52 PM