[donnrogers]

Starting about a month ago, when I started my car the volt meter on the dash would show low voltage. I'd rev the car a few times and it would kick up to where it usually shows. Then I started noticing the meter spiking and the engine accelerating while I was sitting in park. The battery "idiot light" would come on too. I went over to autozone and they did a test. They said the battery was not holding a good charge; so I replaced it. A couple days later, I couldn't get the volt meter to kick up even after reving the car. As soon as the juice in the battery was gone the car died. I'm thinking bad alternator. I replaced it. New one is not charging the battery. Old one is not charging the battery. Now I'm wondering if there is a short in the wiring from the PCM to the alternator or a bad PCM.
Do you know what pins on the PCM are for the alternator GENMon and GENCom. With the engine off and running both; I checked voltage at the alternator and alternator connection. B+ to ground is 13vdc. Pin 3 is 13vdc. Pin 1 is about 9vdc. Pin 2 is .004vcd.
I don't think the alternator is "exciting."
I need help!
'05 P71 approx. 125k miles. 6g alternator

[Dominick]

Did you check or replace the connector to the internal regulator.
that is one issues with that set up!

[donnrogers]

each alternator had its own internal regulator. Both bench test as good.

[electro]

Originally Posted by donnrogers

...Do you know what pins on the PCM are for the alternator GENMon and GENCom. With the engine off and running both; I checked voltage at the alternator and alternator connection. B+ to ground is 13vdc. Pin 3 is 13vdc. Pin 1 is about 9vdc. Pin 2 is .004vcd...

Donn, your PCM determins the optimal voltage of your charging system, by using Pulse Width Modulation to communicate with the Regulator, mounted internal to your Alternator. Your system uses 2 unidirectional communication lines between the PCM and Regulator, and both these communication lines use PWM. The Generator Communication (GEN COM) line communicates the desired setpoint from the PCM to the Voltage Regulator, while the Generator Monitor (GEN MON) line communicates the Generator load and error conditions to the PCM. The third pin on the Voltage Regulator is the "A" circuit pin, a dedicated battery voltage sense line.

Quote:

B+ Output Terminal Circuit 38 (RD) Generator output voltage is supplied through the battery positive (B+) output terminal on the rear of the Alternator, to the Battery and the electrical system. Circuit 35 (OG/LB) This is the A terminal, battery voltage sense circuit. Circuit 1816 (YE/LB) This is the Alternator Communication circuit (GEN COM). The PCM determines the optimal voltage setpoint for the charging system and communicates the information to the voltage regulator. Circuit 1817 (YE) The Alternator Monitor (GEN MON) circuit communicates the generator load and error conditions to the PCM.

Donn, If this car was in front of me,
1) I would verify any voltage reading that the dash instrumentation gives, by using a handheld voltmeter, connected directly to the battery.

2) I would also scan for codes, as there might be a communication code that might give you some direction. The scanner needs to be capable of reading communication codes.

3) I would not probe the PWM circuits anymore.
These are communication lines, not voltage lines. If you wish to "read" them, find someone with a labscope....