2001chevysuburbangray

I have a 2001 Chev Suburban with lots of miles on it. I recently replace the alternator with a rebuilt alternator. Since that time, I observe the voltmeter dropping voltage from the 14 plus volts down to as low as 12 volts. Sometimes the batt lamp comes on; but sometimes it doesn't. The voltage recovers back up to 14 volts plus. I never experienced this voltage deviation before in the 478k miles on this vehicle. This is the second time I have replaced the alternator. I took the alternator back to the store and they checked it and said it was okay. In this investigation, I have learned that this model suburban alternator also gets a signal from the vehicle ECM during operation. I have also replaced the battery, checked all the batt cables and other charging wiring and connections and find no problems with connection integrity. I suspect that the ECM is causing this but don't know how to test or what I should do..I cannot really find any info that is clear and complete on this system.
ALH

mountainmanjoe

check the voltage with a meter at the alternator post

2001chevysuburbangray

I used an accurate multimeter to check the accuracy of the dash voltmeter and it is correct. This is a real voltage deviation caused by either the alternator voltage regulator or the ECM, I think, but I don't know which.

mountainmanjoe

Right, but where did you check it?

With the engine running (and ideally when you notice the voltage dip), probe the voltage between battery positive and the alternator positive.
Then, probe the voltage between the battery negative and the alternator shell.

The PCM modulates the alternator's field winding current on the grey wire, to compensate for engine RPM. The current is pulsed at 400 Hz with a duty cycle between 10 and 90%.

2001chevysuburbangray

I checked the voltage at the batt pos post and the alternator. The batt pos post was only .1volt less than the alternator output post. This vehicle has never acted like this with the alternator output reducing since I have owned the vehicle from new. It only started doing this when I installed this rebuilt alternator. It always stayed on the 14 plus volts when the engine was running. Why did it start doing this now? I know that this year model does not have the smart charging system; that started with either the 2002 or 2003 year model. I do not know why this started now unless the alternator really is not for the 2001 without the smart charging system.

mountainmanjoe

you need to probe like I described in my last post