My voltage started dropping after my h/c swap. Not sure if that had anything to do with it because it didn't appear until 4 to 6 weeks after the install. The alternator would charge intermitantly and got worse to the point were the car wouldn't start. I charged up the battery and bought an alternator from Autozone and installed. Now the car doesn't even charge intermitantly. I checked everything out in my old alternator except for the transistor that regulates the voltage and reinstalled. Same results no charge. Next I took my new alternator back and had it tested and it is fine. That narrows it down to wiring. There are three wires going into my 98 mn6 alternator and I'm trying to figure out how to test them. Any wiring diagrams or tips?

 

You can only use a PCM specific alternator/gen for your car.
You need to have your original alternator rebuilt, you can not buy a NEW alternator for your car because they do not make them anymore... the PCM tells the alternator what to do and when to do it... it is not your average everyday alternator... A New GM alternator and a New PCM flash from the dealer may finally correct this problem.

here is a little tutorial I made a few years ago for this situation to explain how a C5 alternator works. Note, the alternator has to be year specific.

The PCM monitors the alternator through the red and grey wires

The L-terminal circuit from the generator is a discrete circuit (a discrete circuit has no splices and only one source and destination) into the PCM. The PCM applies ignition voltage to the generator L-terminal circuit. A small amount of current flows from this circuit through the generator windings to ground to create a magnetic field which starts the generator process. When the generator is at operating speed and producing voltage, a solid state switch for the L-terminal circuit in the generator opens and the PCM detects that the initial startup current flow has stopped.

The PCM expects to detect low voltage on the L-terminal circuit prior to the generator rotating at operating speed and conversely expects the circuit to be at ignition voltage potential when the generator is operational. When the PCM detects a fault (circuit shorted to ground, or circuit shorted to voltage), the Driver Information Center will display Charging System Fault.

The generator has an input to the PCM called the F Terminal to indicate the percentage of total capacity that the generator is producing. This signal is detected by the PCM as a duty cycle from the generator and displayed on the scan tool as a percentage. The PCM can monitor the generators output under all conditions to determine if it is functioning normally.

When there is low demand from the electrical system on the generator, a low duty cycle percentage will be displayed. As more accessory load is placed on the generator, the duty cycle output detected by the PCM will approach 100 percent. A normally functioning generating system will never reach 100 percent as indicated on the scan tool.

The L and F terminals are the red and grey

 

I read through your tutorial and think I understand what you're saying. The alternator I purchased was a rebuilt unit and is supposed to work on my car. From what your saying it has to match the PCM in my car to work. Does this mean the PCM in my car is unique to my car or to 97 and 98 vettes? Reason i'm asking is because i've already purchase a rebuilt alternator and will need some good backup info to return it.

 

Many parts houses, including GM Parts direct does not have a handle on this issue...
Your local Parts house will see any GM 5.7 alternator and think its the same one..but it is not...
Your car requires a French Made Valeo 110 amp alternator, not a Mexican made Valeo 130 amp alternator. The PCM will not recognize it.

 

Many parts houses, including GM Parts direct does not have a handle on this issue...
Your local Parts house will see any GM 5.7 alternator and think its the same one..but it is not...
Your car requires a French Made Valeo 110 amp alternator, not a Mexican made Valeo 130 amp alternator. The PCM will not recognize it.