1991 Limited, 4.0 HO. I am getting low voltage in my charging system. The other morning, my wife was driving to work and the lights slowly dimmed until the point she had no lights. She turned around and made it home (a few miles). I tested the voltage with an ammeter and it is showing about 11 volts at the battery. I fully charged the battery and checked again and still shows 11 volts. Is this the battery or the alternator? or something else. I cleaned all the connections and still the same. Is the voltage regulated by the PCM or is it an internal regulator in the alternator?

 

11 volts at the battery with the engine running? If so, low charging voltage is usually the alternator itself failing. The regulator is in the computer, but is pretty hardy.

I'd pull the alternator and bench test it. If you have a company nearby that rebuilds automotive electronics, they are a good option for both testing and repair. Otherwise, have a parts store bench test it for you and pick up a new alternator if testing confirms failure.

 

You can also test the alternator output on the motor. Take your positive lead from your voltmeter and touch it to the output stud on the back of the alternator and ground the black one to the engine or frame. When mine was bad it read like 4V, where it should be 12-14V (I think).

 

Thanks for the info guys. I thought it was probably the alt, but just seeing what you guys thought. I don't have time to test it, I am leaving for a long drive early in the morning (hopefully) so I will have to put on a new alt tonight after work and hope that fixes it.

 

If the lights are dimming and shutting down, I would say the alternator is done, try cleaning the ground from the battery to the frame, take it off and give it a good cleaning, keep your fingers crossed

 

1) You can't check voltage with an ammeter - Amperes & Volts are two entirely different measurements.

2) On your 1991, the voltage regulator is in the PCM, not the alternator. Seems like it rarely fails, so you shouldn't have to lay out for a new PCM (if you bypass the PCM regulator and use an add-on external, you will get a persistent CEL/MIL lamp!)

3) Look for a writeup I've put up here on testing your primary electrical. Do it in the driveway with a voltmeter and a helper in 15 minutes - it's not a quantitative test, but it's a good qualitative test that will tell you what you need to know.