I have a 97 5.9L Laramie 4x4 with about 215k miles.

Lately when I start my truck the voltage gauge shows around 12v, and the GEN light will come on. After driving a bit sometimes the voltage will jump up to its normal 14v, but not always.

I've checked the voltage at the battery with a multi-meter, and it is in agreement with the gauge reading.

My question is how to distinguish whether the problem is the alternator or voltage regulator. Should the alternator always put out 14v and the regulator adjusts the charging voltage to the battery, or will the output of the alternator vary based upon regulator control?

FWIW, I have never replaced either alternator or ECU, but I've only had the truck since it was at 185k. From general appearance both could be OE.

TIA

 

Check/clean all connections first. Might be as simple as a loose connection somewhere. (PCM or Alternator.)

If that all looks good, best bet is to yank the alternator, take it to the parts store, and have it tested.

 

Check and make sure that your serpentine belt tensioner is working properly.

 

How do you bench test an alternator?

 

A good alt rebuild shop and some auto parts stores will have a jig that they mount your alt into, spool it up and check the output.

Many moons ago, I had a truck that would have similar problems as yours. Batt light would come on at low rpm's and be fine at higher rpm's. Turns out it was an-almost broken wire. At the higher rpm's, the engine would torque to the side enough to complete a good electrical connection. A idle, the connection would be ****-poor.

 

Thanks all. I will definitely look at all connections and wires.

 

Keep in mind, the voltage regulator is in the PCM, so, check those connections as well.

 

Back to part of the original post. Should the output of the alternator be a constant voltage, or does it vary with demand? Stated in a different way should the voltage at the alternator output always match the charging voltage at the battery?

 

The pcm should regulate the voltage but i am not sure where it does this, whether internally in the alt or externally. Someone else will chime in.

 

The alternator is switched on and off rather than being linearly regulated. When the regulator senses that B+ has dropped to or below the low voltage threshold it switches the alternator on, and when the regulator senses that B+ has attained or exceeded the high voltage threshold it switches the alternator off.

The easy way to get started is to haul the alternator out and take it to someone who can bench test it. If you've got, say, open diodes in the rectifier, which would one of the failures consistent with what you've described, the bench test will show that. If the thing checks out on the bench, then it's time to resume troubleshooting.