Alternator Trouble

[motormayhem]

I know this has been posted before, but I didn't see a fix for my problem. I have a m1009 and the previous owner said the alternators were replaced not long before I bought the truck. When I drive a I notice the Gen 2 Light is lit. When the truck is first started the light is VERY faint, but as I drive and the engine revs through 1st and then 2nd the light becomes as bright as when the ignition is on but the engine is off. If I rev the engine the light stays the same brightness till about 1/8 throttle and begins to dim. Then it is completely off by 3/8 of the gas pedal (that seems to be the sweet spot where it is glowing then then just shuts off completely). The brightness never changes when more load is put on the electrical system (blower on/off, headlights, radio, etc.) and the charge gauge always stays in the green. I checked the voltage at the alts and they are good. I don't seem to see a brown wire coming from my alt. I am told this is the one to check for resistance/corrosion on? A pic would be nice as I think the wires may have changed colors on me. Also any other ideas would be helpful.

Thanks,
Miles

 

[hobie237]

The first thing I'd do is test the voltage that the alternators are putting out with an actual voltmeter, the 20+ year old idiot lights/gauges aren't the most reliable. If the voltage is satisfactory, the warnings can be safely ignored/dealt with when you feel like it. If the voltage is unsatisfactory, then more investigation is required more quickly.

I forget which wire does what on the back of the alternator, but somebody will chime in with that. The easiest way for you to check, though, would be to see if both alternators have the same wiring coming out of the back. If they're not the same, look for what #2 is missing.

 

[motormayhem]

Ok I checked the volts right after starting (with engine idling). Gen 1 = 14.2V Gen 2 = 27.5V. If I gave it some gas Gen 2 went to about 28.7V.

 

[hobie237]

Ok, so you can safely say that it is an instrumentation issue rather than an actual fault in the alternator. That should be fun to track down. Again, check to make sure the wiring on both alternators is the same.

 

[Michael]

The brown wire is one that goes to the dash light. It is part of the connector with two wires that plugs into the internal voltage regulator. The other wire should be red and goes to the 24v buss. They both need to be clean for the alternator to work properly. This plug can be bought at the parts store if it is in bad condition.

 

[dunedigger]

I have this same problem in my m1009 right now, I had a bad run of replacement alts and still waiting for a replacement. When I put my old one back on the light goes out and works as it should. When I put the new one on the light is on. I am assuming at this point it is the diode in the alternator in my situation? My batts stay up with my new alt as well as the old one.

 

[maritimer]

alsi IIRC the alt's in these truck are supposed to be isolated ground alt's and there have been some issues with parts stores and dealers giveing non isolated ground alt's to cucv owners.

 

[hobie237]

Only the 12-24v one needs to be isolated ground. The 0-12v one can be a regular alternator.

 

[motormayhem]

Ok I printed out the wiring diagram in the tm for the dummy lights, but before I start tracing wires are there any common spots that tend to corrode or wear that would make it quicker to diagnose.