M1008 Gen 1 light always on

[MojaveRider]

Just bought my first CUCV today, '86 M1008. Overall it seems like a very stock, unmolested rig, but it's been sitting and very neglected.

First problem to tackle: the Gen 1 light and the oil pressure light are on all the time, even with the key off. The Gen 1 light is fairly bright, while the oil pressure light is more dim. Turning the key on, the Gen 1 light goes off, Gen 2 light comes on (expected), and the oil pressure light goes to full brightness. I didn't mess with it long, but both alternators seemed to be charging (both batteries were above 12V), though I think one was higher.

My first thought with any square body electrical issue is bad ground. My second thought is maybe a bad diode/regulator in alternator 1.

Has anyone run into a similar issue?

 

[90Jimmy]

The search function is your new best friend sir. This topic has been covered in great detail many times by some very knowledgeable contributors. Good luck with the new hobby,,,

 

[MojaveRider]

The search function is your new best friend sir. This topic has been covered in great detail many times by some very knowledgeable contributors. Good luck with the new hobby,,,

I've done tons of reading, reviewed several threads and wiring diagrams. Unplugging the excitor for alternator 1 makes the light go off, so I suspect it's a short inside the diodes of alternator 1. I had it rebuilt by a local shop and it should hopefully go back on the truck tomorrow.

 

[Barrman]

When you pick it up ask if they reinstalled the isolated ground washers? Those are essential.

 

[MojaveRider]

When you pick it up ask if they reinstalled the isolated ground washers? Those are essential.

This was discussed when I dropped it off: they knew it was isolated ground without me having to tell them, and I plan to take a multi-meter to make sure the case isn't grounded.

 

[MojaveRider]

Alternator back on the truck and the problem is fixed. Gen 1 and 2 lights working as expected now. I did verify it was ground isolated, so good to go.