M1009 Passenger Alternator short to ground

79Vette

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I connected the batteries back up on my M1009 this morning, after the truck had been sitting for several weeks for axle rebuilds. When I connected the rear battery I got a small spark, which I assumed was due to a light I had left on or something. When starting the engine the GEN2 light stayed illuminated, so I shut the truck down and disconnected the batteries.

Testing with a DMM, the case on the GEN2 (passenger) alternator is shorted to ground and +12V (<0.1 ohm). I am going to disconnect all the wiring from the alternator first and look for any issues, but I assume this is an internal alternator problem and am planning to pull it today to have a look inside. Any suggestions on what I should check for?

This was a good running truck I just did a 900 mile off road trip with before tearing the axles down, so I am surprised to find it suddenly "broken". Also please, don't suggest to ditch the passenger alt and convert to 12v. I am quite satisfied with the 24v system and intend to get it working again.

Thanks



UPDATE: The problem does not appear to be in the alternator. With the +12V (chassis referenced, so the ground stud on the alternator) disconnected at the alternator, there is still a dead short between +12V and chassis ground somewhere in the truck. I am going to start digging starting with the +12V distribution point on the firewall, but any suggestions are appreciated.

UPDATE 2: I have traced the problem to this small red fusible link on the right side of the firewall 12V distribution point. Disconnecting this fusible link removes the shorted condition. Pulling up the TMs now to see what this circuit is for, because I haven't the faintest idea.

UPDATE 3: Ok, now I can use some help from the alternator and wiring experts. I determined that red wire goes to the ignition switch and seems to provide power for the dash wiring and other things on switched 12V. I also found an intermittent short in some of the dash wiring and fixed it. However, there is still a problem with the GEN2 alternator (sparks when reconnecting it). I took the alternator off and found this melted washer/insulator; is there somewhere I can buy just that part? The alternator is working fine, so I prefer not to buy a full rebuild kit unless needed
 

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MarcusOReallyus

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I took the alternator off and found this melted washer/insulator; is there somewhere I can buy just that part? The alternator is working fine, so I prefer not to buy a full rebuild kit unless needed

Nice troubleshooting! :) That's your isolation from ground. That is probably the culprit.

Check with member Tow4. He sells rebuild kits, and will probably be able to either sell you that part, or tell you where to get it.
 

Tow4

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You can probably get it at Napa or other autoparts store. You may have to settle for a red one. The stud and insulator are separate items.
 
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