Current Leakage through Alternator

[Stein]

I did the 12V swap months ago using the Roscommon instructions. I have double- and triple-checked that it was exactly to plans.

First issue was that it wasn't charging. I finally figured out the exciter wire to the alternator wasn't showing 12 V when running. Rather than chasing wires I ran a wire to a key-on hot. Problem there was when I shut it off there was still enough residual voltage somehow that the truck would continue to run. Changed it to a key-on hot with a switch in the circuit. Start truck, flick on and off switch which excites alternator, charges fine. Mickey mouse but works for now.

After a month it started discharging batteries when parked. Hooked a test light between positive terminal and positive cable and pulled all fuses. Light stayed on. Unplugged the two wire plug on top of the alternator and light went off. So, the black wire is through an open switch and can't be the culprit and that only leaves the red wire. Checking voltage with truck off and key off it is showing .2 or .3 V at the red wire. Not a lot but enough to discharge the batteries in a day or two.

Any idea why any current would be present at the red wire with the key off?

 

[diesel4me]

Is this an older 10SI alternator?...what kind of vehicle?...

If it's a 10 SI,once the field is energized,its possible for 12V to backfeed and be sent to the ignition coil (or the injector pump fuel solenoid in your case being a diesel)--and keep the engine from shutting off--a diode can be installed in the field excitation wire to prevent that...

It's possible one or more of the diodes in the trio inside the alternator failed,and is allowing current to bleed off to ground and eventually kill the batteries..it'll still charge at reduced output with one or two of the diodes "dead" too...if you have an idiot light you may see it glowing dim at night if one of the diodes has failed...

Also,the thick red wire at the two prong plug on the alternator is always hot,only the field excitation wire is switched on by the ignition..

 

[Stein]

Sorry, forgot to say it's an 86 CUCV. Not sure which alternators they had. There were two, of course, before the conversion. Was 24 V, now 12 V. John told me about doing a diode but I didn't know what value/rating I needed.

I have the other alternator that I took off of the truck, maybe I'll just swap them to see if that takes care of it. I did take both down to the local starter/alternator rebuild house and he tested both of them and said they were both good. This was when I was trying to diagnose the exciter problem.

 

[diesel4me]

The CUCV used two "normal" 12V alternators to get 24V ,one had an isolated ground I think--never had to mess with a military truck before,so I'm not much help as far as how it was wired for 24V...but I do know how my civilian 6.2's are wired,if thats any help..

There was a old Hot Rod Magazine article showing how to put an alternator from a '72 up GM into older cars & trucks that had the externally regulated alternator,and it showed where to put the diode and what rating,to prevent feedback to the coil...you can probably find it on google..

 

[Stein]

The passenger side alternator was isolated ground but part of the swap was to move the direct ground cable from the driver's side alternator that was removed to the passenger side alternator and remove the isolated ground wire from the back of the alternator so it is direct ground now.

 

[Stein]

Swapped alternators today. No change. Guess I'll just unplug the alternator every time for now. I suppose I could rig another switch to kill the 12V to that alternator through the red wire. Just flick them both on to run, off to park. Be easier than popping the hood and pulling the plug each time.