M1008 Killing REAR battery?

[Capt.Marion]

I'm no expert on motor vehicle electrical systems, but this sounds similar to problems I've had before on equipment. Couldn't you just disconnect the alternators from the electrical system, charge both batteries, and run the truck for a while without the alternators? If you don't do anything that draws a high amperage, such as running the headlights, you should be able to drive the truck without a problem for at least a few days without having to charge the batteries.

If your battery doesn't run down with the alternators disconnected, then you should have found your problem...

I have no experience with CUCVs- do they utilize just two standard internally-excited 12V GM alternators?

 

[rose jackets67]

Lavarock - Thanks for the info on the alternator diodes... Like I said, I'd never seen that happen before, but clearly that doesn't mean much Both alternators are currently at the rubuilder.

G-Force - That is EXACTLY the kind of info I'd been looking for. I've never had the need to do this in depth electrical troubleshooting, and had been trying to come up with a more scientific method than simply eyeballing wires and connections. Thanks a ton. I'll definitely be trying this ASAP if the rebuilt alternators don't solve it.

Thanks again to all.

RJ,
Do this, disconnect the + battery terminal on the problem battery. Chsarge the battery for about an hour. Whrn you go and put to therminal back on the post if you get or hear sparks you have a current draw on that battery. Take the terminal off the post and put a test light in series with the terminal and the post on the battery. If the light lights you have a current draw on that battery. Now with the test light in series go and disconnect the + wirs on the alternator. if the light goes out the problem is a blown diode in the alternator. If it doesn't, keep disconnecting + wires until it goes out. But leave each wire off just in caes it is more than one thing. NOTE: radios with memories, digital clocks and if your door is open the dome light will also show up as a draw and light the test light. I used to work on boats and dead batteries was an every day occurrance at the marina. This was how we found the draw. From what you describe though, I would bet the alternator ihas a bad diode.
Good luck and let us know how you make out.

 

[G-Force]

RJ,
Glad i could help out....just one thing, if the + cable off of that battery is the 24+ and not the 12+ that is connected in series with the other 12 volt batteries minus terminal, the test light will light VERY BRIGHTLY for a little while before the bulb burns out due to most test lights being 12 volts and not 24 volts. if that is the case just use a 24 volt bulb with a socket to check it out. Chances are that when your alternators come back I'll bet they found a bad diode and all will be good. let us know......and also....if I get pulled over in Georgia, can I mention your name?????
Good luck
Mike

 

[davo727]

Make sure the rebuilder knows that they are isolated ground alternators and that they remain that way. If they build them like a normal alternator your truck will melt to the ground if you try and run them.

 

[doghead]

Make sure the rebuilder knows that they are isolated ground alternators and that they remain that way. If they build them like a normal alternator your truck will melt to the ground if you try and run them.

DS alternator does not need to be an isolated ground alternator.

 

[rose jackets67]

Hopefully I'm not calling it too early again, but it looks like the rebuilt alternators did the deed.

When I dropped them off, I only told the shop I wanted a complete rebuild. I didn't mention anything about the issues with the truck... when I picked the alternators up early this afternoon, the guy at the rebuild shop immediately told me that one of the alts had a bad rectifier diode and asked if I had a battery going dead. Looks like the diode was the problem after all.

Like I mentioned a few posts back, this is the first time I've ever heard of an alternator causing a parasitic power loss like that. Now I know.

Thanks again to everyone for the advice.

 

[Apollyon67]

Had the exact same trouble and followed the prescribed possible solutions. The end fix was a rebuilding of the passenger side alternator. The triple diode was toast. I figure the amount of time you folks have saved me is, well, a whole lot of sweat and blood. What I have left with christening this latest money pit and feeding the mosquitoes. Thank youall very much for your help.

 

[HASSON1911]

Has anybody thought it could be the battery? even brand new batteries can come with broken cells.

Take her out, and charge her up, if she still loses a charge you got a bad one!

 

[ETHAN SHERMAN]

I need help with a similar issue i have a 86 m1008 the dash was replaced with a civilian won so no dummy lights my 24v gauge wont get above the yellow line how can i fix this also in the morning the gauge is in the red after about an hour its at the top of the yellow and that's as hight as it will go

 

[cucvrus]

The idiot lights are supposed to be operational so that the exciter circuit in the charging system is operational. Something to that effect. If you do not have idiot lamps the alternators or alternator that is served by the light will not charge. Check it out. I hope that is simple enough and accurate.

 

[ChrisG]

So the idiot light completes the circuit for the alternator it is serving...I have no idiot light for Gen1 so does that mean the front battery is not getting charged....sorry to beat a dead horse

 

[cpf240]

So the idiot light completes the circuit for the alternator it is serving...I have no idiot light for Gen1 so does that mean the front battery is not getting charged....sorry to beat a dead horse

In a stock setup, yes, the GEN lights need to be working to get the alts to 'turn on', called exciting. You'll need to get out the voltmeter and start taking some readings and comparing those with what they should be.

 

[ChrisG]

Totally stock setup...guess i will start by making sure the bulb is there and if its any good. Great site what a wealth of info on here.

 

[ChrisG]

Well checked bulb that works...guess I got to dig a little deeper this evening. Any quick points of interest I should look into or reasons why this light would not be working. I will be pulling the dash cluster out to have a look at the printed circuit board to make sure the bulb socket is making proper contact. I do have the wiring diagrams too.

 

[Warthog]

Some things to check on the cluster:

circuit board (film) - contacts can be dirty or damaged
bulbs - contacts can be dirty, damaged or burnt out
bulb holders - contacts can be dirty, damaged or bent

Here are the cluster threads from the CUCV Helpful Threads sticky

http://www.steelsoldiers.com/showthread.php?52821-Instrument-Cluster-Cleanup
http://www.steelsoldiers.com/showthread.php?43351-CUCV-Instrument-Cluster-Repair
http://www.steelsoldiers.com/showthread.php?46054-CUCV-Instrument-Cluster-Decoded

 

[rotorhead]

Hey Chris, don't be so hasty....do you have two alternators? If so the right side charges back battery make sure it's charging. Place a screwdriver flat on back side of alternator if it sticks then its charging (in other words it will stick like a magnet)

 

[ChrisG]

Just had gen2 rebuilt and isolated ground insulators installed (they were not by previous owner rebuild) pretty sure that one is working but will check again. If it is not the socket or connections any other areas I should key in on?
Will this light come on even if gen 1 is malfunctioning and not working or is the light out an indication something is wrong with alternator1 ?

 

[ChrisG]

I have also noticed the volt gauge seems to run somewhat low...never seems to be in the green...would gen 1 not charging cause this to happen...I m gonna guess yes.

 

[cpf240]

I have also noticed the volt gauge seems to run somewhat low...never seems to be in the green...would gen 1 not charging cause this to happen...I m gonna guess yes.

It could, yes. Please take some voltage measurements, as indicated in the threads mentioned by Warthog. Only then can you narrow down the issue, and address it appropriately, rather than just guessing and throwing parts / money at it.

 

[ChrisG]

So it appears that previous owner (he had a parts truck) put a gen2 isolated ground alt in the gen1 position but hooked it up with ground to where it should be on alt one the bolt in back not the post. Think this explains a lot of the issue,gonna take it to alternator rebuild man and have him set it up to work in gen1 position so it grounds to case. Seem feasible?