Strange electrical issue

[General Mcgregor]

Hey fellas!
Hoping for some answers on an issue I'm having with my electrical system.
The other day I went to start my truck (it had been sitting for about two weeks). Turned on the master switch and got no air buzzer, hit the start button and got nothing. I charged up the batteries (thinking that they were just dead) and turned on the switch again. Still no air buzzer but the truck started no problem. I checked my generator gauge and saw that it was reading in the red (discharge), this happens sometimes but it usually jumps right into the green after a couple seconds. This time it stayed where it was, showing excessive draw on the batteries. I checked the batts, one was reading 12.0v and the other was around 9v Overall I had a reading of about 21v between the two. I installed a battery equalizer a couple months ago but if it's not getting a charge then I wouldn't expect it to be working, right? I then tried to polarize the generator but the readings I got from the dash gauge and the multimeter were the same. Strangely enough my horn stopped working also. This is an entirely new issue so I suspect that the lack of charge, low air buzzer and horn operation are connected somehow, unless I'm just that unlucky. All of my other 24v electrical components seem to be working although my lights are very dim. Is there a spot where I should be looking for a short? How would I go about determining if my generator is junk or not? I have a spare but I don't want to swap them out until I have eliminated every other possibility.I tried searching but cant seem to find a precedent to these unique problems.
Thanks in advance! -GM

 

[NDT]

The 9 volt battery is a goner. If you had high current draw, both batteries would be drawing down. Tell us about polarizing the generator, do you have a 25 amp generator?

 

[rustystud]

The 9 volt battery is a goner. If you had high current draw, both batteries would be drawing down. Tell us about polarizing the generator, do you have a 25 amp generator?

Yes, the battery is toast. Even with an equalizer the bad battery will not charge properly. The reason the horn doesn't work is low voltage I'm sure. That solenoid takes a bit of amperage to work at 24 volts. At 21 volts I'm surprised your getting anything else to work except the lights.

 

[mobileauto2000]

Sorry double post,

 

[mobileauto2000]

I'm at a loss here. If your talking about the 1972 m35a2 you should have a alternator not a generator. It would not need to be polarized.
Second a couple bad cells usually won't pull down the system voltage that much with the engine running. Not unless they are shorted.
I suspect that your master switch is failing and the alternator is not being energized, I had this happen to me not to long ago.

 

[General Mcgregor]

The 9 volt battery is a goner. If you had high current draw, both batteries would be drawing down. Tell us about polarizing the generator, do you have a 25 amp generator?

Data tag says 'Generator, DC 60 amps'. After my engine swap I had to polarize it to get anything out of it, afterwards my gauge was well into the green.

Yes, the battery is toast. Even with an equalizer the bad battery will not charge properly. The reason the horn doesn't work is low voltage I'm sure. That solenoid takes a bit of amperage to work at 24 volts. At 21 volts I'm surprised your getting anything else to work except the lights.

Yeah I checked it again today, the low battery was at 8.4 and the high was reading 12.1 volts. Looks like I will at least need a new battery.

I'm at a loss here. If your talking about the 1972 m35a2 you should have a alternator not a generator. It would not need to be polarized.
Second a couple bad cells usually won't pull down the system voltage that much with the engine running. Not unless they are shorted.
I suspect that your master switch is failing and the alternator is not being energized, I had this happen to me not to long ago.

The Generator/alternator is not original to the truck so the year may not be a factor in this case, but now I'm doubting myself. Like I said, the data tag says 'Generator' but I feel like I'm about to be educated.

I checked the voltage at the generator today while it was running and got a reading of 21 volts DC. Could it be reading low because of the bad battery or could my generator be toast?

-GM

 

[welldigger]

You should be getting around 28 volts from the alt. You should also check and make sure your getting 24v to the exciter wire on the alternater. If it doesn't have voltage the alternater will not put out power.

I'm confused as to how ploarizing the alternater fixed anything. I feel that more than likely you bumped the exciter wire and it made contact causing the system to work. Heck the rectifiers in these stupid military units are prone to burning out as it is.

 

[Jeepsinker]

If you only have 21 volts supplied to the gen/alt, you will only get 21 volts out. Replace the bad battery and check it over again.

 

[welldigger]

If you only have 21 volts supplied to the gen/alt, you will only get 21 volts out. Replace the bad battery and check it over again.

Not quite. The exciter wire detects voltage drop. Which then tells the alternater to produce power. The alternater should then put out around 28 volts to charge the batteries.

I do agree you should start with fully charged batteries. If you still only have 24 volts at the hot wire of the alternater then it isn't producing power.

GM, which alternater/generator specifically do you have?

 

[General Mcgregor]

Looks like this one, so it is an Alternator? I guess the fact that it says Generator right on the data tag threw me off a bit.

 

[Jeepsinker]

That is an alternator. Data plate says generator because it is technically a power generating unit.

 

[welldigger]

Ok so yours has the external fan? If so I'm guessing you have a Leece-Neville unit. Though, which brand isn't really important. I was just making sure you didn't have a civilian unit or some other oddball.

Ok as stated before start with fresh batteries and make sure your getting 24v from the exciter wire at the alternator. If yes, crank the truck and see what the output is at the hot wire. Should be about 28 volts.
If both of those 2 tests check out then you can move on to finding a possible short, etc.

 

[brianp454]

The turn signal flasher is notorious for causing strange behavior. Once you sort through the battery and other issues, you might want think about it. They are $48 on Erik's. Turn Signal Flasher, 11613631_Nartron, Control Unit (24 Volt), 11613631 New Style

 

[General Mcgregor]

Okay so first I replaced the bad battery with a known good one (from my jeep) and the buzzer, horn and everything else was operating perfectly. The gauge was still reading in the yellow and my output at the alternator was around 25 volts.
The battery is an Ever-Start Maxx from Wal-Mart, it was still under a 3 year warranty (for the next month) so I brought it back and they honored it with an even exchange
I took my spare alternator (prestolite) from my old engine and swapped it with the (suspected) bad one, installed new battery.
The gauge is now back in the green, ran it for about 20 mins at idle with headlights and convoy beacons on and it seemed to check out okay. Now I need to get the other one tested so I'm not without my spare. Hopefully I didn't damage it when I was attempting to 'polarize' it. Thanks for the help guys! I'll let you know what I find out.
-GM

 

[rustystud]

When you tried to "polarize it" you probably fried the diode pack. Should be an easy fix.