Odd Voltmeter Reading

[cavetech]

Original batts or did you get all new ones? There polarity box under the spare tire check for loose connections there. And they have been known to fail. Cab power goes thru that box

There is one original battery left. Two batteries were replaced before I bought the truck and I bought one new battery.

I didn't know there was a polarity box (whatever that does) under the spare. I will hunt it down in the morning. Thank you for the tip!

Steve

 

[tennmogger]

As long as you have mismatched batteries in a four battery system, you will have charging and longevity issues.

 

[cavetech]

OK, I swung the spare tire out and I'm looking for the electrical box I should check for bad connections. I see a small box (about the size of a computer hard drive) with a few electrical cables attached to it. It is down under an orange cylinder about 12" long that I think is the ether bottle. Looks tough to get at. Have I found the right box? If so, what is it and what does it do?

 

[coachgeo]

OK, I swung the spare tire out and I'm looking for the electrical box I should check for bad connections. I see a small box (about the size of a computer hard drive) with a few electrical cables attached to it. It is down under an orange cylinder about 12" long that I think is the ether bottle. Looks tough to get at. Have I found the right box? If so, what is it and what does it do?

Reverse Polarity Protection Circuit. In simplicity... and nothing is simple... It is done to provide protection against accidental reverse polarity of circuits. (reversed Neg/Pos battery cable for example).

More here - http://electronicdesign.com/power/reverse-polarity-protection-automotive-design

There could be more circuitry for other purposes in the FMTV Rev. Polarity Box ... but it's primary function is as named.

 

[Suprman]

I think its basically a diode array used to prevent damage if the batts are inadvertently hooked up backwards. I have not seen one bad but the mil techs I have spoken with say they have had to replace them. Measure the voltage going in and the voltage coming out. Usually a diode will drop around .7 volts.

 

[tennmogger]

Here's what you are looking for. Lift the flexible covers to see the terminals on the polarity protection device. You can tell if the connections are tight by wiggling the wires. There are labels on those wires but mine are hard to read. I think the 24v pairs are on top and 12v below, but please verify. Of course there is lots of current there if the batteries are still connected so watch those wrenches .

 

[cavetech]

I will get down in there and find it. I'll check for loose cables and any corrosion.

Thanks, all, for helping me find it. I enjoy learning about my trucks.

Steve

 

[FloridaAKM]

When I first got my truck, one of the posts on the battery polarity device had the nut loose on the stud. You could move the cables, when I tried to tighten the nut, no way as the threads were ruined. I tried to remove the nut, but that just broke off the stud, leaving a short functional stud. To fix it, a short terminal was bolted to the stud & the terminal had the cables bolted to them. That fixed the cable connection, but the alternator starts showing 26-27 Vdc upon crankup & drops to 24Vdc after running for a while......that isn't right, Suprmans check out, I have not had a chance to try out due to too many projects.

Good luck with your trucks issues.

 

[cavetech]

I have an answer. I hired an Army truck mechanic from a local military facility. He diagnosed my voltage problem as a bad voltage regulator.

Does anybody have a voltage regulator for my truck?

Here are the details:
1997 M1078
Niehoff alternator model number N1506-1
Voltage Regulator needed is Part# N3030

Thanks, Steve

 

[cavetech]

I finally got a regulator ordered from a distributor very reasonably. So, my problem is hopefully solved.

Thanks, Steve

 

[mkcoen]

I finally got a regulator ordered from a distributor very reasonably. So, my problem is hopefully solved.

Can you post where you ordered it from in case anyone else needs one?

 

[cavetech]

Can you post where you ordered it from in case anyone else needs one?

Yep. I'll do it tonight.

Steve