Electrical issue in my M51A2

[holley68]

My M51A2 transfer fuel pump started getting very weak. I can hear it cut on but not really pumping. But truck cranked fine.

Then truck stopped cranking. Batteries dead. Had them in the truck a years. Walmart 34N 800cca. Charged batteries. Only one would charge on the machine. The other would through an error code after several hours.

Planning on changing both batteries to smoothing larger. Should I check the alternator too? Any suggestions on checking the alternator?

 

[holley68]

The wiring on the truck is a mess. A lot of wires have been cut in random places. Most of the gauges don't work. The plug in on the back of the truck was cut off.

Fuel pump did not work. I have added a fuel pump under the tool box that works for priming. The pump is not need after the truck is started, but is wired into the orginal pump. So I keep it ok to keep power to the lights.

 

[Ajax MD]

People really give electrical systems low priority on their vehicles...until it finally kills the vehicle.

It sounds like you have a lot of sorting out to do. You can start by checking the voltage at the batteries once you get the truck running.
If the voltage is around 24v, the batteries are not charging. You should see closer to 28 volts while the truck is running.

At this point, there are 2 possibilities:
The generator is dead or the wiring between the generator and the batteries has been cut.

Next step: Put the meter directly on the generator leads and check the output while the engine is running. Use CAUTION around the spinning belts and fan! If you have 28 volts, the generator is charging properly, it's just not getting to the batteries and you need to find the broken wiring. If you have "0" it's obviously dead. If you have in the 20's, but less than 28v, it could be as simple as adjusting the voltage regulator output.

As for killing new batteries within a year, you may have a short to ground that slowly drains the batteries. To test this, connect the batteries and put your volt meter on them and carefully watch the voltage over 15-30 minutes. Do NOT start the truck. If the voltage starts out at 24.98 (for example) and it continually drifts lower without ever settling out, then disconnect the negative cable and observe for another 10 minutes or so. The voltage may actually rise a few points, or at least stop declining. If that happens, you have a minor short in the truck wiring.

In general though, it's not good to let the batteries sit for a full year without occasionally topping them up. Either buy a 24 volt battery tender, or buy a pair of 12v tenders and remove the interlink cable between the batteries.

Regarding all the cuts in your wiring harness, there is a complete wiring schematic in the 5-ton manuals for your truck. All the wires have tiny, metal numbering tags on them. Read the number, trace it to the number on the wiring schematic, locate the wire on your truck and splice them back together or replace the harness altogether.

You can grab that manual, here: https://www.steelsoldiers.com/threads/tms-for-the-m39-series-5-ton.77614/

It's tedious, but not difficult.

 

[topo]

My first 24volt vehicle the owner mounted a few small marker lights on the bed and hooked them up to one battery to get 12 volts and it would kill one battery . every thing needs to be 24 volt .

 

[holley68]

Ajax MD, thank you for the useful information. I always remove the interlink cable to keep the barriers from draining. A battery tender is a good idea, just wish the truck was covered and closer to a power source. I will start working on the items you suggested this weekend..

 

[Ajax MD]

Ajax MD, thank you for the useful information. I always remove the interlink cable to keep the barriers from draining. A battery tender is a good idea, just wish the truck was covered and closer to a power source. I will start working on the items you suggested this weekend..

Ah, ok. If you removed the interlink cable then parasitic shorts should not be able to drain the battery. It sounds like they simply self-discharged over the year.
When they sit that way, the battery plates sulfate and they are more or less ruined.

There are various methods described on the internet to de-sulfate batteries and recover them but I've never really had good luck with that.

I forgot to mention- erik's military surplus sells the military Prestolite wire and shell connectors (also known as Packard connectors) to fix the truck's wiring.
I know it's tempting, but try not to use cheap-o, civilian butt connectors and wire to splice everything back together. At least, not for the long-term.

 

[holley68]

Looking at just going to manual gauges since water temp or oil pressure isn't working. Any recommendations?

Also thought about putting pyro gauge on her.