M939 batteries running down while driving operating vehicle

[crazycanuk]

This forum has been great and I have a really strange problem happening on my 5 ton.

If it sits too long, batteries drained. Fix this with a solar panel and 24 volt conversion for trickle charge.

Now, with engine running the gauge stays in yellow upon startup and the longer the vehicle runs the batteries continue to discharge until they literally have less than 6 volts in each battery!

Any idea where to begin looking for a short etc.? This weekend I will be on site and I am going at a minimum pull all the batteries and trace cables and connections. As in most diesels once it starts, you would think it would not drain.

Another note. Even thought he gauge shows red the lights etc. all worked with no slow down. Really strange.

Thanks for the help.

George McCausland

 

[VPed]

It sound like your alternator is not working as far as the part about the voltage dropping while running. The steady battery drain while not running may or may not be related to the alternator but since you seem to have a problem there anyway, why not start there and hope that cures both symptoms.

While it is true that a diesel engine generally needs no power once running, the shutdown solenoid, air dryer heater element, etc. are powered while running. The alternator also gets power from the "ignition switch" while running.

 

[crazycanuk]

Thanks! I am thinking the same thing. I knew there was drain while running, but when it happened was stumped and was not thinking. I will look at alternator this weekend. I will recharge batteries as well. I am thinking the alternator is not working and maybe a rebuild kit or replacement will be needed. Other than that, truck works for both farm and as historical vehicles in parades. Grandkids love it! Thanks again for the input. I will post early next week or this weekend results of investigation.

 

[MWMULES]

You might also have your batteries load tested after you get them charged up. Your meter should be in the green after truck gets running, the alternator is easy to adjust.

 

[lindsey97]

Check the positive battery cable where it passes the back left corner of the engine. Rumor is it rubs there and could cause a short. Have your batteries load tested.

Get a volt meter and check the actual voltage. If I were to guess, I would say the regulator on the alternator has failed.

 

[crazycanuk]

MWMULES, I will do that although they are new, but with the huge drain they just went through I am concerned. IS the alternator adjustment process in the TM, I am not on site so if there is any insight as to how and tools I will need that would help as farm is off grid and a 40 minute drive one way to a local store.

 

[Tow4]

You remove the small pipe plug on top of the alternator at the back near the regulator and use a small screw driver to adjust the voltage. The adjustment procedure has been posted on SS with pictures.

Use a DMM or VOM at the batteries or slave connector when setting the voltage. The dash gauge on my M929 indicates in the yellow with 28.5 volts measured at the batteries.

 

[74M35A2]

If you find the alternator is out to lunch, as suspected, I personally don't recommend even trying to fix it. There are pages of these failed alternators here, and they commit suicide if the batteries are low, even though that is the alternators core assignment. You can replace it with a civilian unit in the price range of $89-$400, depending if you want new vs used, and genuine vs Chinese.

 

[crazycanuk]

If you find the alternator is out to lunch, as suspected, I personally don't recommend even trying to fix it. There are pages of these failed alternators here, and they commit suicide if the batteries are low, even though that is the alternators core assignment. You can replace it with a civilian unit in the price range of $89-$400, depending if you want new vs used, and genuine vs Chinese.

Thanks for the help.

 

[patracy]

If you find the alternator is out to lunch, as suspected, I personally don't recommend even trying to fix it. There are pages of these failed alternators here, and they commit suicide if the batteries are low, even though that is the alternators core assignment. You can replace it with a civilian unit in the price range of $89-$400, depending if you want new vs used, and genuine vs Chinese.

I've always gone with OE alternators. But the next one that I have to replace, I plan on using a civy unit instead.

 

[Csm Davis]

If you find the alternator is out to lunch, as suspected, I personally don't recommend even trying to fix it. There are pages of these failed alternators here, and they commit suicide if the batteries are low, even though that is the alternators core assignment. You can replace it with a civilian unit in the price range of $89-$400, depending if you want new vs used, and genuine vs Chinese.

I am about to put the alternator and starter on mine that 74M35A2 sold me, I have changed to many of the stock ones. And the price of a new stock one is 300+.

 

[crazycanuk]

Thanks for all the help. Sorry for the delay, life sometimes gets in the way. Okay, took out batteries, had them load tested and recharged. Put back into truck and now NO power to switch or dash. I am thinking some critter chewed something while i was gone.Will check out the switch this weekend and the positive cable mentioned here, but even the battery gauge has no power !

Will update the results this weekend.

Thanks again to all.

 

[ghitch75]

did you clean all the battery connections?.......i had low volts at meter and starter turned slow.....after cleaning no slow starter and gauge in the green........ when i mean clean clean them to a shine and reassemble with dielectric grease on the bolts then spray with battery terminal protectant .....


http://www.napaonline.com/p/CRC095046

 

[crazycanuk]

update. Still no power. I am looking closer at batteries first. The local NAPA store said they did a test but I am getting a dead short on two of four batteries. I am going to hook up just 24 V and see if I get dash. It almost looks like one battery in each pair is fried. Since they appear to be the "wired" at opposite ends of series/parallel connections, I am a bit concerned. Will let the group here know the results after this weekend. Been a few weeks since I was able to work on truck!

Need to get it moved ! "

 

[goldneagle]

If you got 2 dead batteries out of 4, why not just use the 2 good batteries in series for 24 volts. We have had many members here testify that they only need 2 batteries in their 5 ton truck to run them.

 

[Artisan]

If you got 2 dead batteries out of 4, why not just use the 2 good batteries in series for 24 volts. We have had many members here testify that they only need 2 batteries in their 5 ton truck to run them.

I have an M916 w/ a 400HP Big Cam Cummins engines.
I get buy w/ ease w/ 2 Sears, I think they are Group 36
batteries.

yep, just 2 little 12V batteries work WELL.
I run A/C and an AM/FM

The only issues I have had, and it is recent, is if I shut
the engine off w/ the A/C on "something" happens and
next time I fire off the truck the A/C will not work. BLAST!

I replaced the ignition switch, I tested all relays to 100%, now
I am looking at Circuit Breakers...

Problem is intermittent...

Try 2 batteries.

 

[74M35A2]

Did you get the polarity correct on reconnect? The PCB prevents a power-up if the battery polarity is reversed (it's job), and I don't believe the cables are color coded. Also, you could have PCB solenoids that are tired. They will prevent a dash power-up if so.

 

[crazycanuk]

vFinal update as the problem is solved. Rodents chewed a bundle of cables. Not once in two different places. 84 solder and shrink tube later, bundle repaired. Hooked up batteries, nothing. So started over. Do batteries still have charge. Yes all 4 had 13 or 12,5 volts. BUT I noticed on the voltmeter on 2 batters a minus sign! Batteries had reversed polarity! Took those two out of equation and truck started immediately. Strange. I will do some research on the reverse polarity as I have NEVER seen that before.

Thanks to all for the help. This one was strange.

 

[goldneagle]

vFinal update as the problem is solved. Rodents chewed a bundle of cables. Not once in two different places. 84 solder and shrink tube later, bundle repaired. Hooked up batteries, nothing. So started over. Do batteries still have charge. Yes all 4 had 13 or 12,5 volts. BUT I noticed on the voltmeter on 2 batters a minus sign! Batteries had reversed polarity! Took those two out of equation and truck started immediately. Strange. I will do some research on the reverse polarity as I have NEVER seen that before.

Thanks to all for the help. This one was strange.

I know reverse polarity happens in NiCD batteries when you run them down too much. It happened to me in the past. That was because we were told to discharge the batteries totally before recharging. That was bad advise by the tool manufacturers. Once the battery reverses polarity it will never come back (NiCD).

I assume those 2 batteries were drained so badly they reversed polarity. Either just use 2 batteries with your truck or get new ones.

 

[VPed]

Decades ago when I was in automotive technical school, I was taught that flooded cell batteries can be charged up reverse polarity. First, the battery has to be completely dead. Then, someone hooks up a charger backwards. If you completely discharge the ones that are reversed, you may be able to get them back to being right with a proper polarity charge. You can use a 12 volt incandescent bulb across the terminals to get to full discharge state.