M939-A2 Battery Sudden Death

[99nouns]

I know this title sounds like a video game title but this just happened today.

Short story;
I drove 50 miles to visit someone and after staying there for 2 hours I was returning back home as I had a quick stop at the gas station 5 miles away from my destination (or 45 miles away from home) and all of a sudden no start. The battery gauge on the dash was in red section when ignition on.

Reason I am mentioning this because I am running 2 batteries instead of 4 but one of the battery is 3 years old 6TL and other is 4 years old 6TL, I am surprised that 4 years old died instantly like that and it didnt get weak or slowly got drained or showed any symptoms of trouble, they were very good starters in perfect functioning condition despite of being 2.

Is there a some type of a glitch of some sort that these trucks might have it just killed the battery or do you think the battery was defective or might I have another underlying cause leading to this problem???

Long story;
When I got the truck in El Paso, TX from Fort Bliss it had 4 batteries in it, being the newbie days, it use to start very weak and I thought this truck has such a gigantic starter that even 4 batteries being old was not enough, also seeing a starter that size first time in my life was a feeder for my thoughts. Then one day I had no start issue where it was only a click but no rev, so I looked up here in SS (which was my first fix ever) someone mentioning you can run this truck with 2 batteries to get 24V and military only puts 4 "just in case", so I got all 4 batteries tested and there were 2 dead batteries (less than 10V) and two low voltage like 11.something volt ok battery. I got the 2 good batteries charged at a part store still being in TX because this happened like next day or 2 which I went to recover with my dad and his RV so I didnt need to hit the highway right away which fixed the problem.

Furthermore my father wanted to reroute from St. Luis instead of driving to FL directly for his own reasons, on the way the one of the other original battery went bad half way to St. Luis like 4 days later which then I had to buy a brand new battery, because when you get stuck with battery problem, you cannot find a vehicle who can jump start you either and that's why I wanted a good battery. That setup went well for a year and 3 years ago last original battery went south in FL not knowing its use and abuse finally I got one more new battery hoping it will out last my lifetime unfortunately to find out otherwise which I might be expecting too much.

So when this situation happened twice in a short period on the way to St. Luis my dad having the wealth of wisdom, he suggested sometimes there is a weird condition where alternator goes bad and instead of charging the batteries, it would begin draining it very slowly which would kill the battery that I never heard of, but when I explained that to one of the parts shop, guy there said he verified the existence of that such problem and he said he can test for free if I took the alternator to him which he needed to normally pull the alternator out and fasten to the tester and measure amp supply and draw rate with a tester while turning it at certain varying RPMs. At that time I was not willing to remove an alternator bigger than my head and half the weigh I am so I suggested if I can start the engine then disconnect the battery leads from the battery and rev the engine to his needed RPM while testing it, he was like "heck yeah" (pros of heaving a diesel engine).

So that test came out good 4 years ago, but because of this happened unexpectedly now my father thinks starter might have faulty solenoid of some sort that it over loads batteries so much that causes unpredictable life out of them damaging them.

I would appreciate if anyone with more experience chime in.

Thank you in advance and sorry for the history lecture.

 

[Andyrv6av8r]

Could be your PCB box. Bad solenoids that need to be replaced.

 

[autoshopteacher]

M817 with 4 batteries did the exact same thing. Stopped for fuel and no start. Pull started it and drove it to my shop. All terminals were clean and well connected. I did a battery load test with each battery isolated and one battery failed. Replaced that battery and no problems since. In your case I don't know if there is an on board cause, but do the isolated test and verify battery condition. Be sure you do a load test. A fully charged 12 volt battery should have a minimum of 12.6 VDC but that won't check its ability to deliver the amperage necessary to function. I have experienced many battery failures that are only the fault of a bad battery.

If you suspect there might be a parasitic drain on the batteries, disconnect the negative cable to ensure there is no circuit, be sure that all power is off and put an ammeter in series with the negative cable to ground. If you have a good DC inductive ammeter you do not need to disconnect the ground cable. There should be no current draw.

Alternators have diodes in them that do not allow reverse current flow and if these diodes fail, you can get a current discharge through the alternator. If your open circuit test as explained above shows a discharge current, start disconnecting electrical items (alternator) from the truck till the drain is eliminated. Fuses are the easiest way to disable circuits if you have them. M817, not many fuses and I have no 939 series experience.
Great hunting.

 

[juanprado]

Start with the basics. Check each battery voltage individually, charge if needed, load test. It sounds like one went south and dragged the other down. Once you have 2 hot good load tested batteries, check alternator output. Just a little drop in voltage is all that it takes to ruin your day.

 

[simp5782]

I know this title sounds like a video game title but this just happened today.

Short story;
I drove 50 miles to visit someone and after staying there for 2 hours I was returning back home as I had a quick stop at the gas station 5 miles away from my destination (or 45 miles away from home) and all of a sudden no start. The battery gauge on the dash was in red section when ignition on.

Reason I am mentioning this because I am running 2 batteries instead of 4 but one of the battery is 3 years old 6TL and other is 4 years old 6TL, I am surprised that 4 years old died instantly like that and it didnt get weak or slowly got drained or showed any symptoms of trouble, they were very good starters in perfect functioning condition despite of being 2.

Is there a some type of a glitch of some sort that these trucks might have it just killed the battery or do you think the battery was defective or might I have another underlying cause leading to this problem???

Long story;
When I got the truck in El Paso, TX from Fort Bliss it had 4 batteries in it, being the newbie days, it use to start very weak and I thought this truck has such a gigantic starter that even 4 batteries being old was not enough, also seeing a starter that size first time in my life was a feeder for my thoughts. Then one day I had no start issue where it was only a click but no rev, so I looked up here in SS (which was my first fix ever) someone mentioning you can run this truck with 2 batteries to get 24V and military only puts 4 "just in case", so I got all 4 batteries tested and there were 2 dead batteries (less than 10V) and two low voltage like 11.something volt ok battery. I got the 2 good batteries charged at a part store still being in TX because this happened like next day or 2 which I went to recover with my dad and his RV so I didnt need to hit the highway right away which fixed the problem.

Furthermore my father wanted to reroute from St. Luis instead of driving to FL directly for his own reasons, on the way the one of the other original battery went bad half way to St. Luis like 4 days later which then I had to buy a brand new battery, because when you get stuck with battery problem, you cannot find a vehicle who can jump start you either and that's why I wanted a good battery. That setup went well for a year and 3 years ago last original battery went south in FL not knowing its use and abuse finally I got one more new battery hoping it will out last my lifetime unfortunately to find out otherwise which I might be expecting too much.

So when this situation happened twice in a short period on the way to St. Luis my dad having the wealth of wisdom, he suggested sometimes there is a weird condition where alternator goes bad and instead of charging the batteries, it would begin draining it very slowly which would kill the battery that I never heard of, but when I explained that to one of the parts shop, guy there said he verified the existence of that such problem and he said he can test for free if I took the alternator to him which he needed to normally pull the alternator out and fasten to the tester and measure amp supply and draw rate with a tester while turning it at certain varying RPMs. At that time I was not willing to remove an alternator bigger than my head and half the weigh I am so I suggested if I can start the engine then disconnect the battery leads from the battery and rev the engine to his needed RPM while testing it, he was like "heck yeah" (pros of heaving a diesel engine).

So that test came out good 4 years ago, but because of this happened unexpectedly now my father thinks starter might have faulty solenoid of some sort that it over loads batteries so much that causes unpredictable life out of them damaging them.

I would appreciate if anyone with more experience chime in.

Thank you in advance and sorry for the history lecture.

Do you have anything on the tied into the 12v side. That could cause a single battery issue. Bad PCB could cause a no charging issue. No telling how long you could have been driving and not known it.

Other key point Mo whenever you go on trips further from home try not to turn the truck off. My rule is unless i am within 15 miles of home or another truck in case of issues my truck does not shut off and i have a good deal of trust with my truck but it only takes that one time. If you are worried about someone just hoping in and driving off then make a quick cable attachment for the steering wheel that is easy to get on and off. Noone will just jump in with bolt cutters. I leave mine running all the time even going into home depot or walmart but chances of someone figuring the caterpillar transmission out when i pull the air feed and leave it in 16th is slim to none.

 

[99nouns]

Could be your PCB box. Bad solenoids that need to be replaced.

I am trying to understand the way things operate so that I can eliminate problems, if the PCB has a problem than can I safely assume that it will fail to charge the batteries?

Wes said;

Bad PCB could cause a no charging issue. No telling how long you could have been driving and not known it.

If so should I have not has some short of weak start attempt like whining crank rather than instant dead battery, not to forget I had a good strong start 10 minutes prior where I only drove to a gas station 5 miles away where battery got dead instantly with no warnings what so ever.

---------

M817 with 4 batteries did the exact same thing. Stopped for fuel and no start. Pull started it and drove it to my shop. All terminals were clean and well connected. I did a battery load test with each battery isolated and one battery failed. Replaced that battery and no problems since.

This is exactly what happened to me, only issue with M939 series is that it is a Automatic Transmission so pull start was not a option
How long did you have that battery one you replaced, was it a original that came with the truck?

If you suspect there might be a parasitic drain on the batteries, disconnect the negative cable to ensure there is no circuit, be sure that all power is off and put an ammeter in series with the negative cable to ground. If you have a good DC inductive ammeter you do not need to disconnect the ground cable. There should be no current draw.

Alternators have diodes in them that do not allow reverse current flow and if these diodes fail, you can get a current discharge through the alternator. If your open circuit test as explained above shows a discharge current, start disconnecting electrical items (alternator) from the truck till the drain is eliminated. Fuses are the easiest way to disable circuits if you have them. M817, not many fuses and I have no 939 series experience.
Great hunting.

I am going to do that soon, thank you for that info, this will be a new learning experience.

Start with the basics. Check each battery voltage individually, charge if needed, load test. It sounds like one went south and dragged the other down. Once you have 2 hot good load tested batteries, check alternator output. Just a little drop in voltage is all that it takes to ruin your day.

Will that drop in voltage from alternator I assume 14.0+ charging rate kill the battery instantly?

Do you have anything on the tied into the 12v side. That could cause a single battery issue. Bad PCB could cause a no charging issue. No telling how long you could have been driving and not known it.

I dont have anything connected as of aftermarket, but I see there is a thin cable connected one of the batteries for 12V side I dont know what purpose which truck came like that.

Other key point Mo whenever you go on trips further from home try not to turn the truck off. My rule is unless i am within 15 miles of home or another truck in case of issues my truck does not shut off and i have a good deal of trust with my truck but it only takes that one time. If you are worried about someone just hoping in and driving off then make a quick cable attachment for the steering wheel that is easy to get on and off. Noone will just jump in with bolt cutters. I leave mine running all the time even going into home depot or walmart but chances of someone figuring the caterpillar transmission out when i pull the air feed and leave it in 16th is slim to none.

Wes thank you for the tip, this was so unexpected, I trusted of having 2 fairly new batteries, but I have had a chance to do some research on 6TLs today and seems like I am not the only one complaining about this issue now, if I have to be honest, I would even be scared the get into your truck and attempt to drive off, it is a whole different animal now, I mean it might look like a normal MV from outside, but in my book your nick name is the MAD WES as in MAD MAX. Your truck is a STATEMENT.

 

[NDT]

4 years on a lead acid 6TL is as good as it gets. Those things are awful. Get some Hawkers AGMs and put them on a defibrillator and you will have much better results.

 

[antennaclimber]

Load test each battery and see if one is bad. Batteries can fail anytime. I have had them fail in diesel trucks without any warning.

And NDT is right, 4 years on a 6TL is about all you can expect. I won't buy them again.

 

[Scrounger]

First cover the basics. Pull and check the batteries. And yes, batteries can suddenly fail.

Simply put as a battery gets old some of the things that happen is sulfates build up in the case and plates get worn. All it takes is for one of the plates to crack or shift and it will short. When that happens, the battery is toast.

As far as batteries go, I run Deka 6TL batteries and usually get 8-10 years out of a set. I install them as a set, check them regularly with a Duo-check tester, and keep them fully charged.

 

[juanprado]

Will that drop in voltage from alternator I assume 14.0+ charging rate kill the battery instantly?

.

on a 24volt system , you need to be seeing 27.8 -28.4 volts out of the alternator/charging system.

Bad batteries can kill your military alternator/ voltage regulator.... Don't ask me how I know....

 

[autoshopteacher]

This is exactly what happened to me, only issue with M939 series is that it is a Automatic Transmission so pull start was not a option
How long did you have that battery one you replaced, was it a original that came with the truck?

I've owned the truck since August, 2010 and have no knowledge of their in-service dates. I have since replaced all batteries. Two failed separately causing total starting death. I started to monitor them and the other two (one at a time) began to display weakness and failed a load test so I replaced them.

Lead acid batteries will fail for various reasons but they will fail sooner if not kept fully charged. I keep a battery tender hooked up at all times.A quick charging system check is to hook up a DVM across the batteries with engine and all power off. Then start the engine and read DC voltage. It should increase some. I don't remember for sure but I think minimum voltage, engine running should exceed 28 VDC, no load; it should exceed the non-op open circuit test. The TM will provide the correct readings.

 

[74M35A2]

Could just be alternator failure? I'd go group 31 batteries and a new civilian alternator, even if all the original mil stuff was fine.

Do you recall the volt gauge position with the engine running? It would help to easily differentiate the issue if you really want to just find and fix what is wrong, rather than throwing a bunch of new parts at the truck.

You can even just use a digital volt meter to tell the condition of either/both. With engine running at 1000rpm, system voltage should be an absolute minimum of 25.7V, but actually closer to 27.0-28.8v range.


A simple way to load test a battery is using the vehicle itself. Make sure the batteries are fully charged, then put the digital meter on the terminals of one battery while somebody cranks the engine with the emergency fuel cut pulled to prevent starting. Voltage should not drop below 10.0V on any one battery within the first 5 seconds of this simple test. A healthy battery is able to maintain its voltage while delivering up to its rated current.

As others here have said, batteries can and do suddenly fail as well. Each battery is six 2 volt generators in series. If they "drop a plate" in any cell, it breaks the chain, and therefore the output of the battery.

 

[99nouns]

on a 24volt system , you need to be seeing 27.8 -28.4 volts out of the alternator/charging system.

Thank you for correcting me, I am 24 volt guy living in 12 volt world... Good info.

Could just be alternator failure? I'd go group 31 batteries and a new civilian alternator, even if all the original mil stuff was fine.

I would like to do that once I find out that my alternator is actually is the one causing problems, even though it is civilian alternator I am sure it wont be cheap at that size, is there part number of what civilian alternator that will fit? Will it need mods?

Do you recall the volt gauge position with the engine running? It would help to easily differentiate the issue if you really want to just find and fix what is wrong, rather than throwing a bunch of new parts at the truck.

Yes it was in green section but lower end very close to yellow. And no I have already figured out throwing parts is never the solution.

You can even just use a digital volt meter to tell the condition of either/both. With engine running at 1000rpm, system voltage should be an absolute minimum of 25.7V, but actually closer to 27.0-28.8v range.

A simple way to load test a battery is using the vehicle itself. Make sure the batteries are fully charged, then put the digital meter on the terminals of one battery while somebody cranks the engine with the emergency fuel cut pulled to prevent starting. Voltage should not drop below 10.0V on any one battery within the first 5 seconds of this simple test. A healthy battery is able to maintain its voltage while delivering up to its rated current.

Great info and tips I will do that a.s.a.p. Currently I will have to get new batteries before I can do any of the tests, I wish I had unlimited access to money like military does.

Thank you for all the great responses.

 

[99nouns]

First cover the basics. Pull and check the batteries. And yes, batteries can suddenly fail.

Simply put as a battery gets old some of the things that happen is sulfates build up in the case and plates get worn. All it takes is for one of the plates to crack or shift and it will short. When that happens, the battery is toast.

As far as batteries go, I run Deka 6TL batteries and usually get 8-10 years out of a set. I install them as a set, check them regularly with a Duo-check tester, and keep them fully charged.

Is there a source you get them from? Some websites I checked says Government use only. Giving me the impression they wont sell to a civilian.

 

[juanprado]

Is there a source you get them from? Some websites I checked says Government use only. Giving me the impression they wont sell to a civilian.

Napa & O Reilly can order them in.Make sure to check date codes and load test before you leave store. They are not fast moving sku that tend to sit a while before getting sold. Napa warehouse manager in another lifetime......

 

[Scrounger]

Napa Auto Parts carry them. The Napa part number is 7256. They are listed under ordnance battery. If you can wait a little while you may be able to save some money. Most Napa’s don’t stock this battery and must order it from the warehouse. Depending on the individual Napa store they get a drop shipment of batteries somewhere between two- and four-weeks’ time. If you can wait for the drop shipment the manager may pass the savings on. Some trivia on this. Larger orders help on drop shipments, the store get “extra” credit and depending on the store the savings is around 17%. One other thing. There is a $36 core charge on 6TL’s.


As far as your truck goes. The batteries that are in it are toast. Until you have new batteries in it there isn’t much one can do. I suspect that once new batteries are installed that will solve the problem.