M939 transmission problem

[Csm Davis]

Today took the M929 for high speed shakedown and everything went good until I stopped for a red light and went to pull off and it will not shift up out of first gear, it will go into neutral but not reverse or anything above first.

Checked oil, and readjusted cable no luck, transmission has worked fine for about 5 miles before this and fine before the engine replacement.

sent from my decrepit fingers

 

[74M35A2]

Sounds like she's lunched. Good thing you have access to other ones!

 

[M35A2-AZ]

About the same thing happened to me with my M925. Started out ok went about 4 miles and I noticed it was trying to shift (so I was thinking). I stopped and tried to start a gain, no move at all. Had to tow her home. Pulled the pan and there was a lot of clutch plate parts in the pan.
So I replaced it. That was fun.

 

[Csm Davis]

Hahaha funny guy! It was shifting fine, I think it might have gotten something caught in the valve body. Anyone have any good ideas?

Sounds like she's lunched. Good thing you have access to other ones!

sent from my decrepit fingers

 

[goldneagle]

Well I know on the CUCV the tranny shifts by vacuum. How does the trany shift on the M939?

 

[Suprman]

Drop the pan change the filter, change the spin on filter and see how it all looks. If it was me I would see what the Allison dealer has to say. Might be something simple that can be repaired from the pan area.

 

[Tinstar]

If you go on the Allision website,
they have a great troubleshooting guide and other info for that transmission.

Maybe be worth a read

 

[74M35A2]

Well I know on the CUCV the tranny shifts by vacuum. How does the trany shift on the M939?

No vacuum on yesterdays diesels, or vacuum pump. This trans does have a modulator cable from the throttle linkage to the trans, but I think regardless of it's position, it should eventually shift, with the modulator cable just changing said shift points. I have heard of stories where guys took their truck to an Allison dealer and something like this was diagnosed and fixed for only a few hundred dollars, and they left saying the truck has never driven better. That would be a slow drive in a 9 stuck in 1st gear though! Go through the MT654CR troubleshooting guide as suggested, and also call an Allison service shop, they may be able to give you some free try's over the phone.

 

[VPed]

You said you checked fluid and readjusted cable, I'm guessing the shifter cable. Have you checked the linkage from the throttle to the transmission? Just a thought that maybe a clevis pin fell out or something.

I know I have had a vacuum line from the modulator on a TH400 come off and yeah, it should shift eventually but in my GTO, that meant up around 5 grand on the tach.

 

[Csm Davis]

You said you checked fluid and readjusted cable, I'm guessing the shifter cable. Have you checked the linkage from the throttle to the transmission? Just a thought that maybe a clevis pin fell out or something.

I know I have had a vacuum line from the modulator on a TH400 come off and yeah, it should shift eventually but in my GTO, that meant up around 5 grand on the tach.

No we checked all the cables but readjusted the one going to the throttle

sent from my decrepit fingers

 

[KaiserM109]

I lost the low gear pack on M923A1 in -20 degree temperatures. I subsequently found out that my truck did a tour in Afghanistan and that the government puts 15W40 oil in those transmissions. I called Allison for advice an the first thing the engineer I talked to said was "GET THAT GOVERNMENT #@%* OUT OF IT!"

 

[Csm Davis]

I lost the low gear pack on M923A1 in -20 degree temperatures. I subsequently found out that my truck did a tour in Afghanistan and that the government puts 15W40 oil in those transmissions. I called Allison for advice an the first thing the engineer I talked to said was "GET THAT GOVERNMENT #@%* OUT OF IT!"

View attachment 617688

Okay couple things this truck had mostly 10wt in it I know because we put several gallons back in to refill it, second if you are going to be operating these trucks in that cold of an environment you really should look at following the TM recommended changes to the fluids.
As to 15w40 in these trucks I have talked to lots of Army mechanics and all of them have said that they have only topped off a few with 15w40 and have not drained and refilled but a hand full because of having to do a complete remove and replace of the transmissions and those were not a common failure part. So if you had a clutch pack fail at -20 sorry to hear but I believe that would be more of you not changing to a cold weather oil than my problem at 80+ down here. As to what oil I will be running and have run for more than 10,000 miles it will remain 10wt.

sent from my decrepit fingers

 

[KaiserM109]

It is not my intension to start a pi**ing match about who knows what about what has happened to a surplus truck, but at the risk of getting a bit wordy, I would like to comment on what people assume has been done to their truck before they bought it.

There is no way you can know what has been done to a thirty year old truck. Maintenance performed stateside tends to be a of high quality, principally because of the availability of parts and expendable items and the fact that motor pools stateside usually have the luxury of the time to do it right.

That is not true overseas at the end of a very long and slow supply chain. In a combat zone there is a lot of “Kamikaze Maintenance” done; that’s maintenance performed just to get through the next mission. I have done my share of doing whatever is necessary to get a truck into a convoy and out the front gate.

When we took the dead transmission out, we saved the oil in case we might want to have it tested. After I talked to the Allison engineer we saved the oil from the pull-out transmission I was putting back in the truck. The Allison guy told me that any quality ATF would work fine and he was aware of the government’s practice of using 15W40. The oil out of both transmissions was considerably thicker than the ATF I put in.

My truck has had a lot of bad maintenance such as wrong and missing bolts, missing washers and badly spliced wires. It looked like stuff done to “just get the truck rolling.” The only thing we can do is assess the truck’s current condition and go from there, and don’t be so trusting of government decisions about equipment. If you are interested in reading about that and some of the things Robert S. McNamara did just to get on with the Viet Nam war, go to http://jouster.com/ and read both parts of the Saga of the M16.

BTW, the transmission blew on the trip from Ft. Benning, GA to Denver, CO, 16 miles from home. In the GL yard I did check the fluids for level and apparent quality. They barely let me do that and pump up 2 flats.

PS Rest in peace Major Dick Culver, USMC.

 

[Two Stroke Madness]

The use of 15w40 oil becomes a sticks and balls issue. I will explain. Viscosity modifiers are used to change a 15 weight oil to a 40 weight as temperature increases. Under magnification these viscosity modifiers look like little coils until they are heated then they straighten out into long sticks making the oil thicker. Oil molecules look like little balls under magnification. Balls lubricate, sticks do not. Automatic transmissions need as much lubrication as possible, so sticks are out of the question. There for the use of 15w40 in these allison transmissions is a big no no regardless of what Uncle Sam says.

 

[74M35A2]

Polymers. Love the name, 2 strokes rock. From my CR500R to my 6-71TA.

 

[Csm Davis]

The use of 15w40 oil becomes a sticks and balls issue. I will explain. Viscosity modifiers are used to change a 15 weight oil to a 40 weight as temperature increases. Under magnification these viscosity modifiers look like little coils until they are heated then they straighten out into long sticks making the oil thicker. Oil molecules look like little balls under magnification. Balls lubricate, sticks do not. Automatic transmissions need as much lubrication as possible, so sticks are out of the question. There for the use of 15w40 in these allison transmissions is a big no no regardless of what Uncle Sam says.

It is not my intension to start a pi**ing match about who knows what about what has happened to a surplus truck, but at the risk of getting a bit wordy, I would like to comment on what people assume has been done to their truck before they bought it.

There is no way you can know what has been done to a thirty year old truck. Maintenance performed stateside tends to be a of high quality, principally because of the availability of parts and expendable items and the fact that motor pools stateside usually have the luxury of the time to do it right.

That is not true overseas at the end of a very long and slow supply chain. In a combat zone there is a lot of “Kamikaze Maintenance” done; that’s maintenance performed just to get through the next mission. I have done my share of doing whatever is necessary to get a truck into a convoy and out the front gate.

When we took the dead transmission out, we saved the oil in case we might want to have it tested. After I talked to the Allison engineer we saved the oil from the pull-out transmission I was putting back in the truck. The Allison guy told me that any quality ATF would work fine and he was aware of the government’s practice of using 15W40. The oil out of both transmissions was considerably thicker than the ATF I put in.

My truck has had a lot of bad maintenance such as wrong and missing bolts, missing washers and badly spliced wires. It looked like stuff done to “just get the truck rolling.” The only thing we can do is assess the truck’s current condition and go from there, and don’t be so trusting of government decisions about equipment. If you are interested in reading about that and some of the things Robert S. McNamara did just to get on with the Viet Nam war, go to http://jouster.com/ and read both parts of the Saga of the M16.

BTW, the transmission blew on the trip from Ft. Benning, GA to Denver, CO, 16 miles from home. In the GL yard I did check the fluids for level and apparent quality. They barely let me do that and pump up 2 flats.

PS Rest in peace Major Dick Culver, USMC.

And yet here we are, not trying to get a good idea of what is wrong with My transmission, which I bet there is a 99% chance that it has nothing to do with what type oil is in it.

sent from my decrepit fingers

 

[Two Stroke Madness]

Sorry, got a little off topic. Pull the pan, check for shrapnel. In my dealings with automatics symptoms that parallel what you described most likely mean a toasted transmission.

 

[lindsey97]

Have you checked the little governor filter, located at the back bottom of the trans directly below the output shaft? It is behind a large cap nut, 15/16" or 1" maybe. Filter is the size of a thimble, stainless screen mesh.

 

[Swamp Donkey]

Have you checked the little governor filter, located at the back bottom of the trans directly below the output shaft? It is behind a large cap nut, 15/16" or 1" maybe. Filter is the size of a thimble, stainless screen mesh.

Very easy to check too.

http://www.steelsoldiers.com/showthread.php?133133-Allison-Governer-screen-filter

 

[Csm Davis]

Yeah we checked that too. Looks like we are pulling the transmission.

sent from my decrepit fingers