M925A2 transmission overheating

[purple_duckk]

I'm at my wits end chasing this down. Any help or suggestions would be welcomed.

I drove my M925A2 home back in May. 330 miles went off without a hitch, but with 30 left the transmission was getting up around 275 on the gauge and it was shifting way early, bogging the engine and doing the speed to 40 even on flat straight roads.

I let it cool down and drove the last 30 miles it acted normal again, no problem.

Once home I did a full fluid change, engine oil, coolant, tranny oil(15w-40 was what was in it so I stuck with that) and changed all the filters including the transmission pan filter. I disconnected the coolant lines going from the radiator to the torpedo tranny oil cooler and poured water into the overflow and it poured straight out so the cooler flows coolant. I didn't see any sediment in the radiator and the coolant poured out of it when i disconnected the line so that doesn't seem to be plugged, the fan comes on when the engine gets up to temp, and the engine isn't overheating so I don't think there's a clog there.

I disconnected the oil line going out of the torpedo cooler, started the engine and it pumped oil so the cooler doesn't seem to have a clog.

So it seems the system is flowing oil and coolant, but I just went on a test drive after heating up the engine, checking the oil and tranny oil. Within 5 miles the tranny temp gauge hit 250 and I could tell I was losing power and bogging the motor. I live in Florida so it's not like I'm climbing crazy hills or anything, just minor grades.

I used the IR thermo gun to temp the transmission while I was idling. The gauge and IR gun showed 150. I drove around the farm a bit and the gauge and gun both showed 200. Out on the road when I started losing power the gauge was showing 250-260ish but the gun showed 177 at the torpedo and transmission case shot through the service trap door in the cab, but 155 at the pan. Not really sure if the gun is off at higher temps, I'll have to test that but either way ATF has a temp limit very similar to 15w-40 and 260-275 is kind of the limit before it breaks down. The engine temp gauge and ir gun both show 160 when the tranny is overheating.

I know people recommend doing a full fluid swap to ATF, but I don't think that minor change will make me be able to drive all day vs 5 miles. Plus I had no issues for 300 miles at first, so I assume there is another major issue I need to tackle first but I'm out of ideas. Any recommendations?

 

[simp5782]

Do you have the old filter if you changed the spin on? If so cut it open and see what is inside

The gauge sending unit is on the fluid return from the heat exchanger.

Just cause you put 300miles on it doesn't mean anything. These transmissions go out on a whim, either losing reverse or 3rd thru 5th for no reason at all. Or just kills the truck when you put it in D.

I mean just run the IR gun all over the exhaust manifold. It should show more than 300 degrees if it's working right.

And when you say quits pulling , ie just sits there a idles, surges like it's not getting enough fuel. Bogs like it has no fuel pressure, does it stall the truck out and kill the truck?

 

[purple_duckk]

Do you have the old filter if you changed the spin on? If so cut it open and see what is inside

The gauge sending unit is on the fluid return from the heat exchanger.

Just cause you put 300miles on it doesn't mean anything. These transmissions go out on a whim, either losing reverse or 3rd thru 5th for no reason at all. Or just kills the truck when you put it in D.

I mean just run the IR gun all over the exhaust manifold. It should show more than 300 degrees if it's working right.

And when you say quits pulling , ie just sits there a idles, surges like it's not getting enough fuel. Bogs like it has no fuel pressure, does it stall the truck out and kill the truck?

I might have it, I'll check and see if it's in the trash still. What would I be looking for?

When I let things cool down everything works as it should until the temp goes up over 250. I'm not losing gears, I can feel it shifting through them, but when it gets hot it shifts through them way too early.

When I say it loses power, I mean I'll have the pedal to the boards and I'll drop in speed to 40 MPH while the engine is just bogged down. It's clearly in too high of a gear for the speed. The turbo isn't whining and it has that sound that I can't describe any other way than shifting your manual into too high of a gear and giving it gas. The truck doesn't stall or even chug, and when I pull off and start from a stop it has great power at first, but it bangs through the gears way too early. If I shift into neutral it'll rev to the moon and make the turbo scream.

 

[simp5782]

I might have it, I'll check and see if it's in the trash still. What would I be looking for?

When I let things cool down everything works as it should until the temp goes up over 250. I'm not losing gears, I can feel it shifting through them, but when it gets hot it shifts through them way too early.

When I say it loses power, I mean I'll have the pedal to the boards and I'll drop in speed to 40 MPH while the engine is just bogged down. It's clearly in too high of a gear for the speed. The turbo isn't whining and it has that sound that I can't describe any other way than shifting your manual into too high of a gear and giving it gas. The truck doesn't stall or even chug, and when I pull off and start from a stop it has great power at first, but it bangs through the gears way too early. If I shift into neutral it'll rev to the moon and make the turbo scream.

Turbo shouldn't make any whistling or spooling with no load on the engine. I.e. in neutral revving it up. Atleast not very loud anyway.

You are looking for excess metal, o ring rubber. Anything odd.

When the transmission Temps gets high and quits pulling. When you pull off and put it into neutral and increase idle. How long does it take to bring the temperature back down to normal?

Pump pressure is key as well. You may install a test port either at the cooler or in place of the temp sending unit and see what it is. Your pump pressure at idle may not be the same and will not be enough as it will need running down the road.

 

[purple_duckk]

Turbo shouldn't make any whistling or spooling with no load on the engine. I.e. in neutral revving it up. Atleast not very loud anyway.

You are looking for excess metal, o ring rubber. Anything odd.

When the transmission Temps gets high and quits pulling. When you pull off and put it into neutral and increase idle. How long does it take to bring the temperature back down to normal?

Pump pressure is key as well. You may install a test port either at the cooler or in place of the temp sending unit and see what it is. Your pump pressure at idle may not be the same and will not be enough as it will need running down the road.

When I pull off, I usually let it idle for 5-10 minutes until the temp is 225 and then I'll shut it off and wait for it to cool down. Usually 35-45 minutes will get me back down to 150 ish and I can get going for a few more miles.

Are you referring to the water pump pressure to circulate the coolant? Because wouldn't a bad water pump effect the engine temp too, not just the tranny temp? The engine is pretty rock solid at 160 the entire time. But at this point, I'll try anything. Do you have a link to a test port so I can get one and put it on?

 

[simp5782]

When I pull off, I usually let it idle for 5-10 minutes until the temp is 225 and then I'll shut it off and wait for it to cool down. Usually 35-45 minutes will get me back down to 150 ish and I can get going for a few more miles.

Are you referring to the water pump pressure to circulate the coolant? Because wouldn't a bad water pump effect the engine temp too, not just the tranny temp? The engine is pretty rock solid at 160 the entire time. But at this point, I'll try anything. Do you have a link to a test port so I can get one and put it on?

I am referring to the transmission pump pressure if it is now flowing enough fluid to keep the transmission cool while its running then it is going to overheat

Look at your filter head it may have a spot for a port in either the lines or the head itself.

A truck I currently am working on had a sampler port which can also double as a pressure valve.

You can probably find a #16 female to male union that has a port that simply can connect between the 2.

You said you had decent flow but flow is one thing pressure is another. 10gpm looks like alot just running out into a bucket

 

[purple_duckk]

I am referring to the transmission pump pressure if it is now flowing enough fluid to keep the transmission cool while its running then it is going to overheat

Look at your filter head it may have a spot for a port in either the lines or the head itself.

A truck I currently am working on had a sampler port which can also double as a pressure valve.

You can probably find a #16 female to male union that has a port that simply can connect between the 2.

You said you had decent flow but flow is one thing pressure is another. 10gpm looks like alot just running out into a bucket

Gotcha, I'll see if I can put a test port on there. What do I put on that test port to measure the pressure? I know if I crack the sample valve on the oil cooler it sprays oil like 10 feet, but I can see what you're saying about pressure being different at idle and driving.

 

[simp5782]

Gotcha, I'll see if I can put a test port on there. What do I put on that test port to measure the pressure? I know if I crack the sample valve on the oil cooler it sprays oil like 10 feet, but I can see what you're saying about pressure being different at idle and driving.

Your oil cooler is only 60psi.

Transmission should be atleast 150psi at idle. Upwards of 300 running

@Swamp Donkey has the complete manual Maybe he can tell us the correct spec

So maybe a 600psi fluid filled gauge. They are cheap from a hydraulic store. Just rig it to a hydraulic hose to fit the pump fitting

 

[Swamp Donkey]

See if these help. Never trust a military vehicle gauge for troubleshooting. A bad ground at the gauge panel or sending unit will cause the gauge to read high. The gauge ground is a very common problem from corrosion and the design of the gauge bracket. I'd verify your actual temperature before moving forward with pressure tests.

I was in this same boat when I first got my truck. After a few minutes of driving the gauge would rise until it was pegged. Ran individual grounds from all gauges to the firewall, bypassing the panel and dash completely. The gauge read normal after that, along with better readings from the other gauges as well.

 

[purple_duckk]

See if these help. Never trust a military vehicle gauge for troubleshooting. A bad ground at the gauge panel or sending unit will cause the gauge to read high. The gauge ground is a very common problem from corrosion and the design of the gauge bracket. I'd verify your actual temperature before moving forward with pressure tests.

I was in this same boat when I first got my truck. After a few minutes of driving the gauge would rise until it was pegged. Ran individual grounds from all gauges to the firewall, bypassing the panel and dash completely. The gauge read normal after that, along with better readings from the other gauges as well.

Awesome, thanks man! I'll verify my IR gun today. If the gun is reading right and the gauge is showing 275 but the gun is showing 170 at the pan and transmission case, could the temp sending unit being bad cause the transmission to shift early like it seems to be doing?

 

[Swamp Donkey]

So I'm not really a transmission guy...which is why I bought the fancy book. Gotta make up for my shortcomings somehow.

A few suggestions though...

When the Trans is overheating, stop on a slight incline and make sure the truck rolls freely in neutral. Maybe there is a bearing going out somewhere or a brake dragging that's causing the feel of "loss of power" and the early shifting.

Make sure the fluid level is correct using the procedure in the TM. Given the amount of fluid used, it can be hard to get the right level on the first try. Make sure the truck is completely level and check it multiple times.

Take your temp gun and check brakes, T-case, differentials and other things when the Trans is hot. See if anything else is abnormally high that might be dragging and causing extra load

I wouldn't worry to much about fluid type right now. Plenty of people run either type without issue even though it's hotly debated. Mine has always had 15w40 in it and I've never had heat problems. Fluid level is far more important.

Trans temp will go up much faster than engine temperature if something is dragging and causing extra load. Maybe the engine temp hasn't had the time to rise before you stop?

Might be a Trans issue, but I can think of several things that would cause your symptoms. I also tend to look at the Trans last though because I'm not a Trans guy, lol.

 

[purple_duckk]

So I'm not really a transmission guy...which is why I bought the fancy book. Gotta make up for my shortcomings somehow.

A few suggestions though...

When the Trans is overheating, stop on a slight incline and make sure the truck rolls freely in neutral. Maybe there is a bearing going out somewhere or a brake dragging that's causing the feel of "loss of power" and the early shifting.

Make sure the fluid level is correct using the procedure in the TM. Given the amount of fluid used, it can be hard to get the right level on the first try. Make sure the truck is completely level and check it multiple times.

Take your temp gun and check brakes, T-case, differentials and other things when the Trans is hot. See if anything else is abnormally high that might be dragging and causing extra load

I wouldn't worry to much about fluid type right now. Plenty of people run either type without issue even though it's hotly debated. Mine has always had 15w40 in it and I've never had heat problems. Fluid level is far more important.

Trans temp will go up much faster than engine temperature if something is dragging and causing extra load. Maybe the engine temp hasn't had the time to rise before you stop?

Might be a Trans issue, but I can think of several things that would cause your symptoms. I also tend to look at the Trans last though because I'm not a Trans guy, lol.

I appreciate the help. I'll test a few things out today when I can break free.

 

[purple_duckk]

So I'm not really a transmission guy...which is why I bought the fancy book. Gotta make up for my shortcomings somehow.

A few suggestions though...

When the Trans is overheating, stop on a slight incline and make sure the truck rolls freely in neutral. Maybe there is a bearing going out somewhere or a brake dragging that's causing the feel of "loss of power" and the early shifting.

Make sure the fluid level is correct using the procedure in the TM. Given the amount of fluid used, it can be hard to get the right level on the first try. Make sure the truck is completely level and check it multiple times.

Take your temp gun and check brakes, T-case, differentials and other things when the Trans is hot. See if anything else is abnormally high that might be dragging and causing extra load

I wouldn't worry to much about fluid type right now. Plenty of people run either type without issue even though it's hotly debated. Mine has always had 15w40 in it and I've never had heat problems. Fluid level is far more important.

Trans temp will go up much faster than engine temperature if something is dragging and causing extra load. Maybe the engine temp hasn't had the time to rise before you stop?

Might be a Trans issue, but I can think of several things that would cause your symptoms. I also tend to look at the Trans last though because I'm not a Trans guy, lol.

Ok, so I measured the temps of everything now that I know the IR gun works.

At start up the gauge reads 150 and the pan, tranny cooler, engine block and radiator reads 100.

After driving around a bit I got the gauge to read 270 before the loss of power. I pulled over integrally and the transmission case read 170, the pan and cooler read 177 and the radiator read 165.

None of the hubs or pumpkins were hot and neither was the transfer case or the differentials.

I pulled the little wire plug off the tranny temp sensor and the gauge read 0, but I still had the loss of power and early shifting.

I let the whole truck cool down for 5 hours and it did the normal thing of being fine up until it heated up, then the tranny couldn't hold a great and I was left to 40 mph max.

Rolls freely in neutral and the oil level sure looks right both at cold and hot. It IS a little hard to see because the oil is new and clean.

 

[jonesal]

My M923, which has the same transmission as yours, shifted way late from 1 to 2, and way early on the remaining shifts. It was all fixed after I adjusted the modulator cable per the TM. The cable appeared new on the vehicle when I bought it, but I don't think it had been adjusted. If your adjustment slipped, that could possibly explain the early shifting problem. As far as power fade, perhaps you have multiple issues going on?

I hope you find your issue. I use mine around the farm too. They are fantastic vehicles for the money.

Al Jones
Brookings, SD
M923

 

[purple_duckk]

My M923, which has the same transmission as yours, shifted way late from 1 to 2, and way early on the remaining shifts. It was all fixed after I adjusted the modulator cable per the TM. The cable appeared new on the vehicle when I bought it, but I don't think it had been adjusted. If your adjustment slipped, that could possibly explain the early shifting problem. As far as power fade, perhaps you have multiple issues going on?

I hope you find your issue. I use mine around the farm too. They are fantastic vehicles for the money.

Al Jones
Brookings, SD
M923

Good thing to check out. Do you happen to have the TM chapter of the adjustment off the top of your head?

The power fade 100% feels like just because of it being in way too high of a gear.

 

[jonesal]

TM 9-2320-272-23-1, WP 0014 is related to lack of power and transmission. TM 9-2320-272-23-2, WP 0268 has some but the main adjustment is in TM 9-2320-272-23-3, WP 0365.

Good luck!

Al Jones
Brookings, SD

 

[purple_duckk]

TM 9-2320-272-23-1, WP 0014 is related to lack of power and transmission. TM 9-2320-272-23-2, WP 0268 has some but the main adjustment is in TM 9-2320-272-23-3, WP 0365.

Good luck!

Al Jones
Brookings, SD

Awesome, thanks! You saved me a bunch of time opening a few hundred disjointed pdf files I have saved. Hopefully this is it!

 

[jonesal]

Awesome, thanks! You saved me a bunch of time opening a few hundred disjointed pdf files I have saved. Hopefully this is it!

I'm paying it forward. Lot's of great folks here have helped me as well. I also used to write TM's decades ago so I like 'em, a lot!

Please report back on anything you find. I think I'm one of the lucky ones that hasn't had any trans issues... yet?

 

[HDN]

I'm paying it forward. Lot's of great folks here have helped me as well. I also used to write TM's decades ago so I like 'em, a lot!

Please report back on anything you find. I think I'm one of the lucky ones that hasn't had any trans issues... yet?

I'm following this out of curiosity. I was under the impression that the MT654 was pretty reliable for the 5-ton. Having not done research on that bit, is reliability actually an issue?

 

[jonesal]

I'm following this out of curiosity. I was under the impression that the MT654 was pretty reliable for the 5-ton. Having not done research on that bit, is reliability actually an issue?

I was just going off what others have said in other threads. Perhaps I should have worded that differently to reflect that. I did not mean to cause any worry. That being said, I suppose that only problems are posted here with everyone else that have fine transmissions don't post - if that makes sense - perhaps making the perception that there might be problems. I'll go back to my hole now...

Al Jones
Brookings, SD