M939 transmission problem: engine dies when I hit brakes

[ichudov]

I have an M923A2 truck that I am working on. (it sat around for a few years)

Right now, I have it on blocks so that wheels can spin freely.

It has the following problem (among some unrelated issues):

1) I can put it in gear, and accelerate. (remember, it is on blocks so the truck does not move, the wheels spin in the air)

2) However, when I hit the brakes to bring the wheels to a complete stop, however gradually, eventually, the engine DIES as the wheels stop turning. It is as if the torque converter clutch is not disengaging properly.

What could it be?

Notes:

A) in the past, it also had a problem of "dying when put in gear". (as opposed to what I described above, dying when brakes are hit). I messed a bit with the solenoid on the side of the transmission and supplied it with 24v directly. Trying this a couple of times got something unstuck and resolved the problem of dying when put in gear. As of now, this solenoid is plugged as it is supposed to. The solenoid was tested and was found to function properly.

B) The protection box on this truck is bad. I bought a replacement, but it had the effect of turning off when running, which could possibly damage some electronics. Does the transmission have any sort of an ECM that could have possibly fried?

Also, is there some way to disable the torque converter clutch? If yes, will the truck still be driveable, perhaps with reduced fuel economy? Would transmission fuel overheat?

Thanks!

 

[Jbulach]

Check to make sure the button on the end of the transfercase high low shifter is not stuck in.

 

[simp5782]

Shift to neutral when coming to a stop see if it continues doing it.

Is your abs light on?

 

[ichudov]

The ABS light IS on.

 

[simp5782]

The ABS light IS on.

Check the fuse right behind the light to make sure it's not blown. Your pcb shorting out might have fried the ABS module though. This was the reason for putting abs on the trucks was for a stall while hard braking. A malfunctioning treadle or relay valve can alsp throw the light on. I think you have a treadle valve that is sending too much air with application at any pressure on the peddle.

To test:

Rig up a gladhand with a 0 to 120 air pressure gauge. Hook it to the rear service gladhand and monitor the pressure with someone applying the pedal and different variants of force on the pedal. See if it's maxing out at 100+ psi all the time. IF IT IS rig up the gladhand to the front service side and apply air from a compressor at different pressure levels and see if the stall continues. You will not apply the brakes on your truck using the pedal. You will use the air from the compressor for that.


The transmission has no computer control on it

 

[ichudov]

Regarding questions about ABS: I will fix the ABS, and most likely, it is the ABS controller that causes ABS light, which I just purchased.

But, I thought that an automatic transmission should NEVER stall the engine, no matter what. Is that correct?

Can I somehow altogether disable the torque converter clutch, so it does not give me problems?

 

[simp5782]

Regarding questions about ABS: I will fix the ABS, and most likely, it is the ABS controller that causes ABS light, which I just purchased.

But, I thought that an automatic transmission should NEVER stall the engine, no matter what. Is that correct?

Can I somehow altogether disable the torque converter clutch, so it does not give me problems?

No. It is not an electronic lockup.

The button on your transfer case handle puts the transmission into 5th gear for ease of shifting. Does nothing with the torque converter.

M939 trucks prior to ABS would cause stalling during braking.

M939/A1/A2 ABS vs. Non-ABS Trucks

I would like to address braking with a non-ABS truck. My 923A2 truck had the ABS MWO and would not lock up the brakes in a panic stop. I was driving my 925A2 on the farm road a few weeks ago and wanted to see what happens when you lock the brakes up on a non-ABS truck so I would be more prepared...

www.steelsoldiers.com

 

[ichudov]

Can I disable the torque converter lockup?

Do something so that the torque converter is NEVER locked up?

I do not care about fuel economy.

Thanks

 

[simp5782]

Can I disable the torque converter lockup?

Do something so that the torque converter is NEVER locked up?

I do not care about fuel economy.

Thanks

Do you feel the torque converter locking up in 2nd gear when you are running it? Does it downshift out of the lockup?

Should be a 250/300rpm drop/raise with the lockup shift point.

It locks up in 2nd gear before the 3rd shift.

There is a piston inside the transmission that controls the lockup based on fluid pressure

 

[ichudov]

Let me make the clearest indication that I have a problem.

The truck is on blocks and the wheels can spin freely.

If I put it in gear WITHOUT pushing the brake, it goes in gear -- with a thud -- and the wheels spin in the air.

However, if I push on brake and then put the truck in gear, the engine encounters resistance and dies.