Cat 938G rough shifting transmission

[Jonathan Wurz]

If flat out in 4th speed is supposed to mean fast, it does not with this machine as it can barely get past 15 miles an hour on level ground. Normally I drive around in the 2nd and 3rd. It does not seem like ground speed has an effect on the foaming. It usually starts after driving into a pile or when I do a stall test. Basically anytime the torque converter has to work harder than just normal driving around.

 

[George M]

hello good morning, were you able to find the cause of the foam in the transmission

 

[Jonathan Wurz]

hello good morning, were you able to find the cause of the foam in the transmission

nope

I have it apart right now and just replaced everything in the torque converter, TBH I saw nothing that looked worn or damaged. I pulled the TC relief valve out of the control valve and saw it was severely damaged. I am unsure if this could be part of my problem. Maybe the cat guys could weigh in.

 

[Nige]

I pulled the TC relief valve out of the control valve and saw it was severely damaged.

Got any photos.?

 

[Jonathan Wurz]

I have a very shitty camera, but I'll try to get one where you can see the damaged part. The bottom has been worn on the sides and is no longer round or the correct size.

 

[Jonathan Wurz]

I realize you can't feel this like I can, but it feels like the end was cut into a circle with a hacksaw.
On one of the pictures I have it in a 12mm socket so you can compare it to something round.

 

[Nige]

Oh she ferked all right......
The outside looks like a chipmunk has been chewing on it.
What about all the parts around it that it interacts with.? Any signs of wear on any of them.?

 

[Jonathan Wurz]

No, this part is the only plastic part in the control valve....everything else seems undamaged

 

[Nige]

Is the worn outer diameter of the plunger where the arrow is pointing.?
Did you realize that #63 (P/N 6Y-2103) is all one piece and is not supposed to come apart.? The only thing that is separate to #63 is the O-Ring #64. If the guts of the valve are falling apart in your hand it's toast.

 

[Jonathan Wurz]

My mistake, what I have is not the TC relief valve. I can't find any information in my service manual on what it is P/N 7T-8810. Can you tell me?

 

[Nige]

My mistake, what I have is not the TC relief valve. I can't find any information in my service manual on what it is P/N 7T-8810. Can you tell me?

That is the differential valve. Along with the differential relief valve it is designed to maintain the 50psi differential pressure between P1 & P2 during a transmission shift, and additionally to prevent excessive P1 pressure immediately after a transmission shift.

I think you may have found something. Excessive speed clutch pressure (P1) during and/or immediately after a shift is a possible cause of rough shifting between speeds. Remember that the direction clutches use P2 pressure.

 

[Jonathan Wurz]

Would I gain anything from removing the TC relief valve, and how do I remove it?

 

[Nige]

Would I gain anything from removing the TC relief valve, and how do I remove it?

I would replace it while you are in there. It should just knock out, it's only held in by an O-Ring.

 

[Jonathan Wurz]

Got it out, thanks

 

[Nige]

Digressing. I know a long time back i suggested that you could try an SAE50 TO-4 oil in the transmission. Considering the damage you found in the differential valve then when you reassemble it all I would say put SAE30 oil back in it.

 

[Jonathan Wurz]

I was thinking SAE30 to see if there's any change and then trying the TMS TDTO if there is none. Your thoughts on the TMS now?

 

[Nige]

I was thinking SAE30 to see if there's any change and then trying the TMS TDTO if there is none. Your thoughts on the TMS now?

I agree. If changing more than one thing fixes the problem you have no idea which one of those changes actually fixed it. I would say SAE30 for now and keep the idea of the multigrade in your back pocket for later.

 

[Jonathan Wurz]

So I've been pressure-testing the gear pump and I might have something. I plugged the inlet and outlet ports and I have a tire stem in the inlet plug. When I put 60 psi into the pump I can hear air leaking in the bell housing. (The transmission is separated from the motor.) Is this normal or could the bushings in the gear pump be leaking enough to cause the foaming?

 

[Nige]

So I've been pressure-testing the gear pump and I might have something. I plugged the inlet and outlet ports and I have a tire stem in the inlet plug. When I put 60 psi into the pump I can hear air leaking in the bell housing. (The transmission is separated from the motor.) Is this normal or could the bushings in the gear pump be leaking enough to cause the foaming?

It's possible. 60psi is probably a bit much for leak-checking a suction. What happens if you put 10psi on the inlet.?

 

[Jonathan Wurz]

It takes about 2-3 minutes to go from 60 PSI to 0 PSI