Cat 938G rough shifting transmission

[Jonathan Wurz]

Have you been into the converter inlet relief valve and inspected the internals.?
The TC inlet pressure should not rise above 130psi, even with cold oil. The function of the IRV is to control that pressure.

Yes I did, I replaced the relief valve so unless I messed something up it should work fine, but the P3 pressure only goes down to spec after it is heated up to 180 degrees.

I have been trying to confirm or disprove my theory of the oil foaming from being sprayed through the relief valve at 200 psi. I was alternately driving the loader and letting it idle for 20 min every time I drive it the oil is cooled to about 110*F. After driving the the oil is aerated, but if it idles for 15 min, the oil has absolutely no bubbles. This could probably still be a suction leak that doesn't leak at low RPM. I will road the tractor to check the oil after running at high RPM with the oil at operating temperature.

Either way I think it's a bit of a red herring because the only function of the valve is to bypass the powertrain oil cooler when the oil is cold.

I assumed it would help lower the pressure when the oil is cold.

 

[Nige]

but the P3 pressure only goes down to spec after it is heated up to 180 degrees.

Something is wrong. P3 should NEVER exceed 130psi even with cold oil.

I assumed it would help lower the pressure when the oil is cold.

Not correct I'm afraid. It is simply a method of bypassing the cooler under circumstances where the oil temperature does not require it to be cooled.

 

[Nige]

If the aeration of the oil goes away when it warms up the question is whether or not the pump noise is always absent provided that there is no aeration of the oil.?

Maybe a test with a multgrade TO-4 oil might be worth a try. Cat TDTO-TMS oil is a 10W/40 TO-4. This would give you the benefit of the 10W viscosity rating with cold oil.

Potentially another option is to drain the existing SAE30 TO-4 oil and throw some SAE10W hydraulic oil in the transmission as a TEST. You don't have to engage any gears, simply start the engine with the machine cold and monitor both pump noise and converter inlet pressure at Low & High Idle. that will give you an idea how the transmission/converter should behave with a multigrade TO-4 oil in it without the expense of actually buying the oil.

 

[Jonathan Wurz]

Maybe a test with a multgrade TO-4 oil might be worth a try. Cat TDTO-TMS oil is a 10W/40 TO-4. This would give you the benefit of the 10W viscosity rating with cold oil.

So if this would work, does this indicate that everything in the control valve is working properly?

 

[Nige]

So if this would work, does this indicate that everything in the control valve is working properly?

No it does not. It's nothing more than a Band-Aid to control the converter inlet prressure within the required limits when the oil is not up to normal opperating temperature and to attempt to prevent oil aeration - if in fact it is proved that the high pressure with cold oil is actually the root cause of the aeration.

Personally I would suggest using some 10W hydraulic for a test first. See above.

Still need an answer to my question above in relation to the pump noise always being absent when there is no aeration of the oil.

 

[Jonathan Wurz]

Still need an answer to my question above in relation to the pump noise always being absent when there is no aeration of the oil.

Will check this now.

 

[Jonathan Wurz]

No, the noise does not go away after the bubbles disappear.

I also just realized that the bubbles only disappear in the sight glass. This should have been obvious. Since the tractor is sitting in one spot with nothing to move new aerated oil into the sight glass, the bubbles just float to the top in the sight glass and it appears as if the oil is no longer aerated.

It is very likely that the sound is coming from the oil being aerated, and it is always aerated after the engine is running for a few minutes. This would disprove the idea that it is being aerated from cold high pressure oil.

I guess I'm back to the suction leak theory. It has been obvious that you never thought different. For me it is the hardest thing to check, if the most obvious, so I've been trying to check the easier stuff first.

P3 pressure is around 200 PSI when the oil is 70*F. Torque converter outlet pressure is 150 at 70*F. These pressures are all normal once the oil is at operating temperature.

I have been watching this and cannot recreate this pressure, right now it is at 145PSI with cold oil. Thanks for humoring my ideas without too much hostility.

Will post back if I find anything.

 

[Nige]

I have been watching this and cannot recreate this pressure, right now it is at 145PSI with cold oil. Thanks for humoring my ideas without too much hostility.

The goold old "measure twice, cut once" mantra.
TBH it's hard to do something like this when you are not right next to the machine looking, listening, measuring, etc. Frustrating for you I'm sure but also frustrating for the people who are trying to offer you assistance.

If you go back a few posts @HATCHEQUIP made a good suggestion. Try increasing the quantity of oil in the transmission WAAAAAY above the maximum and check what happens to the pump noise. If there is a suction leak inside the case somewhere and you get the oil level above where the leak is then the noise, aeration, etc, etc. should disappear. If it does you have confirmed a suction leak, then just have to nail down the exact location........

 

[Nige]

Going back over the whole of the thread I managed to get more info regarding the operation of the converter inlet relief valve. Thanks to the person who supplied the info - you know who you are.

"The 6Y-2103 is a relief valve for oil entering the converter during cold oil conditions. If functioning properly it should not relieve any oil at operating temperatures. The specification on the 6Y-2103 relief valve is based on the higher pressure created by the lower temperature oil. The 6Y-2103 has a relief pressure of 130 psi. Once the oil reaches operating temperature then the valve is not in use. When the oil is colder it can createe a higher system pressure that can be damaging to the converter."

Interesting to note that, from the above script, provided the relief valve is working correctly the converter inlet pressure should NEVER EXCEED 130psi even when the oil is cold. It might just be an idea to double-check the accuracy of the gauge you are using.

See the attached. I recall that at some point you replaced that valve. Did you install a 6Y-2103 or the updated 148-1928.? The complication is that the 6Y-2103 Valve is NOT cancelled, therefore if you asked for that former Part Number you would not be automatically directed to the replacement P/N by a dealer parts person.

 

[HATCHEQUIP]

Cant help but think he is sucking air and thats whats foaming the oil and causing the wear on the plastic pieces from the air and pulsations + playin hell with these pumps

 

[Nige]

Cant help but think he is sucking air and thats whats foaming the oil and causing the wear on the plastic pieces from the air and pulsations + playin hell with these pumps

I think you're right. It's a PITA not being able to be there next to the machine and listen to it then try stuff to nail down whatever's going on.

 

[Nige]

Another snippet. After the DEC 1999 build date of your machine the Tube item #24 was updated from 6Y-8508 to 252-5129. This happened in APR 2006. Unfortunately there does not appear to be any documentation relating to the reasons for the change that I can see.
That doen't happen to be the pump suction tube by any chance does it.?

 

[HATCHEQUIP]

Id say you might have it , tube or oring.

 

[Jonathan Wurz]

That doen't happen to be the pump suction tube by any chance does it.?

That is the suction tube, it does have new o-rings but I didn't know that the tube has been updated.

I added a bunch of oil, and I am pretty sure there is no suction leak INSIDE the transmission. I measured the height of the oil when it was running and it is high enough to cover everything under suction. This did not test the suction tube.

 

[HATCHEQUIP]

And did the noise or anything change? And how much oil did you add?

 

[Nige]

I measured the height of the oil when it was running and it is high enough to cover everything under suction. This did not test the suction tube.

By this do you mean that the oil level was high enough to submerge the extreme top end of the Tube #24.? If it was in fact that high then you have “tested” the Tube because if it was spilt or cracked then the oil would have entered it via the crack as well as being sucked up the inside of it from the magnetic screen, and the pump noise should have completely disappeared.

 

[Jonathan Wurz]

And did the noise or anything change? And how much oil did you add?

No, noise is the same as far as I can tell.
8 Gallons

By this do you mean that the oil level was high enough to submerge the extreme top end of the Tube #24.? If it was in fact that high then you have “tested” the Tube because if it was spilt or cracked then the oil would have entered it via the crack as well as being sucked up the inside of it from the magnetic screen, and the pump noise should have completely disappeared.

No, this would be impossible since the tube is outside the trans case. I meant to say everything inside the transmission that is under suction. I was just going to replace the tube until I saw the price. ($897) I might still but I'll try to take it of and pressure test it first. I think I did that at some point already but I can't be sure.

 

[Nige]

Sorry I got the impression that the tube was internal to the transmission, not external. A hand sprayer with soap solution should indicate if it is sucking air. Alternatively remove it, blank both ends, then pressure test to maybe 10psi.