Cat 938G rough shifting transmission

Jonathan Wurz · Oct 23, 2021

Shifting from F to R it will jerk forward before moving in reverse and vice versa, when shifting F-N-F it will jerk forward, pause then move forward. If I leave it in neutral for 5-10sec then change direction, it will shift without the jerk. Shifting between gears is very rough except for the 4th gear which is very smooth. It gets worse as transmission oil gets hotter.
While the oil is cold, shifting is perfect. Even high speed directional changes are very smooth. I do have some more info
but but I'm not sure what's relevant and what isn't. 1999 4YS01211

Nige · Oct 24, 2021

Some basics first.
What type & grade of oil are you using in the transmission.?
Pull the transmission dipstick. Is the oil dark & cloudy or clear & bright.?
When was the oil and filter last changed.?
Has an old filter ever been cut open to look for particles in it.?
Has the magnetic screen on the transmission suction (illustration below) ever been removed, inspected, & cleaned.?

Jonathan Wurz · Oct 25, 2021

A Cat mechanic told me it might be a broken spring in the clutch pack, so I took it apart and replaced all the springs, (there were no broken ones) clutch plates, friction plates, (some were badly worn and a little warped, likely from rough engaging or incomplete disingaging??) and seals. Then I cleaned the magnets in the suction filter (there were particles from the worn friction plates) and replaced the screen. I reassembled everything and and changed the oil and filter. The oil is Cat sae30 transmission oil and it was not dark & cloudy before oil change. I have never opened a filter to check for particles. I replaced 3 solenoid shifting valves and switched them around, but I didn't notice any difference.

Nige · Oct 25, 2021

Was the shifting issue you describe in your OP present before you tore into the transmission.?
If yes, is it the same as it was before you went into the tranny, better, or worse.?
Did you replace all the transmission clutch piston seals.?

While you were in the transmission as you mention above did you go into the control valve at all.?

Jonathan Wurz · Oct 25, 2021

Shifting issue was present before I tore it apart, it didn't get better or worse.
I replaced all the piston seals.
I did take apart one part of the control valve. It was a valve of some kind and I have no clue how it works. I made sure it was clean, there were some o-rings that seemed to be in good shape.

Nige · Oct 25, 2021

I think you may have to go into the control valve. There is a screen inside that valve and an orifice inside the dump valve which is part of the control valve.
There is also a screen inside the flow control valve which is mounted in the transmission case.
Any small piece of debris (like the ones you found in the magnetic screen) could be rattling around in a screen and impeding the oil flow.

I'll attach some parts illustrations marked up to show the screens, etc.
Also the disassenmbly & assembly procedure for the control valve. If you don't feel comfortable pulling the complete control valve apart then I suggest that you hand the job off to someone who is. One other thing is that every O-Ring, seal, or gasket that you touched to get into the control valve, or inside the valve itself, will need replacing whether they look to be in good order or not. By now they are probably hard.

I think if it was me I would look at the screen in the dump valve first.

Cmark · Oct 25, 2021

Have you confirmed what the transmission oil temperature is reading?
More than once I have had harsh shifting caused by the temperature sensor giving erroneous readings. In a nutshell, it fools the ECM into thinking that the oil is at a different viscosity than it actually is.

I'm not sure if you can check this through the monitor or if you need to connect ET. I don't really have the time to look into it right now. Nige always seems to have plenty of time on his hands though

Jonathan Wurz · Oct 26, 2021

Nige said:
I think if it was me I would look at the screen in the dump valve first.

Taking apart the dump valve now. Thanks for all the help.

Cmark said:
I'm not sure if you can check this through the monitor or if you need to connect ET.

I'm not sure what ET is, and my loader does not have a monitor. I will find a way to check this if I don't find anything in the control valve. Thanks

Nige · Oct 26, 2021

Chris, no transmission temperature input to the autoshift control. See below. This is quite an old machine (1999) with minimal electronics on it. The converter temp sender signal goes straight to the TC temp gauge and the warning light panel.

Jonathan Wurz · Oct 26, 2021

I removed the dump valve and the differential valve, disassembled them, and cleaned everything, including the screen. I now have perfect directional shifting, but gear changes under a load are still very rough.

Nige said:
Chris, no transmission temperature input to the autoshift control. See below. This is quite an old machine (1999) with minimal electronics on it. The converter temp sender signal goes straight to the TC temp gauge and the warning light panel.

Thanks.

Nige · Oct 26, 2021

It sounds as though you are heading on the right track. Did you find anything in particular as regards particles.?
Have you had a look in the flow control valve in the transmission case yet.? It's in the Transmission Case & Parts attachment a couple of posts up.

Do you have any means of checking transmission pressures.?

Jonathan Wurz · Oct 26, 2021

I didn't see any particles, but I cleaned the screen plug with 30psi compressed air. There might have been some particles that I missed.
I was able to get to dump valve without removing the cab, but not the flow control. Not sure when I'll have time to remove the cab, could be next week or next month.
Might be able to find the time and the tools for checking pressures, do you have the spec sheets handy?

Nige · Oct 27, 2021

Attached the test procedure along with the specifications.

Jonathan Wurz · Oct 27, 2021

Nige said:
Attached the test procedure along with the specifications.

Thanks, I checked the pressures, P2-300, P1-350, P3-140,

Nige · Oct 27, 2021

Those pressures aren't a million miles away from spec, depends how accurate the gauge is I guess.
I'd still have a question mark over the flow control valve in the transmission case. You can have on-spec pressure without having the correct flow.
This may involve removal of the control valve assembly and complete cleanout of the whole thing. I realize that requires removal of the cab and is not the easiest thing in the world to do, but your experience so far of some improvement in tranmission clutch engagement with only limited cleaning suggests it's the right way to go.

Jonathan Wurz · Oct 27, 2021

I'm planning on doing that next week, but I might have to wait for the parts. I decided to just skip the guessing and just replace all the spools, screens, o-rings, etc.
I'll get back to you when I know more, but until then maybe you could take a look at this post 938G torque converter pressure. Thanks.

Nige · Oct 28, 2021

Jonathan Wurz said:
I decided to just skip the guessing and just replace all the spools, screens, o-rings, etc.

Normally spools would only suffer from wear as a result of high operating hours or damage caused by metal particles from a failure somewhere else passing through the control valve.
If you have a spool that is so badly scored/damaged to make it unusable there is a good chance that the bore in the valve body will be damaged making it also scrap.
Agree with replacing screens, O-Rings, orifice, maybe also springs, but that would depend on the total hours. How many hours is on the machine.?

Jonathan Wurz said:
I'll get back to you when I know more, but until then maybe you could take a look at this post 938G torque converter pressure. Thanks.

You never posted a link to the other thread. However it might be a good idea to check torque converter outlet pressure in addition to what you checked already.

Jonathan Wurz · Oct 28, 2021

The machine has 12600 hour, If you think the spools are in good shape I could save over $1000
Here's the link to the other thread.https://www.heavyequipmentforums.com/threads/938g-torque-converter-pressure.91698/

Jonathan Wurz · Nov 25, 2021

Finally found time to take it apart and put the new parts in. I am very disappointed in the shifting, and I don't think that it has improved . Before taking it apart I noticed that the oil in the tranny sight glass was aerated, so I thought I was loosing power through the torque converter because of the air in the oil. I replaced the seals in the suction line (1P-3707,5F-8000) and it seems like I have more power, but the oil in the sight glass is still aerated even if I only run the engine for 2 min. Is there any other way for air to enter the system, and can the aerated oil cause rough shifting? Thanks for any help you can offer.

Nige · Nov 28, 2021

Jonathan Wurz said:
can the aerated oil cause rough shifting?

In my opinion yes. Having replaced the O-Rings at both ends of the suction line with no result another possibility is the transmission pump itself sucking air via the O-Ring (#15) in the pump body. After that I think you are down to something metal cracked or split allowing air into the system. TBH at 12k hours the pump is probably getting pretty tired by now.

Regarding the oil is there any way that you could observe what happens to the oil level in the sight glass when you start the machine from cold and before the oil starts aerating.? Does it stay pretty much the same or does it drop significantly.?

This is one of the cases where not being there leaves everyone other than yourself at a disadvantage.
Did you completely remove, disassemble, & clean every part of the transmission control valve and also did you remove & clean the flow control valve in the transmission case.?

Is the directional shifting still OK after all the recent work you did on it and just the speed changes are still rough.?

Cat 938G rough shifting transmission

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