Cat 120H Transmission

Juan Manuel Terra · 2025-08-14T20:57:36.000Z

I’m working on a 120H, serial number P5FM04921. Initially, the motor grader had no power in reverse. It also had another issue: when engaging reverse in any gear and pressing the clutch, the machine would move forward. We decided to disassemble the transmission and found that the last reverse clutch disc (Clutch 3) was out of the splines, slightly bent and worn. The piston was also quite worn. We decided to replace all the discs in that clutch pack and the piston. After reassembly, reverse regained power, but the issue persisted — pressing the clutch while in reverse still caused the grader to move forward. We ran some tests, and after a while, forward gears 2, 3, and 6 stopped working. We disassembled the transmission again and this time found the same issue, but in Clutch 2. We performed the following tests: Swapped solenoid valve 2 with valve 4, and valve 3 with valve 4 — no change. Disconnected the solenoid valve for Clutch 3 (A). In reverse, pressing the clutch no longer made the machine move forward, which led me to suspect a possible leak from that valve to Clutch 2. While in reverse and experiencing the issue (moving forward when pressing the clutch), I measured pressures on Clutches 1 and 2, but did not detect any pressure. Also, I have tested the electrical signals and they are OK. Any ideas on what could be causing this?

Started by Juan Manuel TerraAug 14, 202546 replies

Omar SimentalJunior MemberSep 27, 2026#41
I checked the screen orifice and looked good I'm going to do more test and check if there are any internal leaks in the housing and if also take apart the control valve again
NigeLegendSep 27, 2026#42
Thanks for that. I'll look at a breakdown of the control valve. FWIW those plastic screened orifices I just recommend replacing them, especially on a high hour machine. I know you have swapped two complete solenoid valves with no result but have you confirmed the presence of 24v power at the solenoid valve for #2 clutch when the appropriate transmission speed (2F, 3F, or 6F) is selected.?
Omar SimentalJunior MemberSep 27, 2026#43
Ok I'll replace the plastic screens and I'm going to check again voltage at the solenoid valve I really just checked that they were getting voltage but not how much I'll do that tomorrow and let you know thanks
NigeLegendSep 28, 2026#44
I would suggest to check the voltage to the solenoid valve before you start pulling the control valve apart again. You will either need the rear wheels removed or have it up on blocks because to test it correctly the engine will need to be running.
Omar SimentalJunior MemberSep 30, 2026#45
Sorry for replying late, just checked the voltage it's 23 24v and no pressure at port C for clutch 2 got a second control valve from an other machine going to do some test maybe tomorrow any suggestions appreciated. Does any body have a hydraulic schematic for the transmission. And just a note that same company has around 6 machines 12h same serial number and 4 have transmission problems.
NigeLegendSep 30, 2026#46
So long as the control valve from the other machine is known to perform correctly then I think you might be on to something. If it fixes the problem then I think you should look closely at the current control valve for a crack in the body as discussed earlier. I’ll see if I can find something illustration for the transmission hydraulic circuit. If you have 6 similar machines and are going to be setting up transmissions there is a plate that is used for initially setting the individual clutch pressures. You can justify it and without it pressure adjustments are a roll of the dice. Maybe that’s why so much transmission troubles.? Maybe another thing you should be looking to invest in is a service manual.
NigeLegendSep 30, 2026#47
Nige said: I’ll see if I can find something illustration for the transmission hydraulic circuit. Click to expand... See the attachment. Nige said: there is a plate that is used for initially setting the individual clutch pressures Click to expand... The Part Number is 139-7050. Called a "cover plate".

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