Cat 120G Transmission Failure and the Realities of Rebuilding a Power Shift System

The Caterpillar 120G motor grader is a staple in road construction, mining, and site preparation. Known for its robust frame and hydraulic controls, the 120G features a power shift transmission with six forward and six reverse speeds, controlled by clutch packs and hydraulic valves. While the transmission is built for durability, years of hard use or poor maintenance can lead to internal failures that are costly and complex to repair.

In one case, a unit with serial number 87V1469 showed signs of transmission failure shortly after purchase. The machine could move forward under no load, but lost drive when the blade was engaged. In reverse, the tires spun but the machine failed to move effectively. Inspection revealed blackened transmission oil, a burnt odor, and metal and friction material debris in the filter—clear signs of clutch pack breakdown.

Initial Repairs and Ineffective Fixes

The first attempt to resolve the issue included:

• Cleaning the transmission filter
• Replacing the hydraulic oil with SAE 30 as a test fluid
• Installing a new transmission oil filter

Despite these efforts, the machine continued to lose drive under load. This indicated that the problem was deeper—likely within the clutch packs or internal oil passages—and that a full teardown was necessary.

Expert Diagnosis and Rebuild Strategy

Experienced technicians noted that black oil and burnt smell typically point to clutch pack failure. The presence of friction material in the filter confirmed this. In such cases, pressure testing is often futile. A complete transmission rebuild is the recommended course of action.

Key steps in a proper rebuild include:

• Obtaining the correct parts manual (e.g., SEBP1045) for component identification
• Disassembling the transmission and cleaning all oil passages thoroughly
• Replacing all clutch discs, piston seals, springs, and snap rings
• Inspecting and replacing bearings, thrust washers, and gears as needed
• Cleaning and testing the control valve group, replacing springs and spools
• Verifying cable linkages, shift forks, and detents for proper function
• Checking the transmission pump (e.g., 3P0891 or 3P0892) for wear or scoring

One technician shared a story about a 143H grader that failed repeatedly due to incomplete cleaning during previous rebuilds. Residual debris in the oil passages caused clutch pistons to stick, leading to recurring failures.

Post-Rebuild Issues and Further Troubleshooting

After a full rebuild, the machine still failed to move in gears 1, 2, and 3, but operated normally in 4, 5, and 6. This pointed to a low-speed clutch pack issue. Pressure readings showed 285 psi at the pump and 80 psi at the speed clutch in neutral.

Recommendations included:

• Using compressed air to test each clutch piston for seal integrity
• Inspecting the control valve for contamination or spring failure
• Measuring clutch pressure in each gear to identify leaks or low engagement
• Verifying shift fork alignment and detent operation
• Confirming that all replacement springs match OEM specifications

In one Texas repair case, a technician discovered that a non-OEM hydraulic sleeve had been installed without a proper oil passage, preventing differential lock engagement. Small details like this often cause major problems.

Best Practices for Transmission Rebuilds

When rebuilding a Cat 120G transmission:

• Replace all seals, O-rings, and gaskets with new components
• Measure spring lengths and replace any out-of-spec units
• Inspect all snap rings, thrust washers, and bearings for wear
• Clean every oil passage, especially clutch piston ports and valve bores
• Test clutch pistons with air pressure before reassembly
• Ensure cable linkages and control levers move freely and engage properly

Documenting part numbers, spring specs, and pressure readings during the rebuild helps prevent future issues and supports warranty claims.

Conclusion

The Cat 120G transmission may be mechanically straightforward, but its reliability depends on precision, cleanliness, and attention to detail. A proper rebuild restores performance and prevents costly downtime—but shortcuts and missed steps can lead to repeat failures.

In heavy equipment repair, the real challenge isn’t disassembly—it’s ensuring that every rebuilt system works flawlessly under pressure. And when the oil starts to smoke, it’s not just a symptom—it’s a warning. Listen carefully, rebuild thoroughly, and the machine will reward you with years of dependable service.