CAT 955 Transmission Not Shifting Causes and Solutions

[MikePhua]

The CAT 955 Track Loader Legacy
The Caterpillar 955 track loader was a staple of mid-20th century earthmoving, combining the digging power of a dozer with the versatility of a loader. First introduced in the 1950s, the 955 evolved through multiple series—H, K, and L—each refining powertrain, hydraulics, and operator comfort. By the time the 955L was released in the late 1970s, it featured a turbocharged diesel engine, torque converter drive, and a three-speed powershift transmission.
Caterpillar sold tens of thousands of 955 units globally, with strong adoption in construction, logging, and land clearing. Even today, many 955s remain in service, especially in rural operations and restoration fleets.
Understanding the Powershift Transmission
The powershift transmission in the CAT 955 uses hydraulic pressure to engage clutch packs that control gear selection. Unlike manual gearboxes, powershift systems allow shifting under load without disengaging the drive. The transmission typically includes:

• Three forward speeds and one reverse
  
• Torque converter coupling to the engine
  
• Hydraulic clutch packs for gear engagement
  
• Control valve body with shift solenoids or mechanical linkages
  

When the transmission fails to shift, the issue often lies in hydraulic pressure loss, clutch pack wear, or control linkage misalignment.
Common Symptoms of Shift Failure
Operators may encounter several signs that the transmission is not functioning properly:

• Machine stuck in neutral or a single gear
  
• Engine revs but no movement
  
• Harsh engagement or delayed response
  
• No change in gear when shift lever is moved
  
• Audible whining or grinding from transmission case
  

In one case, a landowner in Georgia reported his 955K would not move despite engine power and hydraulic function. After inspection, the transmission oil level was low, and the filter was clogged with clutch debris.
Hydraulic Pressure and Fluid Integrity
The powershift system relies on clean, pressurized hydraulic fluid to actuate clutch packs. Key factors include:

• Fluid level: Must be checked with engine running and transmission warm
  
• Fluid type: Caterpillar TO-4 or equivalent is recommended
  
• Filter condition: Clogged filters reduce pressure and flow
  
• Pump output: Worn pumps may fail to build sufficient pressure
  

A technician in Alberta diagnosed a 955L with no forward movement. The transmission pump had worn gears, producing only 80 psi instead of the required 250–300 psi. Replacing the pump restored full function.
Clutch Pack Wear and Internal Leakage
Over time, clutch discs wear down, reducing friction and causing slippage. Internal seals may also degrade, allowing pressure to bypass the clutch circuit. Signs of clutch failure include:

• Burnt smell in transmission fluid
  
• Metal particles in filter or sump
  
• Inconsistent gear engagement
  
• Transmission overheating
  

Rebuilding clutch packs involves disassembling the transmission case, replacing friction discs, steel plates, and seals. While labor-intensive, it can extend the machine’s life by thousands of hours.
Shift Linkage and Control Valve Issues
Mechanical linkages between the shift lever and valve body can become misaligned or worn. In older machines, bushings and pins may loosen, preventing full valve movement. Hydraulic control valves may also stick due to contamination or internal wear.
Recommended steps:

• Inspect linkage for free play and full travel
  
• Clean and lubricate pivot points
  
• Remove and inspect valve body for scoring or debris
  
• Replace worn springs and seals in control spool
  

In one restoration project, a 955H had erratic shifting due to a bent shift rod. Straightening the rod and replacing the detent spring restored smooth operation.
Electrical Components in Later Models
Some late-model 955s incorporated electric solenoids to assist gear selection. These solenoids can fail due to corrosion, broken wires, or coil burnout. Diagnostic steps include:

• Checking voltage at solenoid terminals
  
• Testing coil resistance (typically 10–20 ohms)
  
• Inspecting connectors for moisture or damage
  
• Replacing faulty solenoids with OEM-rated units
  

A contractor in Arizona found that his 955L’s reverse gear was inoperative due to a failed solenoid. Replacing the unit and cleaning the harness resolved the issue.
Preventive Maintenance and Long-Term Care
To keep the CAT 955 transmission in working order:

• Change transmission fluid every 500 hours
  
• Replace filters every 250 hours
  
• Monitor fluid temperature during operation
  
• Inspect shift linkage quarterly
  
• Use clean fuel and air filters to reduce engine strain
  

For machines stored seasonally, run the transmission monthly to circulate fluid and prevent seal drying.
Operator Stories and Field Fixes
In 2015, a forestry crew in Oregon used a 955K to clear firebreaks. Mid-season, the loader stopped shifting into second gear. The crew diagnosed a clogged filter and low fluid level, fixed it on-site, and resumed work within hours.
Another story comes from a ranch in Montana, where a 955H was restored after sitting idle for 12 years. The transmission was flushed, clutch packs rebuilt, and shift linkage replaced. The machine now runs daily, pushing feed and clearing snow.
Conclusion and Recommendations
The CAT 955’s powershift transmission is robust but sensitive to fluid quality, pressure, and mechanical alignment. When shifting fails, a methodical approach to diagnosis—starting with fluid checks and ending with clutch inspection—can restore function without full replacement.
Recommendations include:

• Maintain fluid cleanliness and correct pressure
  
• Inspect and adjust shift linkage regularly
  
• Rebuild clutch packs when engagement becomes erratic
  
• Replace worn pumps and solenoids as needed
  
• Document service intervals and component replacements
  

With proper care, the CAT 955 remains a reliable workhorse, proving that even decades-old iron can still move mountains.