Caterpillar 931C II Transmission Loses Power in Second and Third Gear

[MikePhua]

Quick answer
A Caterpillar 931C II track loader that fails to pull in second and third gear—both forward and reverse—likely suffers from internal clutch pack wear, hydraulic pressure loss, or torque converter inefficiency. First gear remains functional at high RPMs, suggesting partial hydraulic engagement and mechanical integrity in the low-speed clutch.
Caterpillar 931C II background and transmission design
The Caterpillar 931C II was introduced in the late 1980s as a compact track loader designed for grading, excavation, and material handling. It features:

• A 3204 four-cylinder diesel engine
  
• Torque converter with powershift transmission
  
• Three forward and three reverse speeds
  
• Wet clutch packs for gear engagement
  
• Hydraulic steering clutches and brakes
  

The transmission (part number 7G7600) uses hydraulic pressure to engage clutch packs corresponding to each gear. The torque converter multiplies engine torque and allows smooth gear changes under load.
Symptoms and operational behavior
In this case, the loader:

• Engages second and third gear but barely moves at low RPMs
  
• Pulls well in first gear forward and reverse at high RPMs
  
• Shows no fault in fluid level, filters, or parking brake
  
• Has a nonfunctional left steering pedal unless fully depressed
  

These symptoms suggest that the transmission engages mechanically but lacks sufficient hydraulic pressure to fully apply the clutch packs for second and third gear.
Hydraulic pressure and clutch pack engagement
Each gear in the powershift transmission is activated by a dedicated clutch pack. Hydraulic pressure must reach a threshold—typically 250–300 PSI—to fully engage the pack and transmit torque.
Possible causes of failure:

• Worn clutch discs: Reduced friction prevents torque transfer
  
• Leaking seals or o-rings: Pressure bleeds off before full engagement
  
• Weak hydraulic pump: Insufficient flow or pressure at higher gears
  
• Blocked valve body: Debris or varnish restricts spool movement
  
• Torque converter wear: Reduced multiplication at low RPMs
  

One technician noted that torque converter machines require high RPMs to move effectively. If the converter is worn, higher gears may fail to engage under load.
Steering clutch and brake interaction
The left steering pedal activates the brake and disengages the left clutch. If it only works when fully depressed, the clutch actuator or brake linkage may be misadjusted or worn.
Recommendations:

• Inspect the clutch actuator rollers and linkage
  
• Adjust brake pedal travel and spring tension
  
• Check for hydraulic leaks at the steering clutch housing
  

Improper clutch disengagement can cause drag or uneven turning, especially in tight maneuvers.
Diagnostic strategy without pressure gauges
For owners without hydraulic gauges:

• Observe gear behavior at full throttle—if third gear barely moves, pressure is likely low
  
• Listen for engagement clicks or spool movement during gear shifts
  
• Check for fluid discoloration or burnt smell—indicates clutch slippage
  
• Inspect filter for metal debris—suggests internal wear
  

If possible, install a temporary pressure gauge at the clutch test port to confirm pressure during gear engagement.
Preventive measures and repair options

• Replace transmission fluid and filters every 500 hours
  
• Use OEM-spec hydraulic oil to maintain viscosity and pressure
  
• Rebuild clutch packs if pressure is confirmed but torque is low
  
• Clean valve body and inspect solenoids for sticking
  

Continued use in failing gears may cause further damage to clutch plates and torque converter components. Early diagnosis reduces rebuild cost and downtime.
Conclusion
Loss of pulling power in second and third gear on a Caterpillar 931C II likely stems from hydraulic pressure loss or clutch pack wear. First gear functionality at high RPMs confirms partial system integrity. With careful inspection and pressure testing, the issue can be isolated and resolved before major transmission failure occurs.