Restoring Charge Pressure in the Clark HR328 Transmission on a TJ450A Skidder

[MikePhua]

The TJ450A and Clark HR328 Transmission Legacy
The Timberjack TJ450A line skidder, built in the mid-1980s, was a staple in North American forestry operations. Known for its rugged frame and reliable drivetrain, the 450A was often paired with the Clark HR328-4 transmission—a powershift unit designed for heavy-duty off-road use. Clark Equipment Company, founded in 1903, had long supplied transmissions for logging, mining, and construction equipment. The HR328 series featured a charge pump system that maintained hydraulic pressure for clutch packs and lubrication circuits.
Symptoms of Charge Pump Failure
Operators encountering charge pump issues on the HR328 typically report:

• No transmission pressure at startup, especially in cold weather
  
• ATF leakage from the regulator stop valve, which may stop once warmed
  
• Pressure gauge flickering briefly to 50–100 psi, then dropping to zero
  
• No oil flow from the regulator valve’s test port, even with the engine running
  

These symptoms suggest a failure in the pump’s ability to prime or maintain suction, often caused by air ingress, low fluid level, or internal wear.
Inspection and Troubleshooting Strategy

1. Check Regulator Valve O-Rings and Gaskets
   Replace all O-rings between the regulator valve and housing. Even minor leaks can prevent suction buildup. Inspect the safety valve and replace its O-ring if worn.
   
2. Inspect the Suction Path
   Look for collapsed hoses, cracked fittings, or loose clamps on the suction side. A restriction here can cause the pump to cavitate or fail to prime.
   
3. Clean or Replace the Filter and Screen
   Although some HR328 variants lack a sump screen, any inline filters should be cleaned or replaced. Contaminants can block flow and reduce suction.
   
4. Open the Pressure Port During Startup
   Temporarily removing the pressure port plug can help the pump prime by reducing backpressure. This technique is useful when the pump is mounted externally.
   
5. Overfill the Transmission Temporarily
   Adding extra ATF above the recommended level can help the pump prime by ensuring fluid reaches the suction inlet. Once pressure is restored, return to normal fill level.
   
6. Inspect the Pump Internals
   Disassemble the charge pump and check for worn gears, scoring, or shaft play. Even if the pump “looks good,” internal tolerances may be compromised.
   

Field Anecdotes and Practical Advice
A skidder operator in British Columbia faced repeated cold-start pressure failures. After replacing all regulator valve O-rings and overfilling the transmission, the pump began to prime—initially squealing, then stabilizing. He confirmed that the HR328 lacked a sump screen, contrary to some manuals, and emphasized the importance of sealing the regulator valve housing.
Another technician in Pennsylvania noted that pressure drop after startup often points to suction restriction. In his experience, cracked suction pipes and collapsed hoses were the most common culprits. He recommended checking for whine or cavitation sounds as early indicators.
Preventive Maintenance Recommendations

• Inspect suction hoses quarterly, especially after cold weather use
  
• Replace regulator valve O-rings annually, or during any transmission service
  
• Use high-quality ATF, and avoid mixing brands
  
• Log pressure readings during startup, and monitor for trends
  
• Keep spare O-rings and gaskets in field kits for remote repairs
  

Conclusion
Restoring charge pressure in the Clark HR328 transmission on a TJ450A skidder requires a methodical approach to sealing, suction integrity, and fluid management. While the pump itself may appear functional, minor leaks or restrictions can prevent priming. With careful inspection and strategic overfilling, operators can revive pressure and extend the life of their transmission system. The HR328 remains a durable unit when supported by proactive maintenance and field-tested troubleshooting.