Transmission Testing and Troubleshooting in the Terex 72-51B Loader

[MikePhua]

The Terex 72-51B and Its Mechanical Heritage
The Terex 72-51B wheel loader was part of a lineage of robust mid-sized loaders developed during the 1970s and 1980s, when Terex was still under the General Motors umbrella. Known for their rugged frames, high-capacity buckets, and straightforward mechanical systems, these loaders were widely used in quarries, logging yards, and municipal operations. The 72-51B featured a Detroit Diesel 6V-71 engine paired with a powershift transmission, delivering reliable torque and responsive shifting under heavy loads.
Terex, originally founded as a division of GM in 1933, had by the 1980s become a global name in earthmoving equipment. Though production numbers for the 72-51B were modest compared to Caterpillar or John Deere, the machine earned a loyal following for its simplicity and serviceability.
Terminology Notes

• Powershift Transmission: A hydraulic-actuated gearbox that allows gear changes without clutching, using pressurized fluid to engage clutches.
  
• Torque Converter: A fluid coupling that multiplies engine torque and transmits it to the transmission.
  
• Clutch Pack: A set of friction discs and steel plates that engage or disengage specific gears.
  
• Transmission Pressure Test: A diagnostic procedure that measures hydraulic pressure at various ports to assess clutch engagement and fluid flow.
  

Symptoms of Transmission Issues
Operators of the 72-51B may encounter:

• Delayed gear engagement or slipping under load
  
• Machine moves in reverse but not forward
  
• Transmission heats up quickly during operation
  
• Audible whining or grinding during gear shifts
  
• Loss of drive after warm-up
  

These symptoms often point to internal clutch wear, valve body malfunction, or hydraulic pressure loss. In one case, a contractor in Alberta reported his loader would only move in reverse. After testing, he discovered low pressure in the forward clutch circuit due to a leaking seal.
Conducting a Transmission Pressure Test
To diagnose transmission issues accurately, a pressure test should be performed at key ports:

• Locate the test ports for forward, reverse, and clutch circuits
  
• Warm up the machine to operating temperature
  
• Connect a hydraulic pressure gauge rated for 0–600 psi
  
• Shift through gears and record pressure readings at each port
  
• Compare results to OEM specifications (typically 250–300 psi under load)
  

If pressure is low in one circuit but normal in others, the issue is likely isolated to a clutch pack or valve spool. If all pressures are low, the problem may be the pump or torque converter.
A fleet manager in Georgia used this method to identify a worn pump shaft in his 72-51B. Replacing the shaft restored full pressure and eliminated gear hesitation.
Common Causes and Solutions
Transmission problems in the 72-51B often stem from:

• Worn Clutch Packs: Friction material degrades over time, reducing engagement strength.
  
• Valve Body Contamination: Dirt or metal shavings can block spool movement.
  
• Pump Wear: Reduced flow and pressure due to internal scoring or cavitation.
  
• Seal Failure: Hydraulic seals may leak internally, causing pressure loss.
  
• Electrical Solenoid Issues: If equipped with electric shift assist, solenoids may fail or short.
  

Solutions include:

• Rebuilding clutch packs with OEM or aftermarket kits
  
• Cleaning or replacing valve body components
  
• Installing a remanufactured pump with updated tolerances
  
• Replacing seals and gaskets throughout the transmission
  
• Upgrading wiring and connectors for solenoid reliability
  

A quarry crew in British Columbia rebuilt their 72-51B transmission using a kit sourced from a vintage parts supplier. After installation and testing, the loader returned to full duty hauling aggregate.
Preventive Maintenance and Best Practices
To extend transmission life:

• Change transmission fluid and filters every 500 hours
  
• Use high-quality hydraulic oil with anti-wear additives
  
• Inspect shift linkage and electrical connectors monthly
  
• Monitor transmission temperature during heavy use
  
• Perform pressure tests annually or during major service
  

A municipal fleet in Michigan added transmission pressure checks to their seasonal maintenance schedule, reducing breakdowns and improving loader uptime.
Operator Anecdotes and Field Wisdom
A retired operator in Montana recalled his 72-51B losing drive during snow removal. After checking fluid levels and performing a pressure test, he found a cracked valve body casting. Welding and machining the part restored function, and the loader continued working for another five years.
In Argentina, a logging crew used their 72-51B to load timber in steep terrain. They noticed sluggish forward movement and traced the issue to a clogged suction screen in the transmission sump. Cleaning the screen restored flow and improved responsiveness.
Conclusion
Transmission testing in the Terex 72-51B is a vital diagnostic step when drive issues arise. With a clear understanding of hydraulic circuits, clutch behavior, and pressure benchmarks, operators and technicians can pinpoint faults and restore performance. Though vintage, the 72-51B remains a capable machine when maintained properly—and its transmission, while simple by modern standards, rewards those who know how to read its signals and respond with precision.