Troubleshooting and Restoring the Clark AT700B Transmission

[MikePhua]

Clark’s Transmission Legacy in Heavy Equipment
The Clark Equipment Company, founded in 1903, became a major supplier of transmissions and axles for construction and mining machinery throughout the 20th century. Among its most widely used products was the AT700B powershift transmission, a robust unit designed for loaders, graders, and industrial tractors. Built for high torque and continuous duty, the AT700B was favored for its simplicity, modular design, and field serviceability.
Though Clark was eventually absorbed into Dana Corporation, its transmissions remain in service across North America, Africa, and Southeast Asia. The AT700B, in particular, is still found in older loaders and custom-built machines, often requiring specialized knowledge to maintain and repair.
Terminology Notes

• Powershift Transmission: A hydraulic transmission that allows gear changes without clutching, using pressurized oil and clutch packs
  
• Torque Converter: A fluid coupling between the engine and transmission that multiplies torque and allows slippage
  
• Clutch Pack: A set of friction discs and steel plates used to engage specific gears
  
• Valve Body: A hydraulic control unit that directs oil to the clutch packs based on gear selection
  
• Modulator Valve: A pressure-regulating valve that affects shift timing and smoothness
  

Common Symptoms and Initial Diagnosis
Operators dealing with AT700B issues often report:

• No movement in forward or reverse
  
• Engine revs normally but transmission does not engage
  
• Delayed or harsh gear shifts
  
• Transmission fluid foaming or overheating
  
• Gear engagement only at high RPM
  

These symptoms typically point to hydraulic pressure loss, clutch pack wear, or valve body malfunction. In one case, a loader in Saskatchewan failed to move after startup. The issue was traced to a stuck modulator valve and low fluid level caused by a cracked suction hose.
Inspection and Pressure Testing
To diagnose the AT700B effectively:

• Check transmission fluid level and condition (should be red or amber, not milky or burnt)
  
• Inspect suction and return hoses for cracks or leaks
  
• Use a pressure gauge to test clutch pack engagement ports
  
• Remove and inspect the valve body for debris or sticking spools
  
• Verify torque converter output pressure and stall speed
  

If pressure is below spec (typically 200–250 PSI for clutch engagement), the pump or regulator may be failing. A technician in Nevada discovered that his loader’s pump shaft had sheared, causing zero pressure despite a full fluid reservoir.
Repair Strategies and Component Replacement
Common repair actions include:

• Replacing worn clutch discs and steel plates with OEM or aftermarket kits
  
• Cleaning or rebuilding the valve body with new seals and springs
  
• Installing a new torque converter if stall speed is inconsistent
  
• Replacing the transmission pump and checking drive gear alignment
  
• Flushing the system and installing a new filter to remove contaminants
  

For machines operating in dusty or high-load environments, consider installing an external cooler and magnetic drain plug to extend fluid life and catch wear particles.
Recommendations for Preventive Maintenance
To keep the AT700B running reliably:

• Change transmission fluid every 500 hours or annually
  
• Replace filters at each fluid change
  
• Inspect hoses and clamps monthly
  
• Perform pressure tests during routine service
  
• Use high-quality TO-4 fluid with anti-foaming additives
  

For machines stored in cold climates, warm up the transmission at idle before engaging gears to prevent seal damage and clutch drag.
Stories from the Field
A contractor in Georgia rebuilt his AT700B after losing reverse gear during a demolition job. Upon teardown, he found that the reverse clutch pack had delaminated due to overheating. After installing a new pack and upgrading the cooler, the transmission performed flawlessly under load.
In another case, a mining operator in Chile retrofitted his loader with a pressure monitoring system and alarm. The system alerted him to low clutch pressure before failure, allowing him to shut down and replace a leaking modulator valve without damaging the transmission.
Conclusion
The Clark AT700B transmission remains a durable and serviceable unit when maintained properly. Its mechanical simplicity and modular design make it ideal for field repairs and long-term use. Whether diagnosing pressure loss, rebuilding clutch packs, or upgrading cooling systems, a methodical approach ensures reliable performance. With attention to fluid quality, pressure integrity, and component wear, the AT700B can continue powering heavy equipment across demanding job sites.