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The Clark Transmission Legacy
Clark Equipment Company, founded in 1916, became a major supplier of transmissions and drivetrains for industrial and construction machinery throughout the 20th century. The Clark 28000 series transmission was widely used in heavy-duty loaders, haulers, and military vehicles during the 1970s and 1980s. Known for its rugged design and modular clutch packs, the 28000 series was built to handle high torque loads and frequent gear changes in harsh environments.
Its powershift architecture allowed for smooth directional changes and gear selection without manual clutching, making it ideal for machines operating in confined or repetitive cycle conditions. Despite its durability, the transmission’s clutch behavior can become unpredictable over time, especially in machines with high hours or inconsistent fluid maintenance.
Terminology Notes
Operators of older Clark 28000 transmissions often report symptoms such as:
Field Case: Loader in Northern Alberta
A contractor operating a 1970s-era loader with a Clark 28000 transmission noticed intermittent loss of forward drive. Reverse worked normally, and fluid levels were within spec. After removing the transmission pan and inspecting the clutch packs, the forward clutch showed signs of glazing and uneven wear. The modulating valve was also sticking due to sludge buildup. After replacing the clutch discs and cleaning the valve body, the transmission regained full functionality. “It was a classic case of hydraulic neglect,” the technician said.
Diagnostic Strategy
A snow removal crew in Maine rebuilt their Clark transmission after losing reverse drive during a blizzard. The issue was traced to a broken directional clutch spring and a clogged filter. After replacing the spring and flushing the system, the machine ran reliably for another 2,000 hours. “It was a small part with big consequences,” the operator said.
In Georgia, a forestry team added a transmission temperature gauge to their loader. They noticed spikes during long pushes and began using lower gears with shorter cycles. The transmission lasted two more seasons without issue. “Heat is the silent killer,” the foreman noted.
Parts Availability and Support
The Clark 28000 transmission remains a workhorse in vintage heavy equipment, but its clutch system demands attention. Delayed engagement, slipping, and overheating are signs of deeper hydraulic or mechanical wear. With proper diagnostics, timely rebuilds, and disciplined fluid maintenance, these transmissions can continue performing in demanding conditions. In the world of legacy drivetrains, the 28000 proves that old iron still has torque to give—if you treat it right.
Clark Equipment Company, founded in 1916, became a major supplier of transmissions and drivetrains for industrial and construction machinery throughout the 20th century. The Clark 28000 series transmission was widely used in heavy-duty loaders, haulers, and military vehicles during the 1970s and 1980s. Known for its rugged design and modular clutch packs, the 28000 series was built to handle high torque loads and frequent gear changes in harsh environments.
Its powershift architecture allowed for smooth directional changes and gear selection without manual clutching, making it ideal for machines operating in confined or repetitive cycle conditions. Despite its durability, the transmission’s clutch behavior can become unpredictable over time, especially in machines with high hours or inconsistent fluid maintenance.
Terminology Notes
- Powershift Transmission: A hydraulic clutch-based gearbox that shifts gears under load using pressurized fluid.
- Clutch Pack: A stack of friction and steel plates that engage to transmit torque in a specific gear or direction.
- Directional Clutch: The clutch pack responsible for forward or reverse motion.
- Modulating Valve: A hydraulic valve that controls clutch engagement pressure for smooth transitions.
- Torque Converter: A fluid coupling that multiplies engine torque and delivers it to the transmission input shaft.
Operators of older Clark 28000 transmissions often report symptoms such as:
- Delayed engagement when shifting into forward or reverse
- Slipping under load in specific gears
- Harsh or jerky transitions between gears
- No movement despite engine revving normally
- Transmission overheating during extended operation
Field Case: Loader in Northern Alberta
A contractor operating a 1970s-era loader with a Clark 28000 transmission noticed intermittent loss of forward drive. Reverse worked normally, and fluid levels were within spec. After removing the transmission pan and inspecting the clutch packs, the forward clutch showed signs of glazing and uneven wear. The modulating valve was also sticking due to sludge buildup. After replacing the clutch discs and cleaning the valve body, the transmission regained full functionality. “It was a classic case of hydraulic neglect,” the technician said.
Diagnostic Strategy
- Check transmission fluid level and condition (look for burnt odor or discoloration)
- Inspect suction screen and filter for metal debris or sludge
- Use pressure gauges to test clutch engagement pressure at test ports
- Monitor torque converter outlet temperature under load
- Remove modulating valve and inspect for scoring or sticking
- Test solenoids and electrical connectors for continuity and voltage
- Inspect clutch discs for thickness, warping, or delamination
- Transmission Fluid: SAE 30 or ISO 68 hydraulic oil, depending on ambient temperature
- Operating Pressure: ~250–300 psi at clutch pack ports
- Torque Converter Outlet Temp: Below 220°F during normal operation
- Filter Change Interval: Every 500 hours
- Clutch Disc Thickness: Replace if below 0.120 inches
- Modulating Valve Response Time: Less than 1 second from signal to engagement
- Change transmission fluid and filters every 500 hours
- Flush system annually or after contamination events
- Inspect clutch packs during seasonal service or rebuild intervals
- Avoid aggressive gear changes under load
- Monitor for early signs of clutch slippage or overheating
- Use OEM-spec fluids and avoid mixing brands
- Keep electrical connectors sealed and clean
A snow removal crew in Maine rebuilt their Clark transmission after losing reverse drive during a blizzard. The issue was traced to a broken directional clutch spring and a clogged filter. After replacing the spring and flushing the system, the machine ran reliably for another 2,000 hours. “It was a small part with big consequences,” the operator said.
In Georgia, a forestry team added a transmission temperature gauge to their loader. They noticed spikes during long pushes and began using lower gears with shorter cycles. The transmission lasted two more seasons without issue. “Heat is the silent killer,” the foreman noted.
Parts Availability and Support
- Clutch packs, modulating valves, and filters available through legacy Clark suppliers and aftermarket vendors
- Rebuild kits include friction discs, steel plates, seals, and springs
- Technical manuals provide pressure specs and valve diagrams
- Transmission shops can bench-test valve bodies and torque converters
- Electrical components like solenoids and connectors are interchangeable with other Clark models
The Clark 28000 transmission remains a workhorse in vintage heavy equipment, but its clutch system demands attention. Delayed engagement, slipping, and overheating are signs of deeper hydraulic or mechanical wear. With proper diagnostics, timely rebuilds, and disciplined fluid maintenance, these transmissions can continue performing in demanding conditions. In the world of legacy drivetrains, the 28000 proves that old iron still has torque to give—if you treat it right.
