Troubleshooting the Sundstrand M46 Tandem Hydraulic Pump in Bobcat Skid Steers: Pressure Loss, Valve Plate Wear, and Repair Strategies

[MikePhua]

Introduction: When One Side Loses Its Push
Hydraulic systems are the lifeblood of skid steers, and the Sundstrand M46 tandem pump is a critical component in many older Bobcat models. When one side of the machine loses forward drive pressure, it’s not just inconvenient—it’s a sign of internal imbalance that can compromise performance and safety. This article explores a real-world case of asymmetric pressure loss, explains the mechanics of the M46 pump, and offers diagnostic and repair strategies for technicians and operators.
Symptoms of Pressure Imbalance

• Forward pressure on the right side drops to 1,800 psi.
  
• Reverse pressure remains strong at 4,000 psi.
  
• Hydraulic lines on the right side are capped off.
  
• Relief valves have been swapped with no change in behavior.
  

This pattern suggests a directional issue isolated to one half of the tandem pump, likely internal rather than external.
Understanding the Sundstrand M46 Tandem Pump
The M46 is a variable displacement axial piston pump commonly used in hydrostatic drive systems. In a tandem configuration, it powers both sides of the skid steer independently, allowing for precise control of forward and reverse motion.
Key components include:

• Valve Plate: Controls fluid flow direction; wear or damage here can cause pressure loss.
  
• Swash Plate: Adjusts piston stroke length, controlling displacement.
  
• Piston Barrel: Houses rotating pistons that pressurize fluid.
  
• Relief Valves: Protect the system from overpressure; not the root cause in this case.
  

Likely Cause: Damaged Valve Plate
A damaged or worn valve plate can cause uneven pressure between forward and reverse directions. The valve plate interfaces with the rotating piston barrel and directs fluid flow. If scoring, warping, or erosion occurs, fluid may bypass or leak internally, reducing effective pressure.
Diagnostic Steps

1. Pressure Testing
   • Compare forward and reverse pressures on both sides.
     
   • Use a calibrated gauge to confirm readings.
     
2. Relief Valve Inspection
   • Swap valves between sides to rule out valve failure.
     
   • Confirm spring tension and seat integrity.
     
3. Visual Inspection of Valve Plate
   • Disassemble the pump and inspect the valve plate surface.
     
   • Look for scoring, pitting, or uneven wear.
     
4. Swash Plate Angle Check
   

• Ensure the swash plate moves freely and symmetrically.
  
• Binding or misalignment can affect displacement.
  

Repair Recommendations

• Replace the valve plate if wear is evident.
  
• Clean and lap mating surfaces to restore sealing.
  
• Reassemble with new seals and torque to spec.
  
• Flush the hydraulic system to remove debris.
  
• Refill with manufacturer-recommended hydraulic fluid.
  

Field Anecdote: Milwaukee Mechanic’s Insight
A seasoned technician from Milwaukee recalled a similar issue on a Bobcat 743. After chasing relief valve problems for days, he finally tore into the pump and found a valve plate with deep scoring from contaminated fluid. A replacement plate restored full pressure, and the machine returned to service within hours.
Preventive Measures

• Change hydraulic fluid every 500 hours or annually.
  
• Use high-quality filters and inspect them regularly.
  
• Avoid operating in dusty or muddy conditions without proper shielding.
  
• Monitor pressure readings during routine maintenance.
  

Conclusion: Precision Matters in Tandem Systems
The Sundstrand M46 tandem pump is a robust unit, but even small internal flaws can cause major performance issues. When directional pressure loss occurs, especially asymmetrically, the valve plate is a prime suspect. With careful diagnostics and methodical repair, operators can restore full functionality and extend the life of their Bobcat skid steer. And as always, the best repairs begin with understanding—not just parts, but the stories they tell under pressure.