Diagnosing Low Charge Pressure in the CAT 140G AWD System

[MikePhua]

Persistent low charge pressure in the all-wheel-drive system of the Caterpillar 140G motor grader often stems from internal bypassing, failed pump couplings, or malfunctioning relief valves. Accurate diagnosis requires isolating each charge pump, verifying flow, and inspecting the relief and cooler bypass circuits.
Caterpillar 140G Background and AWD System Overview
The CAT 140G motor grader, introduced in the 1970s and produced through the 1990s, was a cornerstone of Caterpillar’s road-building lineup. Known for its mechanical reliability and operator-friendly layout, the 140G was widely adopted by municipalities, contractors, and mining operations. Later variants included an optional all-wheel-drive (AWD) system, designed to improve traction and grading performance in soft or uneven terrain.
The AWD system uses a pair of charge pumps piggybacked on the main hydraulic pump. These pumps supply pressurized oil to the AWD motors and associated valves. The system is designed to maintain approximately 450 psi with AWD disengaged and 350 psi when engaged at idle. A drop below these thresholds indicates a fault in the charge circuit.
Terminology and Component Overview

• Charge Pump: A low-pressure hydraulic pump that supplies oil to the drive motors and maintains system pressure.
  
• Relief Valve: A pressure-regulating valve that opens when system pressure exceeds a set limit, protecting components from overpressure.
  
• Cooler Bypass Valve: A secondary relief valve that allows oil to bypass the cooler if pressure exceeds 60 psi, often masking upstream faults.
  
• Case Drain: A return path for internal leakage from hydraulic motors or pumps, used to monitor bypass flow.
  
• Dead-Heading: Blocking a pump’s output to test its ability to build pressure without flow.
  

Symptoms and Initial Observations
In one case, a 140G AWD system failed to build more than 60 psi, regardless of relief valve adjustment. The operator bypassed downstream valves and routed flow directly from the charge pumps to the filter and relief valve, yet pressure remained low. This suggested a fault upstream of the relief valve or internal bypassing.
Diagnostic Strategy and Testing Procedures

• Inspect the relief valve for internal damage. A weak or broken poppet spring can cause premature opening. Disassemble and test spring tension manually.
  
• Verify cooler bypass function. If the main relief fails, the system may default to the 60 psi cooler bypass valve, limiting pressure regardless of pump output.
  
• Isolate each charge pump. Disconnect the tee where the two pumps merge. Cap one line and run the other into a bucket. Crank the engine and observe flow. Repeat for the second pump.
  
• Dead-head the system briefly by capping the outlet from the filter. Crank the engine without starting and monitor pressure rise. If no pressure builds, one pump may be backfeeding through a failed coupling or internal leak.
  
• Check the drive coupling between the main pump and the piggybacked charge pump. A sheared or stripped coupling can cause one pump to spin freely without generating flow.
  

Field Anecdotes and Practical Advice
A technician in Alberta traced a similar issue to a failed input shaft in the AWD transfer gear group. The shaft had worn splines, allowing intermittent drive to the charge pump. After replacing the shaft and reassembling the group with proper bearing preload, charge pressure returned to normal.
Another operator discovered that one of the charge pumps was spinning backward due to a misassembled gear train. This caused it to act as a drain path for the other pump, effectively nullifying pressure.
Preventive Measures and Recommendations

• Replace relief valve springs every 4,000 hours or during major hydraulic service.
  
• Monitor case drain flow from each motor and pump. Excessive flow indicates internal leakage.
  
• Use a flow meter to measure pump output at specified pressures.
  
• Keep a schematic on hand for tracing complex tee and bypass circuits.
  
• Log pressure readings during seasonal inspections to detect gradual degradation.
  

Conclusion
Low charge pressure in the CAT 140G AWD system is rarely caused by a single component. Instead, it often results from a combination of relief valve malfunction, internal leakage, or mechanical failure in the pump drive. By isolating each pump, verifying relief function, and inspecting the cooler bypass, technicians can restore full AWD performance and extend the life of this classic grader.