Caterpillar 953C Hydraulic Pump Failure: Diagnosis, Disassembly, and Field Repair Strategy

A newly acquired Caterpillar 953C track loader experienced a complete failure of its implement hydraulics—specifically, the loader functions ceased while the machine continued to travel normally. Pressure gauges placed on the two diagnostic ports beneath the sight glass showed zero movement, suggesting a total loss of hydraulic pressure in the implement circuit. This pointed toward a failed implement pump, but further investigation was needed to confirm.

Pump Type and Location

The 953C uses a vane-type hydraulic pump for its implement system. Vane pumps are known for smooth operation and compact design but can suffer catastrophic failure if internal components shear or seize. The implement pump (part number 149-4699) is accessed by removing the floor plate and belly pan, allowing direct access to the pump housing and suction tube bolts. This design simplifies removal but still requires careful disassembly to avoid contamination.

Initial Diagnostic Steps

Before removing the pump, technicians performed several checks:

• Spin-on hydraulic filters were cut open to inspect for debris. No significant contamination was found, though the inspection method (using a sawzall) may have obscured finer particles.
• The main pressure relief valve was removed and disassembled. It appeared clean, though minor aluminum shavings were found—likely from the outer housing during removal.
• The pump made a faint whine, similar to air purging after an oil change, suggesting partial rotation or internal damage.

These findings narrowed the fault to either a broken pump shaft, internal vane failure, or a disconnected coupling.

Relief Valve and Solenoid Considerations

The pressure relief valve, located below the oil filler, is a common failure point. If stuck open or if its O-rings are blown, it can cause a complete loss of pressure. In early 953C models, a 24V solenoid valve was used to control dual-pressure settings. However, in this case, the machine lacked the solenoid, simplifying the diagnosis.

Technicians noted that disassembling the relief valve is straightforward—marking threads and counting turns allows for accurate reassembly. Pressure settings can be verified using a gauge after reinstallation.

Pump Removal and Internal Inspection

After removing the floorboard and belly pan, the pump was extracted and disassembled. Inside the cartridge, the shaft appeared to be spinning freely—suggesting that the rotor or cam ring had failed. This type of failure is rare without accompanying noise or debris, but not impossible.

Key observations:

• No significant metal fragments in filters
• Shaft rotation without pressure generation
• Clean relief valve and manifold

These signs pointed toward a mechanical failure inside the pump—likely a sheared rotor or broken drive coupling.

Hydraulic Tank Drainage and Cleanliness

Draining the hydraulic tank is a messy but necessary step. The recommended procedure includes:

• Removing the filler cap and checking the seal
• Extracting and cleaning the strainer
• Opening the drain valve and capturing fluid in a suitable container
• Reinstalling the plug and inspecting for leaks

To improve access, technicians suggested driving the machine onto timbers or positioning it over a pit—especially when removing suction tube bolts.

Field Anecdote: Filter Pleat Inspection

A seasoned millwright shared a tip: when cutting open filters, inspect the bottom of the pleats, as debris often settles there first. Squeezing the filter in a vice can help expose hidden particles. This method has revealed early signs of pump failure in other machines, even when the outer filter appeared clean.

Final Thoughts

The Caterpillar 953C’s hydraulic failure was ultimately traced to internal pump damage—likely a broken rotor or cam ring. While the absence of debris and noise made diagnosis challenging, methodical inspection of filters, relief valves, and pump internals confirmed the fault. This case underscores the importance of understanding vane pump behavior, using proper filter inspection techniques, and approaching hydraulic failures with a structured diagnostic plan. With careful disassembly and part verification, even a sudden implement failure can be resolved efficiently in the field.