Komatsu PC40MR-1 Control Issues? Understanding the 708-3S-04531 Pilot Pump
If your Komatsu PC40MR-1 or PC45MR-1 excavator feels stiff, or if the joystick controls are unresponsive while the engine is running fine, you are likely looking at a Pilot Pump failure.
The 708-3S-04531 is a gear-type pilot pump that provides the low-pressure oil needed to actuate the main control valves. Without this "trigger" pressure, your machine won't move, no matter how much power the main pump has.
Technical Fitment Guide
This pilot pump is essential for the PC40/45 series equipped with the Komatsu 4D84E (or Yanmar equivalent) engines.
| Machine Model | Component Type | Engine |
| PC40MR-1 / PC40MRX-1 | Pilot / Gear Pump | 4D84E |
| PC45MR-1 | Pilot / Gear Pump | 4D84E |
3 Signs Your Pilot Pump (708-3S-04531) is Failing
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Heavy Joysticks: You have to pull the levers much harder than usual to get a response.
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Delayed Action: There is a noticeable "lag" between moving the joystick and the excavator bucket reacting.
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Low Pilot Pressure: If you hook up a gauge to the pilot test port and it reads below factory specs (typically around 3.5 - 4.0 MPa), the gear teeth inside the 708-3S-04531 are likely worn out.
Pro-Tip: Check the Pilot Filter First
Before you replace the pump, always check the Pilot Filter. A clogged filter can mimic a failing pump. However, if you find metallic silver flakes in the filter, it’s a confirmed sign that the pump's internal gears are grinding and it’s time for a new 708-3S-04531.
