TL;DR
Hoist or suspension faults usually trace to a low strut nitrogen charge, hydraulic leak, or failed position sensor. Check strut height and hoist pressure before entering the load zone.
What you might see
- payload monitor showing uneven corner weights
- body raises or lowers slowly
- ride height below minimum at one corner
- hoist hydraulic pressure alarm active
Likely causes
Low nitrogen charge in a front or rear suspension strut
Hoist hydraulic cylinder seal leaking, causing slow body movement
Body-up limit switch or position sensor failure giving a false fault
Hoist control valve sticking or contaminated with hydraulic debris
Required tools
- Nitrogen service cart with regulator
- Calibrated hydraulic pressure gauge
- Body prop (safety stand rated for body weight)
- LOTO kit
Safety first
- Never work under a raised haul truck body without the manufacturer-rated body prop installed. Body weight can exceed 100 tonnes.
- High-pressure nitrogen in suspension struts. Follow the strut charging procedure exactly to avoid overpressure.
- Falling material hazard from ore residue in the truck body. Clear area before raising or lowering.
Procedure
- 1
Park the truck on level ground with the body fully lowered. Apply the parking brake and lock out the engine start.
Warning: Never work under a raised truck body without a purpose-built body prop. Body weight is several hundred tons. - 2
Read the payload management system display for individual corner weights. A corner reading zero or erratic points to a sensor or strut fault at that corner.
- 3
Measure the static strut height at each corner and compare to the specified loaded and unloaded heights in the Caterpillar documentation.[1]
- 4
If a rear strut is undercharged, connect a nitrogen service cart to the strut charge valve and recharge to the specified pressure and height.[1]
- 5
With the engine running, activate the hoist to raise the body halfway. Measure hydraulic pressure at the hoist circuit test point.
- 6
If hoist pressure is low, trace the hoist cylinder rods for oil weeping indicating a cylinder seal failure.
- 7
Check the body-up limit switch wiring for chafing or connector corrosion in the dusty mine environment.
- 8
If fault persists after strut and hydraulic checks, retrieve the stored fault codes from the machine ECM for further diagnosis.
Sources
Caterpillar Caterpillar 793F / 797F Haul Truck general technical documentation, Caterpillar
Caterpillar haul truck service and maintenance manual, suspension and hoist system procedures (general)
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