TL;DR
Milky or foamy differential oil means coolant ingress, usually from a failed oil cooler core. Pull the oil, confirm the contamination source, and replace the cooler before returning to service.
What you might see
- oil sample showing coolant contamination
- milky or foamy differential oil
- drive train oil temperature above normal
- recurring differential debris alarm
Likely causes
Oil cooler core in the differential cracked or leaking coolant into the oil
Water ingress from a stream crossing or poor water drainage in the mine
Seal failure on the wheel station allowing water from tire washing operations
Incorrect oil type or mixed lubricants creating emulsification
Required tools
- Oil sample kits for each differential and wheel station
- Pressure test kit for oil cooler
- Oil drain containers
- LOTO kit
Safety first
- Drain plugs on large haul truck differentials hold a very large oil volume. Position drain containers accordingly.
- Lockout the engine start before opening any drive train component.
Procedure
- 1
Pull an oil sample from the differential and each wheel station. Send samples for spectroscopic analysis and coolant glycol test.[1]
- 2
If the glycol test is positive, drain the contaminated oil immediately. Do not continue operating with coolant in the drive train oil.
- 3
Remove the differential oil cooler and pressure-test it externally with compressed air in water. Bubbles confirm a leak.[1]
- 4
Inspect the differential housing interior for rust or water staining indicating the cooler has been leaking for some time.
- 5
Replace the oil cooler if a leak is confirmed. Flush the differential housing with clean oil before refilling.
- 6
Check each wheel station seal for water marks or seal lip damage.
- 7
Refill with the correct specified oil grade and volume. Run the machine for one hour under light load and pull a follow-up sample.
Sources
Caterpillar Caterpillar 793F / 797F Haul Truck general technical documentation, Caterpillar
Caterpillar haul truck service manual, drive train oil sampling and cooler inspection procedures (general)
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