Fault Phenomenon: Caterpillar 785D mining truck transmission oil level rises abnormally, and transmission oil is found in the engine oil (if using a common cooling circuit) or oil film appears in the coolant.

Troubleshooting Process:
– Checked cooler: internal baffle or tubes in plate or tube-type cooler perforated.
– Checked pressure: transmission oil pressure higher than cooling medium pressure.
– Checked oil sample: laboratory analysis confirms fluid mixing.
– Checked exterior: no external cooler leakage.

Root Cause: The internal baffle or tube bundle in the transmission oil cooler perforated due to corrosion or vibration fatigue, causing cross-contamination between transmission oil and coolant (or engine oil).

Required Parts: Replace transmission oil cooler core, replace all contaminated fluids.