Fault Phenomenon: Caterpillar D9T dozer transmission oil temperature rises very quickly, even with the cooler functioning normally.

Troubleshooting Process:
– Checked bypass valve: spool stuck in open position.
– Checked spring: spring broken.
– Checked oil quality: debris causing spool sticking.
– Measured temperature difference: no temperature difference across cooler inlet/outlet.

Root Cause: The bypass valve next to the transmission oil cooler is stuck open due to debris, allowing oil to return without passing through the cooler.

Required Parts: Transmission oil cooler bypass valve assembly.