John Deere 644G Parking Brake Comes On when The Transmission oil warms up after use for about 30mins to 60mins

[hillside876]

What is the tcu

The transmission control unit.

**FIXED-ish** My technician was able to bypass the TCU using relays and diodes; it has been working without any known issues for the past few weeks. Since he was able to get it working without the TCU, I think I'll get a replacement TCU to fix it properly.

 

[Mikepd177]

Hi, hopefully, someone else had a similar experience and can give me some guidance. I have a 1993 644G loader with about 9300hrs, The Unit works fine under light load for a longer period. Once the unit starts working and the transmission oil indicator gets to about 1/2 or a little above the warm markings the park brake light comes on and locks the machine. Once the transmission oil cools, I can use it again until the oil warms up. I'll list what I've tried below.

- Change Transmission oil and filter
- Change the Oil Cooler Filter
- Clean the oil cooler with low-pressure water
- removed the thermostat from the bypass valve

Try rebuilding the park brake pump. Had a similar problem. Chased down all the pressures and solenoids for a half a year till I took apart that pump.