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

[hillside876]

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

 

[Diesel Dave]

Welcome to HEF hillside .
I assume the park brake comes on with no warning at all... that can’t be good !
The 644G park brake system pressure is produced by the pilot/brake pump which is supplied with hydraulic system oil not transmission oil.
From your problem description it sounds like the park brake solenoid electrical coil is failing when it gets hot.
There are 2 (possibly 3) identical solenoid valves on the pressure reducing valve assembly. Swap the park brake coil with the boom down coil and see if the park brake works without issues.
See screenshot below.
.

 

[hillside876]

thanks, I'll try this later and give update

 

[Bigbert]

I'd rather see an issue in the electrical system. Mechanically, hydraulically there is no reason for such a phenomenon. Check wires, plugs of park brake system, transmission connectors for dirt, dust, humidity.

 

[hillside876]

Hi, I swapped the solenoids and the loader worked longer and the transmission temp went to 1/2 way, more than ever before. unfortunately, the parking brake still came on. After this, I just unplugged the parking brake solenoid and the machine worked for almost 2 hours nonstop with the temp staying in the middle and never got hot. I did notice the manifold did get extremely hot. I plan to buy a new solenoid and see if that helps, if not is it safe to use with the parking brake disconnected? I mainly work on flat ground and boom down when parked.

 

[Diesel Dave]

The transmission controller unit shouldn’t allow the transmission to go into gear without a signal from the park brake pressure switch. Did you bypass the pressure switch as well ?
NO ONE here is going to tell you it is safe to put the loader to work with a disabled park brake.

 

[hillside876]

The transmission controller unit shouldn’t allow the transmission to go into gear without a signal from the park brake pressure switch. Did you bypass the pressure switch as well ?
NO ONE here is going to tell you it is safe to put the loader to work with a disabled park brake.

no, the only thing I disconnected was the 2-wire electrical plug for the parking brake solenoid.