JLG 1250 AJP Drive Troubleshooting

[Ricardo Catarino]

Hi everyone.

I'm currently working on a JLG 1250AJP that is giving me a huge headache.
Numerous problems with boom control, CANBUS faults, Overload sensors and oil leaks that are all sorted out.
But there's a drive problem that we haven't been able to solve yet.
The machine uses a variable displacement, Sauer H1 tandem double pumps to feed the drive motors. One pump feeds left side and the other the right side.
As soon as you throttle the engine to 1800 rpm and release the brakes the wheels on the left side start to spin forward, even with prop. coils c1 and c2 disconnected from pump control. Tried to null, center the pump both mechanical and at controls to no avail.
I removed the pumps and charge pump to be checked at an expert workshop that reported a badly worn pump that needed replacement so I ordered a new one that cost 6000€.
The pump arrived yesterday so I properly installed, filled and bled the pumps. Gave a couple cranks without letting the engine fire, too fill, bleed the hydraulic system and get charge at nominal pressure.
Fired the engine and tested the thing.

Drive still going forward on brake release without command...

Couple of notes:
Machine is jacked of the ground.
Most valves on traction manifold replaced by another working unit, including hot oil shuttle, relief, flow/divider, check valves.
Left motors heating up pretty fast due to high pressure running on braked wheels.

Already sunk a lot of money on this. Any help would be greatly appreciated.
Thanks in advance

Best Regards

 

[BigWrench55]

Have you swapped the directional coils from one pump to the other to see if the problem moves to the other side?

 

[Ricardo Catarino]

Did it before taking the pumps off to be checked by an hydraulic system expert house.
Anyway, all 4 coils are disconnected right now and when breaks are released the left side immediately drives foward. Pumps are brand new.

With coils connected, I did the drive and steer calibration and JLG analyser giving the "everything ok".

Thanks for the help brother.

 

[BigWrench55]

I understand. But you had the problem before you installed the new pump and have the same problem with the new pump. If your null is set correctly and you had eliminated electrical interference. Then I would think that the directional valve is leaking by.

 

[Ricardo Catarino]

Updating for future troubleshooting.

The problem was in the hydraulic swivel. Internal gaskets were blown causing two ports to leak between them. Pressure to steering comes from another pump and was leaking to the drive circuit.

It was blown between ports 2 and 3 for diagram reference.
Port 2: steer pressure
Port3: drive, left side foward

Thanks.