JLG 340AJ brakes stuck on, hydraulic issue

[Toddgarage]

Working on a JLG boom lift 340 AJ. It developed this issues rather abruptly. The brakes will not release at low or mid engine speeds. If you select full engine speed, the brakes release enough to travel the machine in either direction. At low or mid engine speed the brakes drag enough to cause the tires to visibly shutter, but the drive motors can overcome the brakes enough to move. Here in Michigan it’s cold, and this problem happened outside last week while working. As the hydraulic temps warm up, the brake issue gets worse.
I’ve done a solid 2 days of diagnosis and trouble shooting. Including a very helpful phone call with a tech from JLG who was very knowledgeable.
Details:
1625 hours, Kubota diesel, H1 S. Danfoss main pump, clean hydraulic fluid.
Charge pressure from port M3 is variable with engine speed. It’s 100psi at idle, 200psi at mid rpm and 350 at full throttle. The brakes only released at 350psi.
Tested pressure at brakes, it also varies with engine speed. JLG tech says charge pressure should be 375psi all the time. Small adjustments to the charge pressure adjustment screw don’t make significant changes to the pressures.
*Removed the charge pump filter and found small green debris in it. Soft, not pipe dope.*
In the diverter valve (aluminum manifold assembly in undercarriage). I have replaced the spool o-rings for both the brakes valve and traction lock valve. Checked coils for voltage and ohms, this looks good. The oscillating axle cylinders also don’t seem to work. These are supplied hydraulic pressure from the same diverter valve block.
The JLG tech is hinting at a bad charge pump in the main hydraulic pump. The variable pump is used for the 4 wheel drive motors, which work. There are 2 additional auxiliary pumps mounted on the main pump.
Has anyone had a similar issue with their pump, brakes or hydraulic system on their boom lift. The debris in the filter… maybe a seal was blown out by excessive pressure during cold operation?

Thank you fellow mechanics!

 

[Acecadet]

I would try removing the charge pump adjustable relief and check for debris or something being broken in there.

 

[Toddgarage]

I did removed the adjustment valve. No visible damage to the assembly, both springs in good shape.
Usually 1 or 2 turns will make something happen on the gauge. This time… no luck

 

[Acecadet]

Did you try capping the line supplying the aluminum block to rule out any leakage there?

 

[Toddgarage]

Not yet. That was also a suggestion made from the JLG tech rep. His plan was to completely isolate the main pump and test pressures.
waiting on -16 plugs/caps to do so.

 

[Toddgarage]

The JLG tech rep did suggestion something similar.
His suggestion was to isolate the main pump. Keep the suction and return to tank hoses in place. Remove all the other hoses and cap/plug all removed hoses. Gauge the 3 test ports and check the numbers against the specs. Need some -16 plugs and caps and for the temps to rise.

 

[Toddgarage]

I noticed Danfoss has 4 updates or “redesigned” parts that are directly related to the charge pump on the H1P pump. Most likely I’ll send the pump to a rebuilding service and stop hoping for an easy fix.
Picture shows debris in the charge filter. Every pleat has green rubber… bummer

 

[Acecadet]

Maybe watch this video first. Danfoss uses a lot of green o rings in that pump. Might be as simple as a blown o ring. After watching these videos I know I would feel confident enough to tear into one. Just a thought.

 

[Mechanicsville804]

@2:50 of the above video I see that green/teal color used on the High Pressure Relief valve.

 

[Toddgarage]

It’s easy to spin those in and out. That’s the certainly worth trying.
Machine is outside getting ice cold. I’ve got other busted junk to fix at the moment. I’ll be back on the JLG soon! Updates asap.

 

[Toddgarage]

Finally got around to looking into this.
Both high pressure relief valves (possible combo check valves) have green o-rings but are in good shape
The charge pressure relief valve assembly was also checked. There are minor scratches along the body. The only o-ring was the cap to pump body sealing boss o-ring.
Maybe there is internal o-rings or other areas to look at?
The search continues

 

[skata]

Seems like an oring got lose somewhere, and sent thru and chewed by the pump?

 

[Toddgarage]

Lesson to be learned here for the guys and gals who spin wrenches;
This machine has taught me a few important lessons.
1: JLG has exceptional support staff on when you call them. This machine was refurbished before we got it, at that time the hour meter is reset. So the pump, motor and machine have significantly more hours than the 1600.
2: This Danfoss H1 pump has a handful of update parts. Sounds like there were some issues. Seal revisions and others.
3: follow procedures! And here is why….

I had to fully drain the hydraulic reservoir tank to remove the high pressure relief valves and charge pressure check valve. (The charge pressure was already way low and variable with pump speed which is not good).
I should have checked the procedure for bleeding the pump, pre-filling it, etc. I did what I normally do for gravity feed or suction only pumps. And I cranked the motor over without starting. Thinking that was good enough, I started the motor. Big mistake.
The pump wasn’t completely empty of air. Ran the machine and everything works besides travel. The variable displacement main pump is responsible for drive motors and charge pressure for brakes. By running the pump without the proper bleed procedure, I put the final nail in its coffin. Off to the rebuild center it goes.
There is a filter near the center of the machine that should catch any trash generated by my mistake. When that day comes, I’ll try to remember to post a follow up.
Cheers and stay safe.

 

[Mechanicsville804]

Thanks for the follow ups! it happens to the best of us, we cant know everything!