JLG 120HX drive issue

[ve6jy]

1996 JLG 120HX sn 229xx

Has been working well for several years since I acquired it second hand. A swing issue was fixed (thanks to this forum) and no issues until a few days ago. Used around my acreage, I just drove to a location, went up came down and shut it off. A few hours later went to move it and it won't drive. Nothing in forward, reverse, hi/lo creep etc. All other functions are fine. Engine revs up and seems to load a bit. Once got a bit of a jerk motion from the wheels but generally nothing. Doesn't drive from either basket or handheld. Axle lock lights green, everything looks ok. No circuit breakers popped. All wiring and boards are very clean, dry and nothing obvious burnt etc. Pouring over the manuals etc and looking for places to check but thought I'd see if anyone had a similar experience and knew some quick spots to check. Thanks for any help and I'll keep you guys posted as to any progress and test results.

Thanks, Don

 

[laycobeau]

Do you having steering control? power for steer and travel come from enable

 

[ve6jy]

Yes both front and rear steer are fine.

 

[TheFixer]

I would look at the counter balance valve that controls the brakes. If the engine is loading, than you are probably getting pressure, so I would first find the pressure test quick disconnect and put a gauge on it. You should see probably 3000psi when the drive valve is functioned first to eliminate the valves and the valve controls. If that is good than you probably have to check the pressure on the brake actuators which is controlled by the counter balance valve. You probable need to pull a line at a brake and put in a tee for that test.

 

[ve6jy]

A bit of an update..... and thanks for the help so far. I don't have the stuff to check the pressure but certainly could cobble something together. There seem to a lot of quick connect styles - what brand does the JLG use? Aeroquip?

I've determined (I feel fairly sure about this) that I get adequate drive pressure to the wheel motors - the engine starts to load down as the proportional drive controls are advanced and one can see the hoses to the drive motors move a bit. The brake solenoid gets its 13 volts correctly and one can feel/hear that it is doing something. You can also feel some motion in the hydraulic lines connected to it but one can't feel anything on the brake hose running to the wheels. I jacked up the unit and balanced it so only 2 wheels (1 rear and 1 front) are on the ground. Neither of the tires in the air turned, so I changed the balance and got the other 2 tires in the air. One of those turned when the drive control was advanced. But it jerked abruptly at the start a bit and it was like it needed a fair bit of force to overcome the brake - maybe this one was weaker than the others.

So it seems like everything points to inadequate or maybe no pressure getting to the brakes. Looking at the parts diagram such as at http://www.gcironparts.com/GCIronPa...atID=93&prodID=57106&ssID=8617&highlight=2605
looks like item 46 - shuttle valve and item 49 check valve might be the ones involved? Any suggestions as to which one might be more likely to fail? Maybe the check valve? I did try tapping them in case something was stuck, didn't help. My experience is in electronics so this is all a learning experience. Sure would be nice to have the hydraulic equivalent of a DC clamp on Ammeter in the electrical world that could just measure hydraulic flow by clipping something around the hose!

Again, thanks for any insight!
Don


120 HX brake solenoid

 

[ve6jy]

I've been a while getting around to measuring the pressure but I did today. Just past the brake solenoid (so on the brake side) I get ~4300 psi. So the brake solenoid is doing its thing fine, I'd say. This pressure comes from either of the fwd or rev main pressure lines so the same pressure must be there as well. These lines tense up and move (alternately) under forward and reverse control. Yet nothing turns. Occasionally a very slight jerk at one of the wheels - I have it jacked up so 3 of the wheels are completely off the ground. What is preventing it from moving?

As in my original post, this machine was working fine for several years and just failed.
Any suggestions very welcome!