I've recently purchased a JLG 450 AJ 4x4 boom lift. It is a 1998 model with approx 1400 hours. My issue is the braking system. When the machine is cold or should I say when the hydraulic oil is cold the machine will drive in all drive functions as expected with the exception of trying to load onto a trailer 30 % grade. The front/steer wheels drive up the ramps with little effort but as soon as the back wheels touch the ramps the brakes come on. I've put a test light on the brake solinoid and tested at the moment that brakes engage and find that the solinoid has not been deactivated. So I assume that it is caused by a drop in hydraulic pressure allowing the spring brakes to engage. I've now placed a inline pressure gauge on the pressure line to the brake solinoid and it reads roughly 300 psi when cold and 100 or less when warm. I should also state that when the hydraulic oil gets warm/hot 120 degrees or so the brakes are ramdomly applied when moving the machine at any speed and when the ground gets soft or unlevel the machine becomes very difficult to move due to constant brake engagement. Question 1, I believe the brake solinoid is fed from the charge port on the servo pump, should there be more pressure than what I have? How much pressure should be on the brake solinoid? cold? warm? Question 2, Is this pressure adjustable? I hope I've provided enough information to get the ball rolling and can't wait to get some advice on what to do next. Thanks
Brake pressure does come from drive charge pump. With oil at operating temp you should have 400 psi on charge pump circuit with engine running and machine neutral. There is a pressure regulating valve on the charge pump circuit but I'm not ready to say that it's your problem at this point yet. If you're showing 300 psi on charge pump with oil cold, machine neutral, and only show 100 psi on charge pump with oil warm, machine neutral, I'd be tempted to send the pump to a pump shop and have it checked out. Reason being we have to know for certain pump is operating properly. If it is, it's going to be fun finding your gremlin. If you go to the JLG website you can download manuals for the machine. In the service manual, last two pages, are the diagrams for 4wd drive circuit.
Thank you for the the info, I called my JLG dealer and he said that he didn't know how much pressure i should have but I could look it up in the manuals listed online . I tried to explain that info isn't in the manuals. Not much help at all. I hate to think I have to remove this pump and send it off considering the machine has what I think is low hours. Does anybody know who would do a good job with this pump? I'll try to find the tag this weekend and start the search. Any Ideas how much it would cost to have it rebuilt? Thanks again for the info WILLIE59
You're welcome marbil222. Download your manuals. The diagrams I referred to, last two pages in service manual download, specifies 400 psi for charge pump circuit. There should be a tag on the pump that has the numbers you'll need to get info on pump, likely it's a Sauer Sunstrand pump, but can't say for certain. If you can find tag info on pump, call Ron at Fluid Power International and see if he has any ideas about low charge circuit pressure. Also might give you some idea what it will cost to test pump as well as repair if needed. http://www.fpihydraulics.com/ One thing you should do is remove the element from the charge pump filter and rinse it in a clean pan with varsol or naptha and see what degree of metal fragments come from filter media, gives you an idea if charge pump is coming apart inside.
Update I just wanted to thank you for your help on my braking trouble. We sent the pump out to Nova hydraulics in Indianapolis IN. They did a rebuild on the pump and were back in business. Thanks again for your help!
Sounds awesome marbil222. Thanks for posting update.