Hey guys, I've lurked here for a long time and learned a lot. I've got a question I hope someone has some ideas on as it's driving me nuts. I have an older (1998) Versalift bucket truck - an old AT&T vehicle. One morning, I went to raise the boom and there was a 5 second delay or so before it began to raise normally. I lowered the boom and the problem went away for several raise-lower cycles. Then the boom would no longer raise at all. The hydraulic system is pretty basic. There is a bank of 3 spool valves on the pedestal. They have an electric solenoid but can be operated manually too. Boom lift cylinder is standard double acting with a cylinder-mounted counterbalance valve on the "raise" circuit. The spool valve seems to be operating normally and engaging all the way. I can hear and feel the fluid (at least a lot of it) rushing through the spool valve as I engage it. I can see it trying to lift the boom but there is not enough force to do so. All of the other spool valves in the bank work perfect, and even the "lower" function of this one seems to work. I removed this spool valve from the body and it looks pristine. Not a scratch on it and it slides inside the body with a good fit. I'm at a loss for where to look next. Would a blown piston seal cause this? I would think the boom would still raise as the rod in the cylinder would become displaced as the cylinder filled with fluid? What about the counterbalance valve? Could it cause a symptom such as this? Any easy way to isolate this problem to either of these areas? (attaching a hydraulic drawing. I drew a red line around the boom lift circuit) Appreciate any ideas! John
If you have a helper that can run the controls put your ear to the cylinder and see if you can hear it bypass. If it's that bad you'll hear it. Also, does it have the safety stuff on it? In a former life I worked on a lot of Altec stuff and the outrigger safety switches were a common issue. If you have a way to test pressures you might do that too. Could be a holding valve as well, might have a listen to it. It's most likely on the cylinder near the hoses. Heck, relief valve may have trash in it as well and that's what you're hearing. I don't see a holding valve on that schematic but I'd be surprised if it didn't have something like that on it. Let us know what you find. Junkyard
Thanks Junkyard, I tried the listen trick but I can't really hear anything definitive. There is definitely a counterbalance (holding) valve. You can see it just to the left of the cylinder in the schematic. Fluid is (I think) either leaking past this valve or the piston seal. Just with I could tell which one!
On almost all electronically controlled machines the joystick does two things, upsgrokes the pump and sends power to the correct spool. Have you tried running the boom up while operating the swing or another function. There should be two microswitches on the boom controller. I'd make sure those are functioning properly.
Hi knepptune, this controller is old school... no electronics. There is a solenoid on each spool but you can operate them manually. I am confident the spool is operating all the way.
If. My bad. I'd look into the holding valve first. Piston seals typically won't just go without warning.
If the cylinder will not move at all in the up direction, it could only be the holding valve - if the valve was stuck not allowing oil to enter into the cylinder. Its designed to allow oil in, but not out until released by the down circuit. I would remove the line coming from the other side of the cylinder, then activate to raise. If oil comes spurting out of the other side of the cylinder line, you know its simply bypassing the piston seal. If no oil is coming out, you know nothing is going into the cylinder. Start cracking lines and holding vavles loose and see where you have oil.