I have been having an intermittent problem with my T500J towable lift that no one seems to be able to solve and wondered if anyone has come up against anything similar with these lifts. The lift will load up hydraulically, meaning the dump valve will stick closed while in operation and kill the motor. If I energize the coil with 12 volts of power it will reset the dump valve(cartridge) and allow normal operation again, for a little while at least, until the cartridge loads up again. I have replaced the dump cartridge and the coil twice and the machine will work fine for a couple of days, then it loads up again. The ground control module (with a software update) has been replaced and so has the hydraulic fluid tank, just in case air was getting into the system. The harness has been ohm-ed out and there are no shorts in the system. Anyone have any input? Here's a picture of the block assembly: The lift itself: Thanks
Does this happen while the boom is in the process of lifting or does it happen when the lift switch is released? If it happens while a function is in motion then something is blocking oil flow returning to the tank. If it happens after a function switch is released, then most likely the compensator circuit is not unloading to tank when the proportional dump valve closes. This can be caused by the compensator spool sticking, or having a bad o-ring. Also, the 0.011 orifice getting blocked will cause the same reaction. Unless someone jacked-up the main pressure relief valve, the engine shouldn't die even under full load. So it seems that the compensator circuit is suspect. In the schematic, the blue arrows point to the proportional dump / compensator circuit. "A" is the Proportional Dump Valve "B" is the Compensator (load sense) Valve "C" is the 0.011 Bleed Down Orifice. If "C" is blocked after a function is released, pressure will be maintained on the Compensator. This will not permit the compensator to release back to tank. This can cause the engine to stall because it is not permitting the oil to reach the Relief Valve "D". Putting 12 volts to the proportional valve at this time will relieve this trapped oil. This can be verified with a pressure gauge. With the engine not running, put a pressure gauge (10,000 psi to be safe) at the port in the drawing "E". Start the engine and monitor the pressure at the gauge. When the load-up occurs, note the pressure on the gauge. If the pressure is not extreme then the problem is certainly in the Proportional / Compensator Manifold. This being intermittent, it probably is some debris floating around in the Proportional Dump / Compensator circuit.