I started this new thread as a continuation of "1988 Link-Belt LS4300 CII help - Solenoid coil source?" as that link was getting too long as we've solved previous issues as resurrection of our project continues. Since successfully replacing the original Operator Console with a new panel that is a bypass replacement of the original Computer, and using a linear actuator to replace the original throttle control motor, not to mention many other small but significant repairs and replacements of defective parts we've finally peeled enough layers off the onion to get to our last major problem before getting our 35-ton beast moving on its own once again. The motor and hydraulics are running strong with 4500-5000 psi hydraulic pressure available off the pump sections 1 & 2 test ports. But the pilot control pressure measured at the test port under the cab and just above the frame is only showing about 50 psi when it SHOULD be around 600 psi regardless of engine speed or input to the controls. When we throttle up we can get a very slight movement of the swing and travel, and the boom, arm, and bucket movements are slow motion at best. The pilot pressure as we understand it is off the pump #3 section that feeds over to the Control Valve solenoid that then feeds the Swing Brake, Cushion, and 2-Speed Travel solenoid valves. That same pressure going into the Control Valve solenoid after the filter and check valve is apparently also fed to the test port under the cab that then feeds the remote controls (Joy sticks and pedal controls). So that is where I've been measuring the steady 50 psi once the fluid warms up. We've replaced the pilot filter element and checked the check valve to ensure no blockage there. Though we brought up a test tee to measure pressure before the filter it was the wrong size compression fittings for the hose, so back to my friendly hydraulic shop for more plumbing. There is a large valve body attached to the side of the pump assembly that has three pilot lines going over to the main valve assembly and one that goes over to the pilot filter and check valve. That assembly also has the large pressure and tank return piping and hose that feed the main valve assembly. What we're having a problem with is determining why we can't get enough pilot pressure. Our testing and observation keeps pointing to that mysterious valve body on the side of the pumps, and we can't find any reference to it and its functions in our service and Operators manuals. An early morning "dream-state epiphany" hit me that perhaps the problem is NOT the pump side but perhaps a relief valve somewhere downstream that is partially open. This theory is strengthened by my observation that the pilot pressure remains steady at 50 psi regardless of engine speed. To test this I'll put a test tee between the pilot pressure line and the filter to monitor pressure there while I put a cap on the check valve side that leads over to the remote control valve block under the cab where pilot pressure is normally monitored. If pressure goes up then I'll know the problem is downstream. We DO have a local Service Manager for LBX who has been helpful in getting earlier questions resolved but he's going to be gone for the holidays and we'd sure like to get a handle on this pilot control pressure mystery. Meanwhile, we plod on! Yes, that's the pilot pressure filter housing just ahead of the check valve. Pressure then branches up to the Control Solenoid Valve (Control/Pilot oil feed) above the three lower solenoid valves (2-Speed Travel, Cushion, Swing Brake), and also down to the remote control valve block under the cab where the Pilot Pressure test port is located] .
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