Gday, I desperately need some help. Our Magnum seems to want to hold about 2000psi pressure on the pump all the time (once the oil is hot). We have disconnected the signal line to the remotes and to the front suspension, thus eliminating them but it still does it. I have had the compinsator valve off and all seems OK and also had the priority valve (under cab) off and seems to be OK too. When the oil is about 35c it will operate fine as long as you do not exeed 2000psi, as soon as this happens it stays at that pressure until you move the steering quickly to the left and right. Once the oil gets to 45c and more it stays at 2000 psi reguardless. Also at this level the only way to start the engine is to turn the steering side to side as the engine cranks otherwise there is too much pressure and it wont start. Thanks in advance .
There's a whole lot of things on a hydraulic system like that one that could cause your problem, and I know nothing about the circuits or components on an MX255. Have you put a pressure gauge on the pump output line and checked the standby pressure when it's cold and hot? If the standby pressure is normal when it's cold, but jumps to 2000 psi hot, sure sounds like something wrong with pump compensator.
I have worked on the older magnums 89 series, either the compensator isn't letting the swash plate back off, or the swash plate is getting stuck in a spot. If the compensator looks fine, prob should pull the pump off and see if something has failed inside.
If you've had the signal line off where it fits directly on to the compensator then you have effectively discounted every high pressure circuit that feeds in to it and would point to a sticking plunger in the piston pump......which in a short space of time can grind the brass slipper in to the swash-plate and destroy it all.......if there's a case drain on that pump perhaps whip the plug out and see if there's any traces of contamination
My vote is with Richard. Have heard of some Magnums going full tilt and blowing lines.