Got a call from an operator the other day says that the quick drop valve dosen't work on his D7, checked it out and blade will only quick drop above 1600-1700 RPM. Pulled the valve apartand checked for any garbage stuck in the valve or the orfice passages. Didn't find anything. My understanding of the valve is that it works like a regen valve on an excavator. Pressure differential inside the valve moves the spool to connect the rod and barrel ends of the cylinder so the blade falls under its own weight. Thought maybe a weak hyd pump might cause not enough oil to flow to the valve to create the required pressure drop so the spool can't shift. Blade works fine other then the quick drop issue. Anyone else run across this problem before. Thanks
Goodmanstruck, When the blade is lowered without resistance, the weight of the blade moves oil from the rod end of the cylinder, through the quick-drop valve and back to the blade lift control valve. As the oil goes through the orifice, a pressure increase is caused in the rod end of the cylinder. This pressure pushes against the face of spool valve which causes it and plunger to move against the force of the spring. Return oil from the rod end of the cylinder is then added to the flow of oil from the control valve to the head end of the cylinder. This lets the blade drop very fast. The 7H ser 2 we have will quick drop at just over an idle, did you check the free lenght of the springs while you had them out, with all the other functions working ok I would like you said (pump flow may not be enough at low rpm) to help unseat the valve. Or there could be one that is sticking just enough to not allow it to release untill a higher pressure is there, haven't done much cat work in awhile but thats the way it works if I remember correct. Were you able to check any pressures ?.
Hey sheepfoot, thanks for the reply. I didn't get a chance to check pressures yet, I got sucked into a main control valve problem on another machine, but I will check the standby pressure and main pump psi when I get sometime. I think I'm going to have to flow the pump at idle and see the output even though most pump flow specs are at ful rpm. This machine has about 9 thousand hours on it, was a rental that we purchased for use in a quarry job. I just had to rebuild the transmission, they spun out one of the pinion bearings, and shortly after that I was cleaning out the undercarriage for service and found the equalizer bar bearings hammered out.
Hey Sheepfoot, I had a chance to check out the D7 again yesterday and found that if you hold the blade tilt function over while trying to quick drop it works everytime at idle. I'm thinking that maybe it blew one of the resolvers apart, I've seen it before on backhoes and loaders. Without the resolver for that function the pump won't kick in. Only thing I can think of at the moment.
Hi there I know it's 3 years late but I bet the problem was accumulator charge pressure low. Thanks Steve
Steve, Did you ever get a resolution to your mod to the 8L hydraulic control valve for pulling towed scrapers..?
D8 l scraper mod Hey Nige Yes I did mate. We put a priority valve on the apron, when the apron tops out it ejects and it works well. Thanks Steve
Nice one. The simple solutions are always the best.