Was working then quit has plenty oil. The lever in the cab feels fine so I think the spring in control valve inside hydraulic tank is good.Also the drive shaft mounted to the flywheel that turns this pump is spinning with machine on. Am reading that the hoist pump sucks oil from the hydraulic tank pressurizes it then sends it to the control valve inside tank that sends the pressured oil to the hoist cylinders by lever position inside cab. Anyone no how to check hoist pump pressure? Or seen this problem?
Just to get started on this question. What is the S/N of this 769D? I know that there are at least two very different versions of a 769D with completely different set ups reguarding the hydraulics. Lets start there then the chance of getting a helpful reply would be much better. I could make some wild guesses but would prefer to know what you actually have before hand. One other question while I'm at it. You say it was working and now it doesn't. Did it raise the box one trip and the very next trip it wouldn't? Box isn't just way over loaded is it? Sometimes I've seen where they just refuse to lift if too much load or load too far towards front. In that case it sometimes helps to position truck with front end up hill to shift a little weight to the back.
Serial # 5TR957
Well I going to try attaching the test section on the hydraulic system to give you something to start with for now.
Ok I checked the screens at the hydraulic tank and the screen by the transmission cooler they all looked normal. I changed the o ring at the hydraulic filter cause it was leaking from being brittle and worn out. Also tightened all the hose clamps going to the hoist pump. Tried the lever for the dump bed and the bed raised. I did it a couple times and kept working so I will keep all undated if the issue returns. Could the pump been sucking air from the o ring or hose clamps? Thanks for the replies.
The valve is cable or pilot operated
The lever in cab pulls the spool on the control valve in the hydraulic tank. So it is pilot controlled no cable.
The control is pure mechanical, w/bellcranks & levers right from the cab to the valve in the hyd tank. No pilot control. Accumulated wear in the linkage leading to the valve not fully stroking..?
If it were the linkage then I would think it would be consistent? Is there a dump valve?
If my bed won't go up but I put a pressure gauge on the quick disconnect nipple on the lift cylinder. When I try to lift it and it doesn't go up if my pressure is below 2750 psi will that mean I have to add shims or does the pilot operated relief valve test only work if my bed is going up and down so I can test it? I really don't see what the pilot relief valve does in connection to a bed not raising?
Just to clarify here are you saying you did the test and got 2750 PSI and the box still does not raise? If you are seeing 2750 PSI and the body is not moving something is very wrong. Need a bit better description of what you are doing and what the readings are when you are testing. I believe I said back a ways that if the truck is overloaded even with a perfectly operating hydraulic system body might not move. I have personally see this happen on more than one 769 truck. That is what the relief valve is there for, to prevent something failing do to excess pressure. If the truck is overloaded and trying to dump load while backed up a slope it will not raise. Easiest thing would be to position truck with front end facing up hill and then try to dump. If that fails use a excavator to pull some of the load off the truck. I'm just using the information you are limiting me to so I can only guess the truck is loaded. For lack of any other ideas how about posting a picture or two of the truck! The relief valve controls the maximum hydraulic pressure in the system.
Oil started to run out one of the bolts that hold the pump together so I pulled out the pump. The front seal on the pump was pushed out and there was some scaring on the vanes inside pump. Thinking this is why it was not raising the bed up that easy. Will let y'all know how it works after I get this pump rebuilt. Before I install pump going to run some clean oil through it and put some white grease lubricant through the pump.
Okay so I put the rebuilt pump back on and checked the control valve inside hydraulic tank. Everything checked out good. So finally I replaced the o rings that are on the control valve inside the tank these o rings go up against the wall of the tank and have bolts that go through tank to mount the pump and hydraulic hoses on the outside. Now the bed no raise problem is fixed. I came to the conclusion these o rings were the problem glad I replaced them. Just wanted y'all to know this fix.
Good to know you have it working again, but a couple things still have me confused. You say "bolts that go through tank to mount the pump". Are you saying the pump is inside the tank? According to SIS the only thing inside the tank is the control valve and the suction screens. The in the previous post you say "scaring on the vanes inside pump". According to what my SIS shows this truck S/N has a gear type pump not a vane type pump. Or did you mean to say scoring on the pressure plates on either side of the gears? Just trying to clear up some details for anyone else who might have a similar machine and finds this thread in the future.
Yes I meant the control valve inside hydraulic tank is mounted by the bolts that go through the outside of the tank then go through the holes in the tank to mount the control valve in place inside hydraulic tank. And yes the hoist pump has vanes that you can see when you remove the hydraulic hose on top and bottom. But I had installed it repaired and the problem continued till I replaced the o rings on control valve.
Vanes are not typically visible, YMMV, is it possible you were seeing gear teeth?
What I was going by was the S/N 5TR957 that was given for the truck in question. This is what Cat SIS shows for that version of the 769D. If it has 8 1/2 inch bolts like the one circled in red above and is an aluminum housing then it is your basic Tyrone single stage gear pump. On the other hand if it looks more like the one below with cast iron housing and just four bolts from the end where the red circled one is below then it would be a vane pump. Note this is just a generic picture of a Vickers pump, the ports might not be in the same relation to each other and the input shaft on a Cat pump would in all likelihood be splined not a straight shaft with a key as in the above picture.
Yes definitely the first picture and what am calling vanes are probably the 2 rotors you can see from that top hole in the picture. Just spin the front shaft on the pump and you will be able to inspect any scaring on the 2 rotors in that hole.
What you are calling "rotors" are just a simple pair of gears. As like I said it is a gear type pump. Here is a picture of the break down of the pump and you will note that circled in red is # 3 and it is called an "Idler Gear". The other gear is referred to as a "Shaft, drive" as it is the one that is splined in the the drive assembly on the truck. One more thing as this machine has had a failed pump that would have contaminated the oil I would be taking extra care to clean the system. I'm only guessing you cleaned the inside of the tank while replacing the o-rings on the control valve and also replaced the oil with new oil? If oil was reused the best thing would be if you had access to a filter cart to clean up the oil. Either way I would also suggest running a ultra high efficiency filter, part number 132-8875, for a couple cycles. Cat suggest even on a new machine to run this filter for the first 8 hours of operation then to switch to the standard filter, part number 1R0719. Of course if the "Plugged Filter" light comes on before the 8 hours you should change it at that time. If not on an oil sample program that would also be a good idea, you might have seen this problem coming be fore failure if you did. Good luck!