648 gIII overheated hydraulics

Beardguy40 · October 07, 2018, 03:45:54 PM

I've read a couple different scenarios of overheated hydraulics, on jd 648 skidders. I have 2004 648gIII dual arch. I checked everything with a temperature gun, on a cold start. At 10 minutes idle time my pump was at 137-147 degrees and climbing, with continuous pressure on my grapple squeeze. I replaced the pump, and still have the same problem, but worse. The heat being generated is between the filter housing and the pump. My oil cooler is running at a cooler temp, my valve bank, steering valve, hoses, and grapple are at normal temperature. After one trip my temperature was at 220 degrees, and the hose from the top of the pump to the hydraulic housing looks like it's trying to collapse(not 100%) My question is there a screen in the filter housing that could be plugged up, or in the return side? Also, I did blow out my oil cooler, but did not flush it internally. Any advise would be greatly appreciated.

kiko · October 07, 2018, 06:36:05 PM

The small hose behind the suction hose is an oil cooler bypass. The fitting where this hose goes into the pump has a relief valve built into it. If stuck open it will not allow oil to travel through the cooler. Also pump pressure is set on the pump to 3100 psi. The main relief on the control should be set higher at 3400 psi. This is a closed center system, so if the main relief is set below pump pressure, it will open and relieve to tank creating massive heat in the pricess. When this condition exist you can hear a hydraulic hiss at the valve that may go away when activing functions.

backwoodsdave · October 07, 2018, 08:01:50 PM

To my knowledge there is no screen in the hydraulic reservoir on the suction, I believe there should be a a screen in the filler hole to keep the trash out, I've seen a few machines where a leaking piston packing in the grapple cylinder will bypass when the continuous hold is on and create a heating problem at idle and gets worse with a hard drag, sometimes you can hear hear the engine labor with the continuous hold on and and hear he engine change when you bump the open grapple button.

Beardguy40 · October 07, 2018, 08:24:08 PM

Wouldn't the Piston packing at the grapple create heat? Everything back there seem to be around 70 to 80 degrees while the pump was 130 or 14

backwoodsdave · October 07, 2018, 08:57:46 PM

Yes generally the heat is created at the leak at the cylinder packing itself, how about the control valve, any hissing, heat build up at the main relief or a particular section ? Oil cooler clean? Fan and shroud in good shape so the air is going through the cooler instead of around it ? As Kiko mentioned there is a check valve to direct the oil through the cooler, below the hyd tank at a tee if I remember correctly.

mike_belben · October 07, 2018, 09:34:05 PM

Closed center makes me think DC. Are you guys calling variable piston pumps a closed center system?

The pump is moving fluid that is not doing work. If it was an issue on the suction side i dont think it would heat fast and that youd find some evidence of frothing and probably a whine as the air cavitates in the gear housing. Dont hold me to that, just a hunch.

Is the machine weak in any way or just running hot?

Skeans1 · October 08, 2018, 06:05:46 AM

Did the pump have the air bled from it?

Beardguy40 · October 08, 2018, 09:38:15 PM

I believe I discovered the problem. This machine has a hydraulic oil cooler pump on the back of the diff pump. When I took the line off at on the return side of the oil cooler just before the return oil filter and started the machine, there was nothing. So I went to the oil cooler pump and took the line off going to the oil cooler. Started the machine. Nothing. Think I got a bad oil cooler pump. If I'm not mistaken, they did away with this set up on newer Glll and H model skidders. I could be wrong on that tho. I'll no more tomorrow when I put the new oil cooler pump on and will let ya folks know what's going on.