743 hydraulic issue

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Sparky87

New member
Joined
Aug 29, 2024
Messages
2
Hi all, new here. Just picked up an 86 743. It's in decent shape, needs some love. The first thing on my list is an issue with the lift arms drifting down. The hydraulic power seems to be fine although I haven't had a chance to load the bucket down yet, however when I raise the lift arms they start to drop as soon as i let off the pedal. I know it's low on hydraulic fluid and I need to change it. Other than that I don't see any obvious signs of leaks. Could that be a symptom of a valve spool needing seals? The lift cylinders don't show signs of leaking. Just hoping someone can point me in the right direction. Thanks
 

foton

Well-known member
Joined
Mar 1, 2018
Messages
1,464
I believe that usally it is the seals in the cylinders that fail vs. the control
 

Joelv38

New member
Joined
Sep 2, 2024
Messages
1
Why won't my 743 start after replacing the lift cylinder? Fluid to low? Safety switch somewhere? Ran fine when parked to replace seals, now nothing happens when I turn key!?
 

spitzair

Well-known member
Joined
Dec 17, 2009
Messages
187
Did you raise the cab at all? Could be the harness became unplugged. Not sure if the 743 is the same but on the 732 there's a double rectangular plug where the cab meets the main frame on the bottom left back corner of the cab as you're sitting in the sear and chances are that one or both might have unplugged themselves? Just a thought...
 

Jim A

Well-known member
Joined
Nov 28, 2022
Messages
121
Worn piston seal rings in boom cylinders or spool valve is worn bypassing.

You can check by raising boom and capping hydraulic ports on pressure side of cylinder ( non rod side) only leave the rod side hose on . See if it drifts down . If it does rebuild cylinders. If not control spool valve worn orings seals are bypassing .
 

ward

Well-known member
Joined
Sep 8, 2024
Messages
47
Per Catterpillar one would use a temperature gun after running the unit for around 10 minutes raising and lowering the bucket, lifting a load would be best the cylinder that is leaking is going to be warmer by about 10 to 20 degrees depending upon the bypassing in the cylinder.
 

ward

Well-known member
Joined
Sep 8, 2024
Messages
47
The tollarance on a spool valve is such that the film of oil on the spool is the high pressure seal, the O ring is just a wiper to keep most of the dust and dirt out and they should be changed when they start to leak a lot.
 
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