Help with D3 Cat

[PD Mech Steve]

Have D3 cat 79U. Transmission pressure drops as soon as throttle is jacked up. With top cover removed, oil is spraying out of small 1/4" hole in the top of the valve body at the right hand end. This is at the load spool. Any ideas anyone?

 

[Bob/Ont]

Steve does the hole have 1/8 pipe thread in it?
Later Bob

 

[PD Mech Steve]

Help with D3

Steve does the hole have 1/8 pipe thread in it?
Later Bob

No it's not threaded.

 

[PD Mech Steve]

It seems as if a valve is opening prematurely and allowing oil to dump back into the sump. However all of the springs in the valve body were replaced prior to me getting the machine including the relief valve in the manifold. I had hoped that someone assembled something wrong when putting back together but it appears that is not the case. I currently have the valve body off and apart and have found nothing out of the ordinary. Again, the machine will build pressure at idle (although only about 250 psi) but as soon as the throttle is raised the pressure drops off to zero.

 

[Nige]

If the valve has had all the springs replaced prior to you getting the machine AND the machine never ran right after replacing said springs then I'd bet a pound to a pinch of **** that the valve is incorrectly assembled.

 

[PD Mech Steve]

I had the same thoughts. However, the problem existed before the valve was disassembled. I have the book on it and am studying the operation.

 

[Bob/Ont]

Steve, have you checked the suction screen?
Later Bob

 

[lantraxco]

I have often had to repair things that had been incorrectly assembled exactly the same way by several mechanics in a row after the first person or persons unknown either guessed incorrectly or simply botched the job the first time. Pictures in service manuals have been known to be incorrect know and then. Might want to double check that any soft lines are connected properly also, funny stuff happens.

Cheers,
Lanway

 

[PD Mech Steve]

Yes I have checked the screen. Also checked and replaced the O-rings on the suction tubes. After reading the theory of operation, it appears that the oil coming from the hole is correct. This oil is leftover from the modulating valve and is returned to sump via that hole. So I am still at a loss as to why the pressure would drop when the machine is throttled up. I am wondering about a rag or something being pulled into the suction tube as the RPM's increase. ??? This transmission was supposedly overhauled about 700 Hrs ago by the local CAT dealer. Looks to me like it hasn't been outa there in quite a few years based on the amount of crap built up around it.

 

[Old Magnet]

My memory is really fuzzy on this topic but I think I recall a similar post where there was an internal pump suction leak (casting problem?) with the early units that caused this problem. Sorry I can't be of more help but maybe this will jog somebody elses memory.

 

[pp13bnos]

Its amazing how quickly, and how much stuff can build up on the belly pan.