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Crawlers, Dozers, Loaders & Backhoes Discussion Forum

Cat D3 transmission losing pressure

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05-23-2020 04:18:44

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Hello all, I'm at a loss here and hoping someone has some insight. I have rebuilt this thing from the ring and pinion up. Everything that I could buy new, I did. Get everything together, and my bran new transmission is not working correctly. Upon startup, I have over 330psi pump pressure. After shifting gears a few times, the pressure gets lower and lower till eventually theres only 50 psi on the pump port. Everything in this tranny is new except the valve body, or transmission control valve. Yesterday I found a used control valve that was "a good one" from the guy, put it on, and couldnt even get 100 psi out of it. I'm at a loss without ripping this tranny out and having someone go through it. Anybody have any ideas?

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05-24-2020 04:08:55

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 Re: Cat D3 transmission losing pressure in reply to Rack1234, 05-23-2020 04:18:44  
The reason it was rebuilt was because after about an hour of working it, it would just stop moving. I was rebuilding the engine anyways, so I figured I may as well rebuild the transmission too. It was also overheating within that hour of work. Found large pieces of clutch material in the filter screen and was leaking more oil than a person could keep in it. I rebuiltbit myself, not from a dealer or cat.

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05-24-2020 03:02:05

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 Re: Cat D3 transmission losing pressure in reply to Rack1234, 05-23-2020 04:18:44  
Start again why was the transmission in need of a rebuild,what was the symptoms,was the pump making pressure,what do you mean by brand new,a new unit from Cat,a rebuilt unit from Cat or an aftermarket unit from outside supplier,had you established what was wrong before the tear down,pressure tests etc.


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05-23-2020 19:59:14

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 Re: Cat D3 transmission losing pressure in reply to Rack1234, 05-23-2020 04:18:44  
in 1993 I redid the transmission on my 910 loader which is the same as the D3. The charge pumps are barely big enough when everything is tight.
Since you had even less pressure with a different valve body that would suggest thats where the problem is. my valve body was badly worn in the casting where the pressure relief spool runs. I rebored the body and made an oversize spoolvalve to tighten it up.

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