2007 535B Cat Skidder Transmission Issues

[knight_hawk00]

Good Evening,
We have a 2007 535B Cat Skidder that we bought with a rebuilt transmission that needed the rear end installed.  It was supposed to be in good condition other than the rear end.  Once we installed the rear end it would not go into gear.  We finally took it to a vendor and they found that a modulation valve spool was in backwards.  They switched the valve and all appeared to be Ok.

We got the machine back and test drove it around the yard.  It seemed to perform well, and would go backwards and forwards in all gears without issue.  However, once we began to work the machine and put it under load, it seemed to loose gears.  Upon closer inspection it would begin to kick into neutral when you went to select gears.  If you waited a few seconds it would go into the gear you selected, but the warmer it got the worse it became, and eventually would not go into forward or reverse.

We returned the machine to the shop and they replaced the flow control valve and thought they had it, but it proved to be ineffective. 

Also of note, we did replace the hydraulic pump early in the process.  The transmission does not over heat at any point in the process.

Can anyone offer any advice to trouble shooting? I have read on line that it may be connected to the brakes, causing it to shift to neutral.  If this is correct can any one elaborate?  Any other ideas?

Thanks,
Appreciate the feedback

[mike_belben]

Youre gonna need a service manual or atleast a few pictures from one that calls out the control pressure port locations and pressure specs under whatever operating conditions, to know if youve got the right pressure going into each planetary servo or not.  

What oil is in it?  Cat is pretty specific about powershift fluid. 

Heres some theory of operations from cat.  Simpler, older one first. 

CATERPILLAR TRACTOR D8 BULLDOZER POWERSHIFT TRANSMISSION PROMOTIONAL FILM 52514 - YouTube

Putting the POWER in POWERSHIFT Transmissions - YouTube

[kiko]

When you say it is kicking into neutral ,what is the shift indicator on the dash indicating? Does the selected gear remain on the dash when it kicks out of gear or does it indicate N.  Also have the  transmission pressures been checked while these issues arise?  There are two orifices in the transmission control valve, one is located inside the modulating valve and it is green and one in the dump valve is red. Also the control valve on the B series and early C's have slugs that get hung in the aluminum bores especially as they get hot.  Typically if there are brake pressure issues a warning light on the dash will appear .Sometimes the issues are hard to pin point on a forum but we can get to the cause.  There is no declutch on this skidder so you would need to shift into neutral when revving to increase hydraulic speed.  I will post some pictures of the control valve slugs tommorow.  Also there is a transmission system relief valve located under the control valve . 

[kiko]

This is the location of the transmission pressure ports.  The quick connect at the top is for the directional clutch's those solonoid are across the top. The lower connecter is the directional clitch. With machine in neutral both of those ports should have pressure with the top connector having approximately 75psi more pressure.


 

[kiko]

These are the two orifice in the valve , the screens can be stopped on either.

 

 

[kiko]

 

 if your modulator valve or any of the slugs look like this. The entire control valve must be replaced.