D6D power shift

This is another one for "Bob"
But you might try this:
Lock the brakes and stall the torque converter in each speed. If the universal joint turns, the transmission clutches are slipping.

If it slips in all or any speed the list of causes are:
1. Low oil pressure caused by:
Low oil level
Control linkage broken, loose or incorrectly adjusted.
Oil pump or oil pump drive failure
Air leaks on inlet side of pump
Leakage within the transmission
Hydraulic system pressure relief valve incorrectly adjusted or stuck open
Load piston, check valve, or differential valve stuck
2. Mechanical failure in transmission
3. Torque converter failure
4. Transfer gear failure or bevel gear failure.

Lets add a few things! Check to see if the torque is flooding( run it a liitle bit and shut it off and pull the drain plug out of the bottom of the torque- should be about a gallon and a half to two gallons, if more possible torque problems but should be heating if so) Check the trans screen see whats in it. Check the hose or coupling from torque to trans screen(may be sucking air) last but not least stick a pressure gauge in the trans pump and see what it reads in all
trans postions (should be around 300 psi) mite narrow the problem down a
little.Those are good tractors and don't give that much trouble unless you
have a total failure. Hope that helps.

[quote="Al Engnes"]Lets add a few things! Check to see if the torque is flooding( run it a liitle bit and shut it off and pull the drain plug out of the bottom of the torque- should be about a gallon and a half to two gallons, if more possible torque problems but should be heating if so) Check the trans screen see whats in it. Check the hose or coupling from torque to trans screen(may be sucking air) last but not least stick a pressure gauge in the trans pump and see what it reads in all
trans postions (should be around 300 psi) mite narrow the problem down a
little.Those are good tractors and don't give that much trouble unless you
have a total failure. Hope that helps.[/quote]

torque converter is not flooding, checked it
inlet screen was clean, installed new one.
filter was normal, changed it.
drained and re filled transmission with cat transmission oil.
no overheating of torque converter.
no loose connections.
pressures all set to specs in the maintenance manual.
I talked to the service manager at cat and he agreed that it sounds like something in the speed control valve, I ordered all the seals and o rings so I can dis assemble it and check for obstructions.

Bart

p.s. thanks for the reply's, I want as much information I can get as I am driving about 250 miles round trip to work on this thing.

If the transmission is "not locking up", you have inadequate oil pressure reaching the clutch pack pistons .. and you should have an overheating torque converter/transmission, if operation of the tractor is continuing in this state for any length of time.

If your pressure check points are reaching the correct pressures, you do not have an oil supply/pressure problem. I would suggest you have a mechanical problem in the linkages, which is preventing full oil pressure from reaching the clutch packs.

Check for correct operation of the entire control linkage system, from the control lever at the seat, right through to the valve bodies, with the engine running (you'll need to have the transmission cover off, and work the lever and check the linkages. Ensure you have the brakes locked when you do this, and exercise great care).

It's not the 1st time I've had a stick or other "foreign body" jammed in the linkages that prevented correct operation.
You also need to check that the pins in the linkages, including those inside the tranny, are all properly secured, and haven't lost cotter pins (split pins) and are partly falling out, and interfering with other parts of the valve bodies, or other castings/brackets, which is preventing proper engagement of the spools.

The fact that the problem is intermittent, is indicative to me, of a mechanical interference problem, that comes and goes, according to the movement/location of the interfering parts.

If the transmission is "not locking up", you have inadequate oil pressure reaching the clutch pack pistons .. and you should have an overheating torque converter/transmission, if operation of the tractor is continuing in this state for any length of time.

If your pressure check points are reaching the correct pressures, you do not have an oil supply/pressure problem. I would suggest you have a mechanical problem in the linkages, which is preventing full oil pressure from reaching the clutch packs.

Check for correct operation of the entire control linkage system, from the control lever at the seat, right through to the valve bodies, with the engine running (you'll need to have the transmission cover off, and work the lever and check the linkages. Ensure you have the brakes locked when you do this, and exercise great care).

It's not the 1st time I've had a stick or other "foreign body" jammed in the linkages that prevented correct operation.
You also need to check that the pins in the linkages, including those inside the tranny, are all properly secured, and haven't lost cotter pins (split pins) and are partly falling out, and interfering with other parts of the valve bodies, or other castings/brackets, which is preventing proper engagement of the spools.

The fact that the problem is intermittent, is indicative to me, of a mechanical interference problem, that comes and goes, according to the movement/location of the interfering parts.

Bart, did you check the pressures in all gears at low idle aswell as full throttle? Should be around 90% of full throttle value.
When at low idle pull each steering clutch lever and see what happens to pump pressure.
Leave gauges hooked up and see the reading when it will not drive.
I suspect as does OZ that you don't have enough pressure then. Likely only Initial pressure. If so it could be dirt in the valve plugging the orifice to the load piston. Come on back with what you find.
Later Bob

[quote="bob"]Bart, did you check the pressures in all gears at low idle aswell as full throttle? Should be around 90% of full throttle value.
When at low idle pull each steering clutch lever and see what happens to pump pressure.
Leave gauges hooked up and see the reading when it will not drive.
I suspect as does OZ that you don't have enough pressure then. Likely only Initial pressure. If so it could be dirt in the valve plugging the orifice to the load piston. Come on back with what you find.
Later Bob[/quote]

I only checked it by the book which has you check at idle and at full governor, so I never checked pressures in gear.
The service manager at cat also suggested putting the gauge on the filter housing and watch the pressure when shifting. he said it should drop briefly than back up to system pressure.
I would assume if there is a valve not seating the pressure would stay lower than operating pressure.
thanks
Bart

Bart, if you checked it at low idle and got pressures on spec, it's now likely leaking inside.
The pressure drop and rise at shifting is called Modulation and should be a fast drop as one clutch emptys and a smooth rise os 1 to 2 seconds as the next clutch fills for smooth engagement. This is the same as letting the clutch peddal up slowly untill engagement on a car or truck.
Inside the trans, the valve body has two sections, the lower one, the Selector Pack has the Selector Spools. I am streaching my memory here, one end of each spool bore has a machined surface. I can't remember what direction or speed to shift to but you will see when in there. Just take off cover and shift around(with engine off) untill the shoulder of each spool comes close to a machined surface. Then adjust the Hook Ends of each spool untill the valve surface and spool shoulder are flush. This needs to be checked and adjusted if neccessary any time you take the valves out and put back in. Also make sure the shifter shafts and bushings going through the trans case are not worn.
The top section likely where your problem is, is the Pressure Control pack, The pump oil goes through the filter and to the Main Pressure control valve on the back of the torque divider housing. The Main pressure relief valve, sets up Max pressure and sends it to the trans valves and steering valve. Unless you shift the trans or pull the steering levers very little oil goes down that hose, only clutch leakage. The most of the oil goes into the torque divider then the cooler and on to trans lube.
The valves in the trans pressure control pack are not pressure relief valves, they are pressure reducing valves. They don't dump pressure like the main relief does they limit pressure going through them by sencing down stream pressure. The modulating valve is shim adjusted to Initial Pressure, and comes up to full pressure when Full Speed Clutch Pressure when oil beyond the Differential Valve works it's way through the Orifice Check Valve and in behind the Load Piston on the back of it's spring. When you shift the pressure behind the orifice check valve drops off, the check valve opens and the load piston moves back releasing the spring.(sharp pressure drop) When in selected gear, the speed clutch is at initial pressure, directional clutch is at 55 PSI lower due to differential valve, Directional clutch pressure seats the orifice check valve and works it's way through the orifice and in behind the load piston causing the Speed and directional pressures to slowly rise. This is modulation and happens with every start up and shift.
Hook up gauges to trans valve and watch it happen as you run machine at low throttle.
Later Bob

Bart - My experiences with the D6C/D powershift tranny is that they are one of the best trannies that Cat built, and rarely give too many problems.

However, the Cat powershift trannies have one potential problem area - the filter is a bypass filter, and if left unserviced for any extended period - and it fills with trash - it dumps the crud it can't handle - right into the tranny oil system. This then, means that the valve spools and the tiny orifices in the pistons and spools can get blocked, and create havoc.

Brake lining material is the biggest bugbear in the above scenario. When the brake linings get old, and way down, they commence shedding lots of little chunks of the lining material. The molded linings don't seem quite so bad for this - it's the severe service, woven, brass-impregnated linings that seem to be worst for shedding lining material when they get old.

These little chunks, if they bypass the filter, are the ones that can get into the spools and pistons and cause problems. There are orifices so tiny in the spools, you need to dis-assemble and clean them thoroughly, and check them extremely carefully, to ensure that oil pathways aren't being blocked, and creating transmission control problems.

The only other problem I've had with these trannies is blown o-rings - mostly between the valve body and the clutch pack housings. This problem is usually a constant problem, though - without the intermittent symptoms that your tranny has.