Car was sitting in a field at a body shop all last winter until we started working on it in April. The body shop put a new cooling line in it but we weren’t sure what they did about replacing the fluid. Engine light was on whenever we started it.

Finally got it road test worthy and noticed, when the engine got warm, it slipped/slammed hard into 3rd gear – but it would at least go into 3rd gear. Read up and found where you pull off the drain plug and only 3 quarts should drop out if it was filled correctly…we got 4 qts. We then put 2qts in and drove it. Still slipped and got a “gearbox fault” error. Drained out fluid again and put 3 qts back in until it finally started overflowing from the indicator plug…just like the manual said to do. Test drove it and found it slips going into 4th gear, but no more gearbox fault errors.

Finally pulled the OBDII codes and got a P0733 “3rd gear ratio error” and a P0734 “4th gear ratio error.” Pulled the TCM and it was dry (saw somewhere that x-types had issues with water getting into the TCM). Cleared the OBD codes and drove it. No new codes but still slips trying to get it into 4th.

Noticed that, with a foot on the brake and in Drive, the tranny does not slip when revving…indicating it is not a clutch. The car wants to go. My buddy has rebuilt transmissions before. He thinks it’s somewhere in the control valve body.

I wanted to get an opinion before letting him dig into that project and burning time and money. Since overwhelming recommendations are to “replace the transmission,” I wanted to take it a step further and find out exactly what does go wrong when these things slip gears. What part(s) go bad?

 

I'd try the valve body first. I had a Jeep Grand Cherokee do something similar to what your X type is doing and the transmission shop was able to fix it in the valve body. Cost about $250.00, a lot better than $2500.00 (and who knows how much more for the Jag).

 

The valve body is pretty simple. Depending on how many miles you have on it you might want to get new plastic check *****, new metal *****, and new solenoids. Make sure when you pull the valve body that you pull all the valves out, make sure they're working and the springs aren't broke and they're not sticking. If they are sticking you can sand them down with some emery cloth. Reassemble and lube generously with a synthetic transmission assembly lube. Now, when you put the valve body back in, you have to secure the manual valve with a clip, undo the shift linkage, turn the shift selector all the way counter-clock wise, and put a upper and lower bolt in. Take off the clip you have holding in the manual valve and rotate the shift selector, making sure the manual valve goes in and out. If it does, install all the bolts and torque. If not, take everything off and try again. You have to have the manual valve engaging.

There are two small o-ring seals that sit behind the valve body, make sure you don't lose these!

 

Thank you for the follow up.

We have the valve body out and bought a set of new solenoids. Question we have now is, how do you actually test a solenoid. We put an alligator clip on a wire to a car battery and tested them like that. Some clicked, some didn't, some only clicked after blowing through them, all have resistance. Is there any specific way to confirm if they work or not? I'm thinking different types have different behaviors. I found some Jag transmission documentation online, but nothing that zero's in to the level of performing surgery on a valve body.

Also, the "Manual Valve", should it have O-rings on it?

 

You need to ohm test them. But, if your car has over 60k on it, you might as well just change them out. Also, it's really important to get that manual valve seated correctly.

 

No, the O-ring seals sit in the transmission. Pull them out, put assembly lube all over them and put them back in, the assembly lube will make sure they stay in place.