The trans on my tj tiptronic one day just started to go in limp mode all the time.
When you turn the ignition on without starting the car, and put it in D, it will flash N.

I have tried to read the codes, it did come up with input speed sensor, i changed both input and output with working ones and didnt fix the problem.

It clunks badly in reverse ( and has done for a while even before the problem) and now it clunks a little bit when you put it in drive. But everytime it goes into drive the N light flashes and it drives in limp mode.

I really need help with this. I have clean all the plugs about 10 times, and changed both input and output sensors and neither worked. My next thought would be the ecu but im not sure. The ecu has been covered in plastic long before this happened.

Thanks SOOOO much all!

Did you check the engine codes as well?
A faulty TPS can make the gearbox do strange things.
You haven't mentioned km on the car, the gearbox may just have failed due to old age.

i will change the tps as i have a new one and see if that helps.

It seems to be going into limp mode before the car is even started.... i thought that might indicate an electrical problem.

also, very occasionally i can put it in drive when the car is off and it wont flash. then i put it in neutral, start it and put it in drive and it works fine, until i turn it off, or it seems putting it in reverse also does it.

Before you muck around with guesses, lift your carpet and check your ECU has not been made wet from a leaking heater

done that. the ecu is dry, and there is plastic over it. took it out and looked at it, no signs that it has been wet before

checked the relay too, seems to be switching fine on and off.

checked the relay too, seems to be switching fine on and off.
Which relay is that?

The one near the ecu. The 4 pin one

Dumb question - have you checked the transmission fluid level?

When was the last service?

Is there an after market cooler or filter fitted? Are any of the lines kinked or otherwise compromised?

You've probably checked already, I just thought we should clear this up so can eliminate the simple stuff.

Moving on; the switches at the actual shifter - do these ever fail? (Serious question) It's all plugged in and clean/dry at the shifter? I have kids chief, so it's not uncommon that I'm scrubbing strawberry milkshake or my own spilt coffee out of the shifter/center console area.

Fluid level checked and was changed not long ago. Fluid is nice red colour and doesn't smell burnt.

I've checked and cleaned the shifter at both ends, and all the plugs multiple times. Still no luck.

Someone out there must have had this problem before. It's just so strange that it's going into limp mode before the car is started.

Is there anything inside the trans that could be switching it into limp mode before the car is started?

ECU is probably not getting the right response from the trans. Solenoids possibly?

How do I check or fix the solenoid?

How do I check or fix the solenoid?

Take it to a trans specialist.

It's not worth taking it to a trans specialist, if I can't fix it, it won't be around for much longer! Anyone else have any ideas, or know how to check test or fix the solenoids?

ok so i have still been driving this car every day for the last 2 weeks ( in third gear mind you) and everytime it starts, before the car moves or anything, it goes straight into limp mode without fail.
Today its changed.

Now it acatually goes into first gear for about 20 meters, sometimes a little more, and then it clunks into third/limp mode. and everytime i restart it now, it goes into gear no problems, for about 20 meters.

Anyone have any new clues now as to what it might be??

Just a suggestion why not try the old disconnect the battery for a while trick?

Hmm, if we're guessing then how about the speed sensors? Input/output shaft.

done the battery trick about 100 times! no luck. Also have changed both speed sensors with 3 different sets and no change.

done the battery trick about 100 times! no luck. Also have changed both speed sensors with 3 different sets and no change.

Did you reset the ECU and check for fresh error codes each time you changed the speed sensors?