Random Gearbox fault PO706

[Showman89]

Hi all, been having an issue with my 99 Daimler V8, 92500 miles.
Had a gearbox fault warning come up on dashboard every now and then, limp mode etc. Usually turning the car on and off again sorts it out and all is well.
Old battery was tired so I fitted a new one, all seemed okay.
In the last two days the warning seems to be coming up more and more often, usually shifting between P and D - sometimes if I switch it off and on again, the warnings gone but the car still feels like it's in a limp mode, as if it's holding back.
So today on my way to work, the fault appeared while I was driving, after going over a minor bump in the road (coincidence, not sure?)
I got to some traffic lights and switched off the car. I went to restart it, and to my horror it was cranking over, but not actually starting. I tried to move the lever to N thinking it could be a park switch, but it was locked in P.
On a couple of occasions while trying to start it, the dashboard threw Engine failsafe mode, gearbox fault, ASC warning, Trac not available, just about every one there is.
Eventually it started and drove off totally normal.
My code reader diagnosed the code as PO706. Only other code shown was for an 02 sensor.
I've not had any nasty bangs or noises with the box and when it's behaving, the car is smooth as silk.
Only thing I've heard is when reversing, I'll hear a noise underneath, like the sound of something catching?
Puzzled and pulling my hair out now as I want to sell the car, but I won't like this.

[Andy P]

Hello and welcome Showman89.

The possible causes of the P0706 code for the gearbox are:

1. Transmission Control Module / J-Gate Module CAN fault.

2. J-Gate Module failure.

I would clean all the electrical connectors at the J-Gate and linear switch.
Also clean the electrical connector at the transmission - leaking oil can contaminate the connector.
Once you have the J-Gate apart check the wires at the top of the circuit board for broken or damaged ones.
Hope that helps and keep us posted with what you find if you can.
Good luck.
Andy

[Showman89]

Thanks Andy, that's great.
Two questions, where is the connector on the transmission located and is it accessible without lifting the car? (I don't have a jack)
Secondly, is the J gate assembly/ski slope the same to remove as the XJ40, as i have done that several times before.

[Andy P]

Hi Showman89,
Take a look at this thread about changing the J-Gate bulbs.
viewtopic.php?f=98&t=12010
It explains taking the J-Gate apart.
You do not have to remove it from the car to access the circuit board.
You will need to lift the car or put it on a ramp to get to the electrical connector which at the back of the gearbox,passenger side.
If you would like the relevant manuals PM me your email address and I will gladly send them to you.
Hope that helps.
Good luck.
Andy