I was on the highway doing about 55-60 when the vehicle appeared to downshift and rpms shot up to almost 4000. I put the car in neutral and issue seemed to vanish. Parked car, and when I returned I had a gearbox fault on dashboard. Drove a few miles with no issues and turned off the car. Started it back up and gearbox fault is gone.
I baby this car and never drive hard, but we have had some awful snow in the past few weeks. Today and yesterday it's raining and the snow is melting. Could this possibly be electrical rather than mechanical? Please help.

 

Hello and greetings Harry ....

Severe winter up your way + 8-9yr old car + original battery? All the more modern Jaguars have quite demanding electronics on board, and I sense you are already aware that this fault may not be what it first seems. Now you may have a genuine gearbox issue, but we have seen so many false fault messages that arise from a dying battery, that it is worth asking you to check that first. Load test the battery voltage to ensure you have 12.6V or higher. A reading of 12 is "no go" and warrants an overnight or 24hr recharge and retest. Readings in the 11s point to a dead battery - whether shorted cell, sulphation - the only relevant issue is the load test voltage with engine not running. It is not unknown for the battery to start to fail under drive conditions - especially if loaded with lights on etc.

On the other hand, if you can't point to the date of a new battery being fitted, and yours is the Jaguar original (Varta?) then at 8-9yrs of age, it owes you nothing - buy and fit a new replacement. Please remember that new batteries are often not fully charged, so check as before. Let us know the results Harry, because the alternate will involve you (or your shop) reading and advising here the fault codes if any.

Best wishes for a clean fix.
Ken

 

Battery is 3 years old. Had it replaced along with new cables.

 

So ... load test battery; advise voltage; read and advise any and all fault codes.

Regards,
Ken

 

Just got car back from dealer.

They pulled code p1571. Brake switch fault.
Also they changed a switch-stop lamp.
They are saying that this is what caused my transmission malfunction.

 

Harry, I had this issue come and go for sometime. After some trouble shooting I found that the TCM under the drivers dash somehow gets wet. Its a known issue with these cars. I pulled the TCM out, took it apart, and saw the corrosion from what must have been a very small amount of water that traveled down the fire wall to the TCM.

I cleaned the TCM with rubbing alcohol and it seemed to fix the issue for a few months till the car down shifted and the transmission fail happened during a rain storm on the highway.

I ordered a new TCM from eBay. When I installed the new unit i moved it more towards the center of the driver footwell, covered it with a freezer bag, and never had an issue since.... That was over 2 years ago.

It can't hurt for you to check the TCM and see if you see any "dried" spots of water at all on the inside of the case.