Does anyone have any experience diagnosing the source of these two codes? They appear intermittently on my 1997 XK8. (4.0 liter non-supercharged) There doesn't seem to be any consistent trigger other than cruising at around 70 mph. The car can go for weeks without triggering the codes. The warning light reads "Poor Vehicle Performance."

They are transmission codes. They cannot be read by a generic scanner but were detected by scanning software from Autoenginuity.

P0790's fault description reads: "Mode switch circuit malfunction."
Possible causes given are: "Mode switch to TCM circuits open circuit. short circuit or high resistance. Mode switch failure."

P1796's fault description reads: "CAN circuit malfunction." Possible causes given are: "Other CAN control module CAN related fault(s). CAN short circuit fault. Control module failure - check for additional DTC(s) to locate control module fault."

My mechanic is aware of these faults and will be taking the car in shortly, but I am looking for any help I can provide him.

Although I have a mode switch replacement ready to put in the car, it makes no difference whether the switch is in the normal or sport mode. The warning light and codes are generated either way

The "Poor Vehcle Performance" light pretty consistently goes off and stays off when the car is stopped and restarted--only to return about a week later. The codes seem to cancel, too.

Although I was not initially aware of any restricted performance, I recently discovered that there was no throttle response over 80 mph.

P0970's default action reads: "TCM adopts Normal Mode."

P1796's default action reads: "When fault is detected: -TCMadopts transmissionmechanical limp home mode (4th gear) -ECM limits engine power."

I am generally already in Normal Mode so I can't speak to P0790"s default action.

Pi1796's default actions are not what I have experienced. The car definitely does not go into limp home mode. Although it might be described as a loss of power, all I detect is a lack of throttle response over 80 mph.

The previous owner reported intermittent warning lights when operating the car on cruise control, but I experience the problem refardless of whether Cruise Control is engaged.

Has anyone seen this problem?

 

It is the transmission range sensor (Most likely). R/H side of the gear box, 2 10MM bolts securing it, shift shaft comes right through the center. Common problem. Jaguar calls it a "rotary switch", MZF calls it a range sensor, Jag scan equipment says Mode switch. It always sets those 2 codes.

 

I have the same car as Robert, and similar codes; P1798 and P0790. The car seems fine, with no engine lights on the instrument cluster and no limp home action.

I have disassembled the switch plinth in the front console (referencing the excellent videos on YouTube) to replace blown bulbs. It was necessary to disconnect the Sport/Cruise control switch in the process. I also had to turn the ignition on in order to remove the gear selector surround. I surmise that having the igniton on but mode switch disconnected flagged the P0970 code. Because the fault was not present the next time the engine ran, it performed normally.

It is my intention to erase these codes and carry on driving. I will probably check that the transmission fluid is fresh soon though.

 

 

Hi Mike,


An update from me is that I have run the XK8 since my last post without recurrence of the problem.


If I were you, I'd cancel the codes and carry on driving.

The noise at 70 around turns could be tyre inflation, suspension geometry or many other things. I doubt they are related to the codes that your guys found.


The rough downshift may have been cured by your fresh oil, but might equally be the result of a worn gearbox.


Cheers,


Malcolm

 

Malcolm,

Thank you for the update.
The bellows boots are worn...that could explain the noise, maybe? I'm having them replaced next week.
The rough downshift has only happened a couple of times and it has been since the fluid was switched out.