Transmission failure light on

[raymondcredie980]

I have an XJS Celebration Convertible 4 litre, the car has done a genuine 43,000 miles and i am the second owner. Around summer this year the transmission failure warning light came on. I have no reason to believe the transmission has a fault, it is working perfectly with no stiffness of the automatic gear selector, in all respects the car is just as it always has been - perfect. A phone call to the closest Jaguar Specialist (56 miles away) revealed that the most common cause of a false warning is a failing battery, upon having this checked it was operating under par, although having no difficulty starting, so i decided to change the battery which was done immediately but the warning light remains stubbornly on! Does anyone have further suggestions please on what may be the cause?

[ptjs1]

raymondcredie980

A failing battery can indeed cause a spurious transmission failure light illumination as I experienced this summer on one of mine. However, you mentioned that you replaced the battery. So a couple of thoughts of where I'd start:

- When you replaced the battery how long did you leave it before connecting the new battery? Was it at least 15 mins?

- Have you scanned the car for engine fault codes? The transmission failure acts as a safety back-up for certain engine problems, in order to protect the car. Even if you cant get the car fully scanned, with a cheap £5 bluetooth obdII dingle abd a £3 phone software package, you could read any engine fault codes from the XJS yourself (everyone should have these useful items)

- When was the transmission oil and filter last changed? The ZF4HP24 box really likes clean fluid.

Cheers

Paul

[raymondcredie980]

raymondcredie980

A failing battery can indeed cause a spurious transmission failure light illumination as I experienced this summer on one of mine. However, you mentioned that you replaced the battery. So a couple of thoughts of where I'd start:

- When you replaced the battery how long did you leave it before connecting the new battery? Was it at least 15 mins?

- Have you scanned the car for engine fault codes? The transmission failure acts as a safety back-up for certain engine problems, in order to protect the car. Even if you cant get the car fully scanned, with a cheap £5 bluetooth obdII dingle abd a £3 phone software package, you could read any engine fault codes from the XJS yourself (everyone should have these useful items)

- When was the transmission oil and filter last changed? The ZF4HP24 box really likes clean fluid.

Cheers

Paul

Hi Paul, thank you for that. Yes, the battery was disconnected for more than 15 minutes because i took it to my supplier for disposal and picked the new one up at the same time, that would be minimum one hour. I had all fluids changed after the light came on, but that was a routine major service and MOT, not as a result of the light showing. I will follow the route of diagnostics i think. Thank you for your kind advice.
Raymond.

[almcl]

Another possibility for this fault which I experienced a little while back, is a failing throttle position potentiometer. Sounds weird, but the gearbox uses the position signal from the pot and if it goes intermittent will throw a fault code.

Mine may well have been self induced following a slightly over enthusiastic use of cleaner in and around the throttle linkage.

Can't now remember where I got a replacement from, may have resorted to the secondhand market although Barratts list the part as 'special order'.