Hi. I've got an irritating issue that seems to be difficult for the dealer (the best of a bad bunch) to fix. (2003 2.5 S-type).

The outside air temperature reading 'drifts' upwards to a high (for UK!) value like 20 deg C (It's 8 deg C today), or up to 50 deg C in the summer. This messes up the a/c and you can have windows fogging.

Disconnecting the sensor and plugging it back in again fixes the problem for minutes and sometimes days, but it always comes back. The car's been to the main dealer 4 times but they can't think beyond it being a harness problem (obviously they've changed the sensor numerous times). They actually ripped the harness apart on one occasion trying to find a broken wire by tugging on individual conductors. They succeeded, because the car died completely on the road the next day due to a wire that they did break in the process!

They are absolutely stumped and had to be persuaded to contact Jaguar and ask for advice, which hasn't emerged yet. They don't seem to understand that any fiddling with the sensor fixes the problem until I drive the car away, when it re-appears!

It seems to me that it's an electronic problem, with the temp reading climbing slowly to a high value over a period of 10-20 minutes. Something like a degraded OP-amp input. Obviously they could change the CC module but I'm reluctant to let them spend my money when they don't know what's actually wrong with it, only to have the fault re-appear in 3 months time.

I've seen this same fault described a number of times on the Internet but no-one ever posts an answer to it. So this seems the obvious place to ask. Ideas, anyone?

 

I've not seen this described before.

It sounds like a ground problem, which causes the input to "float". The workshop manual has the locations

 

I think you have 2 problems, the obvious one of outside air temp not displaying properly, but more importantly the actual inside air temp not being relayed to the HVAC system. I had the latter and the heat/AC would go on and off randomly irrespective of what was commanded.

 

Just the ext sensor or the CCM (or wiring/ground?) going bad (like drifting up) will cause unwanted behaviour, maybe all that's described.

As they've eliminated the sensor and wiring the CCM is probably the next most likely. Maybe hunt for the cheapest you can find (ebay/scrap/etc). Be sure whether you need CCM or RCCM (remote CCM - if you have the touch screen for satnav etc). Likely to be not cheap, and then there's fitting.