I recently had an odd fault with my 03 XK8 and thought I might mention it on here in case anyone else has the same problem. I haven't seen it mentioned anywhere else but please forgive me if this is already a well-known issue.

A few days ago the aircon stopped delivering any heat at all below about 1400 RPM and didn't deliver much at higher engine speeds. At first I thought this might be the heater pump. However, I also noticed that I'd lost a few other things as well.

The problem turned out to be fuse F3 in the engine compartment fusebox, which had ruptured. Referring to the diagrams in JTIS, this fuse feeds circuits 49-57 and also two fusebox relays via circuit 60. The latter two are for the heater pump and front foglamps, so losing F3 deprives you of both. As a consequence of the way the system is wired, you also lose the front and rear foglamp annunciators but not the rear fogs themselves.

Circuits 49 to 57 feed the adaptive speed control, the 'active security sounder', the centre-console headlamp levelling switch and the RH and LH headlamp level actuators. In HID-equipped vehicles they also feed the RH and LH ride-height sensors and headlamp levelling servos.

The root cause of the failure was a short-circuit in the security sounder which ruptured F3. But a loss of heating and other services doesn't immediately and intuitively suggest the security sounder as a possible cause!

And here's the kicker -- exactly the same fault occurred on my XJ8 a couple of months ago. Same cause and very similar consequences.

John

 

My 2000 XK8 risked failing it's UK MOT test recently because in order to pass both the fog lights and their in cabin tell tale lights have to be working.

XKRacer worked on the car and took it for MOT, but couldn't find the tell tale switch fault. Additionally, when I got the car back the heater didn't work.

On your information I've just checked the no.3 fuse in my engine compartment fuse box (5A), and found it blown.

I temporarily swapped it with the 5A in no.2, and both fog light tell tale switch lights now work, as well as the front fog lights. The rear fog lights worked already, as you stated.
It's a bit late for the heater unfortunately, because I've got the throttle body and inlet manifold off, have purchased a brand new heater control valve and am currently trying to track down a reconditioned auxillary heater pump.
Since the old parts are out, I might as well put the new ones in.

On the upside, if my heater packs up in future, it shouldn't be due to the latter two parts having failed.

Thank you so much for sharing your findings.

 

I've added this diagram in case people with the same problem go looking for the security sounder as a source of circuit short.

The diagram was put up by GGG, as a contribution to a thread by someone with the same problem on an XJ8.

Hope you don't mind GGG.