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Q: warning light on 1998 Jaguar XJ8

failsafe engine warning light is on, cruise controll doesnt work as a result of this.
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Does the speedometer work? A failed speedometer with these other two symptoms would indicate a bad Vehicle Speed Sensor. Jaguar are fussy many problems can't be picked up with generic scan tools and need dedicated Jaguar scan tool or at least more expensive Launch, Autologic, or Vedis scan tools to read codes stored in your car's computer.
The most common cause of this believe it or not is the brake pedal switch. On models with cruise there are two microswitches on the brake pedal switch, not one and they have to fully dissengage, ie. open when the brakes are not on.

If you turn off the ignition and wait for 30 secs the failsafe should go off. Listen carefully to a depression of your brake pedal whilst pressing a tiny amount at a time. You should hear two feint clicks. If not pull your brake pedal up as high as it goes and try again. The two clicks should be very close to each other.

I removed the switch from the brake pedal (require 7 and 8mm ratchet spanners), cleaned the microswitches with electrical contact cleaner / lubricant down the rubber seals and sligtly bent the switch levers which made them engage the the tiniest fraction lower down on the brake pedal travel (and I mean tiny). Problem solved. You may need to replace the switch if not.

If your cruise intermittently engages and sometimes just doesn't engage then you are almost certainly looking at this as the problem. There will be no fault codes for this. It is just the ECU not seeing the two brake switch circuits in sync and therefore disabling the cruise and sending the car into failsafe.

Good luck. Fiddly to remove switch requiring patience but actually very easy to do.
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