I have a problem with my jaguar xj40 1990 LHD, the cars bulb failure light is on and the right rear foglight doesn't light up! The bulb is fine and I tested all the connections with a multi meter. Here's the weird part, the voltage straight from the fog light connector gives a reading of 16.5v !!! the cars battery only has about 12v of juice and all the other light connectors give me a reading of ~12v, what is going on?

 

Although this won't explain your voltage reading, I'd be tempted to clean the contacts, and reinstall that bulb (assuming the bulb tests good.) I don't know these systems intimately, but with similar experiences (1980's Fords) the car could only test circuits that were active. I'd be tempted to turn off the front and rear fogs, and take a walk around the car looking at everything else. I do know that even the parking/clearance lights are connected to the circuit that checks for a failed bulb. I don't know (yet) if the rear fogs are included.

 

I've checked the contacts by bypassing the fog light housing, feeding current for the lamp straight from the plug before the fog light housing. On the drivers side this worked and i got my bulb to light. I think that because the voltage is over 16volts from the passenger side, the lamp refuses to light.
Could this be a problem with the bulb failure relay module? I might tear it it open today and post an update if I see anything burnt inside the unit and i'll also read the values of the plug before the BF module.

 

Hi Twitzy187,

Welcome to the Jaguar Forums! It's great to have you with us.

You can download the wiring schematics for your car at this link:

Jaguar XJ40 Electrical Guide 1990

My first thought is that you could test the voltage between each terminal in the fog lamp connector and a good ground to see if you may have positive voltage on both legs. Also, set your meter to ohms and check for low-resistance continuity to ground from each terminal in the connector to be sure you have one good ground and one positive voltage supply.

Please visit the New Member Area - Intro a MUST and post a required introduction so we can learn something about you and your Jaguar and give you a proper welcome.

Cheers,

Don