My rear Fog lamps worked fine when we got our '96 XJS Convertible (4.0L) but somewhere along the line they have taken on some odd behavior and I can't figure out what's up and am hoping some of you may have an idea...

If the headlamps was on the rear fog lamps used to come on instantly when you pressed the dash switch, the illumination in the switch would instantly go from green to orange. Now when you turn it on it stays green and the RFL's don't come on but will after a few minutes of leaving the switch on, the switch slowly goes from green to barely greenish/yellow, turn on the high beams will make the process faster and the switch light will be more orange-ish but not as deep orange as it should be. Tonight I checked the fuse, FINALLY found that stupid hidden relay and swapped it with another car to test it, even swapped the switch itself with our other car, relay and switch both work perfect in the other car and it's relay and switch have the same behavior in the 4.0L car. So it's not the switch or the relay. Fuse is also good, not much else to test on that one, has power on both sides.

I even considered that since I had recently put LEDs into many of my dash indicator lights that maybe the LED wasn't making enough resistance to let the system work right so I pulled the dash and switched the indicator bulb back to a regular bulb but that didn't make any change either.

During all this testing I realized that if I turn on the switch and turn the headlamp switch off then right back on, the RFL's will often (but not always) come on instantly and the illumination inside the switch is orange like it should be. But if you turn the headlamps on and then turn on the rear fog lamp switch you get that odd behavior.

Anyone have any ideas what I'm missing or what else to test?

 

Maybe a short in the switch? If the contacts are dirty it could cause intermittent issues. Try cleaning it.

 

Whelp's, more testing reviled that if the RFL's are on and your on hi-beam, then switch to low beam, the headlamps sometimes (but not always) go completely off. Turn off the RFL switch and they come back on, or switch back to high-beam and they also come back on. Odd. Act's like a bad relay but I tested the RFL relay, now wondering if that relay may get it's activation from one of the main headlamp relays? ...or at the headlamps all on one relay? Anyways, next step is to test all the headlamp relays (after that comes ground connections and that will be a major pain these cars have so many ground points.

 

Interesting, I too am having a problem with my 95 rear fog-lights, only I know what the problem is....basically one of the sockets where the bulbs go is falling apart and one of the lights won't come on. Perhaps your socket has deteriorated as well.

I have been unable to find the part number for the socket to order it new :-(

 

I just got in from further testing and after finding the headlamp relay under the hood on the left side and swapping it with a different relay the problem seems to have went away. Will have to test it more on the road, it doesn't always act on the road exactly the way it does in the garage. Guess the relay was good enough to work the headlamps but whatever interaction it has with the RFL's is where it went screwy.

 

Sounds probable. The RFLs automatically go off when high beam is selected/or prevent high beam being selected when they are switched on. So if this part of the headlight circuit was iffy, it could have affected the RFL operation.
Greg

 

That's odd, I think mine on all my (US Model) cars has always continued to work regardless of if the high or low beams was on, just so long as the headlamps was on at all. I know that's how it works on both of my current cats at least.

 

Maybe this is just a european requirement? It is standard over this side.

Greg