My 89 XJS 12 developed a new problem. when ever I step on the brake the bulb out light on the dash comes on. goes off when I let the brake off. also the cruise control quit at the same time. Coincidence?? Icheck fuses and replaced Brake light bulbs . No change What do I check next?

 

Have a look at the second post in this thread. Does this help?

https://www.jaguarforums.com/forum/x...lights-237621/

Cheers

Paul

 

I think the cruise control shutting off when you step on the brake is part of the design. It should shut off.

I know you said you changed the bulbs, but have you confirmed they are working when you step on the brake? If not, there may be a loose ground that is mimicking a faulty bulb.

 

Are you 100% certain that (a) the handbrake warning light works, and (b) it goes out when you release the HB?
If the HB warning light microswitch is badly adjusted, it might be telling the system that the HB is on, in which case the warning light will light up when you press the foot brake.

Secondly, the brake warning light circuit failure gizmo may be failing and giving you a false signal.

 

I'm confused: Are we talking about brake warning lights, or are we talking about the system that tells you somewhere there is a lightbulb (headlight, taillight, et al) that is lighting up when you hit the brakes?

 

This ^^^^.

Bulb Failure Warning System.

The system has an oddity, though.

The bulb failure icon lights up if you have have failed bulb. No driver action needed. Except for the brake lights. The driver must test the brake lights by applying the handbrake and pressing the brake pedal. If the bulb failure icon lights up, the brake light bulbs are OK.

Cheers
DD

 

It is the bulb out light that is coming on (yellow) the park brake light works proprly

 

I have a confession to make. I completely deleted (after the second round of sensor failure) the rear lights out warming system. When I first started on this car one was dead. I dissected it then and saw that after 30yrs these things must be on their last legs. The big metal strip/tang wrapped in naked copper wire that heats up for function - seems a bad lucas design in the first place, new...

I tried doing some soldering - actually I think the wire might be made of tungsten or something - the wires fought the solder.

I made up some double ended male spade wires, I simply jumped the plug and ran the striped 12v wire, thru the 2-3amp inline, right to the RED light lead. To my surprise it worked - knock on wood. I still get the 30 - 40 sec yellow illuminated check period, I guess for the front lights, then it switches off.

I have heard that the Jaguar bulb is a unique bulb for rear running lights. Could it be that the wrong new bulb was installed and the brakes draw enough current to decrease the amount of current in the sensor,,, thus tripping it?

After the second failure I had had enough. I'll just give the car a walk around checking for lights out, once in a while, like back in the ol days.

 

Well, if nothing else, it's certainly very old school. But let's not be too hard on old Joe Lucas. Lifespan of these BFUs is usually measured in decades. If something on a car lives that long the design couldn't have been all that terrible.





I'm not entirely clear on what you're saying.

Each branch of the bulb failure system for the running lights depends on xxx-amount current draw to work properly. So, yes, incorrect bulbs can cause a problem.

However, the brake light failure circuit works independently. Incorrect bulbs in the running lights wouldn't affect the brake light failure circuit.

Cheers
DD

 

Understood on both points Doug. Understood...

I guess what I was thinking was that if there was a dip in current overall due to the brake lights being activated, with slower running engine due to braking, the amount of current that it takes to bend that bi- metal may drop allowing the contact to close momentarily, lighting up at the dash...? Maybe? What does early signs of failure of these units show up as, anyway? It only takes one to set off the light I think

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