Brought 1985 xj6 in for inspection only to fail for no brake lights.switch was frozen so I freed up and tested positive for 12 volts to switch.no lights.put in jumper through switch no lights.all other rear lights work just no brake lights.
Ready to drive off cliff.

 

Yeah, I just had a ball with a "no brake light" issue a few months ago.


I had the scuttle panel down, the cheek panel off and even the
A post panel down. Oh, and the lens off the driver side rear lamps off.
Jag's innermost workings in the "open".


Good used brake light switch, NOS ignitions witch via David Boger
and reworked connections restored the brake lights!!


Caveat, there are two switches that look alike activated by the brake pedal that function exactly opposite!!! Don't confuse one for the other! One activates the brake lights, only with the ignition switch on, and the other interrupts the cruise control!!!


Carl

 

Carl is right about easily confusing the two switches.

Also, between the brake light switch and the brake lights there's another common trouble spot: the brake lamp bulb failure module. It's in the console as I recall. The S57 Wiring guide will have the details. it'll be easy to check if voltage is going into the bulb failure module but not coming out

Cheers
DD

 

Doug:


I thought the bulb fail unit was in the boot, alongside the ECU,
but, might be wrong.


Agree.


1. S57 is a gotta be.
2. It might be the Bulb fail unit.


Carl

 

You're right, Carl. In the boot. I had "XJS" on the brain.

The "B" terminal on the module is voltage input from the brake light switch. If there's no voltage present when the pedal is pressed I'd suspect a wiring fault between the switch and the module. This assumes the brake light switch at the pedal is operational, of course.

There are two "L" terminals on the module. These are outputs to the brake lights themselves. If there's voltage coming into the module on "B" terminal but no output on the "L" terminals, then the module is kapoot

Cheers
DD

 

Then, jumping L to B restores continuity?


As I "understand" them, they balance one light
against a mate. left to right style. So, a burnt bulb or a broken wire or even a bad connection on one side means "imbalance' and the dash light comes on!! Even mismatched bulbs can do it!!!


My Jeep's dislay does something similar. Even when one of it's two little license plate lights went dark??? Miles to the next service.
counts back from 3K!!


Neat, but not indespensible....


Carl

 

Right!


Yes.

No.

Ummm...sort of

The bulb failures modules simply work on the basis of heat.

When cold, an internal bi-metal switch is closed and passes voltage to the bulb failure lamp on the dash. When the circuit is powered-up, heat from the current opens the switch and the warning lamp goes out...usually in about 15-20 seconds.

The bulb failure modules don't 'know' what bulbs are being used or if they match. They only responds to heat. It's simply a matter of xxx-amount of draw providing xxx-amount of heat. It could be one single bulb or 20 teeny tiny bulbs. If there's enough draw from the circuit, then there will be enough heat generated to open the switch and extinguish the warning lamp.

LED replacements often cause fits because they draw so little current ....and therefore there's not enough heat to open the bi-metal switch.

A mismatched bulb can trick the system, yes, if it draws too little current....as not enough heat will be generated.

Cheers
DD