Hi

I've lost one brake light, and I'm struggling to find out what's holding it back from functioning.

The car has despite it's vintage '77 a bulb failure system, so I know when it started failing. It happened after my fuel tank out and messing around in the boot, but not directly after.

The bulb and wiring is good. The 3 bulbs share same earth connection, and rear light and blink works as it's supposed to.

If I turn on ignition (1st step) and hold down the brake pedal I get brake light in RH side and the Stop failure lamp in the instruments. If I then measure the voltage to the L stop lamp with the connectors disassembled, I have 11.9 V w/o the engine running. Not too bad.
When I then load the connector with my test lamp (a 12V5W lamp in a wire with crododile connectors, voltage drops completely to 0.00V
I also measure 12V at the socket, when the bulb is not in place

I seems to me, that it something is cutting out the stop lamp curcuit when it is loaded ?

I'm not expecting you to know much about preHE electrics, but some of you for sure has more electrical knowledge than me.

Any suggestions ??

 

My first step, personally, would be looking at the bulb failure module itself.

It's a fair guess, but not 100% assured, that the wiring is the same as later models....which is all I have to go with.

The BFM takes output from the brake light switch and splits it to the left and right brake lights. Since one of your brake lights works, we know that the problem is at, or after, the BFM.

Input from the switch, and both outputs from the BFM, are all green/purple wires.

I would check for output voltage from the BFM. Or, jump all three green/purple wires together and see of both brake lights work. If you remove wires from the BFM take good notes as to which terminal they go to, of course.

If no joy then move on to more mysterious and time consuming possibilities.

Cheers
DD

 

Thanks Doug

The Green/purple fits my car, so it seems electrics hasn't changed that much.

The novice question: Where is the BFM located ?

 

On the HEs it's in the console....just aft of the gearshift as I recall. I think you can reach it if you remove the cubby box

Cheers
DD

 

According to the ROM section 86.55.34 you're right. Stop light failure sensor is the name of this unit, so the BFM is split out on more units.
When we are at the topic: The park lamp failure lamp is on for maybe 5 seconds after I turn the light on. I do not see any failing bulbs though, so I guess it's just the Park lamp failure warning sensor that acts up ?

 

Normal for the bulb failure light to glow when the lights are turned on, usually 10-15 seconds or so.

The little bulb failure units have bimetal switches in them. These switches are 'normally closed' and complete a circuit from the light switch to the dashboard warning light, causing it to light up.

When the park/headlights are switched on current passes thru the switches, causing them to heat up and 'open', thus breaking the circuit and causing the dashboard warning light to extinguish.

When a bulb fails, less current passes thru the bi-metal switch. Therefore the switch doesn't get hot enough to open up ....and the dashboard warning light stays on

Cheers
DD

 

Excellent, Park lamp failure system then simply works as it should. Nice to have that clarified.

There will not be more time to spend in the workshop until Tuesday, but I feel confident I'm on the right track after your help

BR Leo

 

To test the BFM swap the left and right stop lights at the module, if it's the same module as the later cars the connectors are separate spade terminals.

If the problem moves to the other stop lamp then the BFM is the culprit.

If the problem stays with the same lamp then it's a wiring/connector or globe issue. A bad connection (high resistance from corrosion) can give you the voltage symptom you described. So clean the connectors and ground.

 

Thanks Warren

Connectors and ground is cleaned at the rear light.
As my picture in 1st post shows, the connectors on the car has got a layer of primer paint at some point, so the ground pin was actually almost fully covered in paint

In case the BFM is found to be the source of failure my plan is to connect the wires directly and leave the BFM out of the loop until I have sourced a replacement.
It's simply too risky to drive with just one brake light !

 

Just found this little write-up:

The brake light BFM is located under the ski slope, approx under the window switches. It has FIVE terminals ("WL" BFM warning output, "E" parking break switch (interrupts circuit ground when actuated), "B" (input) break switch (which DIRECTLY drives the third eye break lamp - if it fails, the BFM won't tell you), and individual outputs for left and right break lamps, both marked "L"). To check this unit without having to unburry it, pull up the parking break, and apply pressure to the break pedal. The bulb failure indicator should light up. If you release the parking break and apply the break, the indicator shouldn't light up (unless of course, there really IS a failure).

Source: I have a 1986 jaguar xjs, my left and right turn signals ...:

 

Result !

The stop light BFM was located as described. I swooped the two upper single lead connectors, and the fault moved to the opposite stop light.
I then created a small harness with 3 connected male spades and connected the 2 single lead connectors to the plug with same color coding wire in the 3-wire connector I had taken of the BFM. Et voila, 2 stop lights working again and ready for the 200 miles trip to Hamburg next weekend

 

Final follow up on this:

I opened the BFM and found, that some solder connection were not OK using my digital multimeter.
I resoldered all joints, mounted the unit again and it works

Pictures are from before resoldering. The bad joints were at the red laquered wire, which is connected in series with the bulbs.