No brake Lights

[PaulR]

Greetings

I have a very puzzling problem with a lack of brake lights.

The brake light switch is located in the union of the brake pipes. The supply side has over 12V on the voltmeter and without any bulbs fitted pressing the brake pedal produces over 12v on the other side of the switch and indeed at the bulb holder.

However inserting a bulb reduces the voltage on the output side of the switch to 0.02.

Connecting a 12v supply direct from the battery to the output side of the switch lights the bulbs indicating that there isn't a problem with the wiring from the switch to the bulbs.

I'm suspecting that it must be the switch but it seems to be performing as it should, Pedal not pressed multimeter reads open circuit, pedal pressed multimeter shows no resitance

HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[RogerH]

Hi Paul,

the switches are known to be iffy. Your fault is puzzling. There must be a partial short to earth inside the switch.

You could leave it in place and fit a 4A switch to the master cylinder bracket. If you do get a BMW metal cased switch.

 

Roger

[Lee Dutton]

I had exactly the same problem and did exactly what Roger suggested (make sure to use the BMW metal cased switch) and have had no problems since.

 

regards Lee

[mike3739]

I had exactly the same problem and did exactly what Roger suggested (make sure to use the BMW metal cased switch) and have had no problems since.

 

regards Lee

This is the way to go I did it on my 6 over two years ago no issues.

 

Cheers

 

Mike

[RogerH]

Hi Paul,

I think I now know the odd things that are going on.

 

The contacts inside the switch are diry/burnt and putting some level of resistance on them.

With no current passing through the closed contacts you will get 12V on both contacts.

 

With the switch output now connected to the lights and apprx 4amps being passed the small resistance drops virtually all the voltage.

 

It's to do wiht Ohms law V=I x R If you have a resistance as small as 3 Ohms then all the voltage is dropped. 4 amps x 3 Ohms = 12Volts.

 

I believe the switch can't be cleaned.

 

Roger

[BrianC]

Had exactly the same problem with my TR3A so gave up and fitted the TR4A switch on the master cylinder frame. End of problem and easy to adjust to allow me to flash the brake lights with a light touch on the pedal whilst accelerating to discourage and confuse tailgaters.

[Badfrog]

Hi,

 

I had this problem some time ago: no brake lights despite good bulbs and a new pressure detector down at the crossing of the lines.

Finally found the problem: bad contact in a bullet connector under the dash, by the left side panel (I've got a LHD car, so think both sides). Approximatively right above the beam foot switch (LHD again). Take it out, solder dubious bullets, clean up and refit: bingo.

 

Badfrog

[AlanT]

Before you condemn the switch just make a resistance measurement across the two contacts. Leave the igition switch off when you do this.

 

Sholud go to less than 1Ohm when you brake.

 

Al.

[wayne taylor]

Keep the switch and swap the car!

[Flyer]

This is high resistance either in the feed to the switch or the switch itself.

 

Bridge the switch contacts, if the brake lights come on then the switch is faulty, if not then the duff connection is before the switch.

[PaulR]

New switch fitted problem gone

Bloody frustrating though

Thanks for all advice

PAul