Brakec lights not working

[Huttey 0]

1999 E39 540 - My brake lights are not working, bulbs are o/k, fuse is o/k, so it appears it is either the wiring or the brake light switch. Can anyone advise exactly where the brake light switch is and how you can test it.

In advance thanks Huttey

[pureboiracer 0]

brakelight switch will be around your brakepedal somewhere. not sure on exact location. u should be able to figure out where when you look.

heres what u need to do.

1. get a multimeter.

2. conect meter between light pos and earth. if it reads 12.5ish volts the fault is between the lights and ground.

if no voltage do the following.

3. conect the multi meter before the switch and earth is somewhere. u should get around 12.5ish volts.

4. test for voltage after the switch when you push the brake pedal. u should get same voltage.

5. if you can find the brake light relay test for voltage at pin 86 or 85. one of them should have voltage when you push the brake pedal. and if you flick the multimeter into resistance. the little horse shoe symbol. and conect the other terminal. (without voltage) to the multimeter and the other wire on the meter to earh you should get a low reading. prefrably under 1 ohm

6. test for voltage in at pin 30 on the relay. and check the resistance of pin 87 and earth. that should indicate what part of the circuit the fault is it. if theres no voltage at 30 when conected to meter and ground then the fault is between there and batery

if there high restistance on pin 87 or the meter says something like ol. then the fault is between there and the lights.

if no voltage at 86/85 then the problembs between there and the switch.

if high resistance on the other pin. then fault between there and earth.

hope this helps. if you need help. call me 0212158039 i am in wellington

[Huttey 0]

brakelight switch will be around your brakepedal somewhere. not sure on exact location. u should be able to figure out where when you look.

heres what u need to do.

1. get a multimeter.

2. conect meter between light pos and earth. if it reads 12.5ish volts the fault is between the lights and ground.

if no voltage do the following.

3. conect the multi meter before the switch and earth is somewhere. u should get around 12.5ish volts.

4. test for voltage after the switch when you push the brake pedal. u should get same voltage.

5. if you can find the brake light relay test for voltage at pin 86 or 85. one of them should have voltage when you push the brake pedal. and if you flick the multimeter into resistance. the little horse shoe symbol. and conect the other terminal. (without voltage) to the multimeter and the other wire on the meter to earh you should get a low reading. prefrably under 1 ohm

6. test for voltage in at pin 30 on the relay. and check the resistance of pin 87 and earth. that should indicate what part of the circuit the fault is it. if theres no voltage at 30 when conected to meter and ground then the fault is between there and batery

if there high restistance on pin 87 or the meter says something like ol. then the fault is between there and the lights.

if no voltage at 86/85 then the problembs between there and the switch.

if high resistance on the other pin. then fault between there and earth.

hope this helps. if you need help. call me 0212158039 i am in wellington

Thanks I will try this out and let you know how I get on

Regards Pete

[Huttey 0]

Well I got a new brake light switch, installed it and now my brake lights are on all the time when the key is on (whether my foot is on the brake pedal or not), the brake lights are not on when the key is off.

Can someone advise where the brake light relay is on an Auto 1999 E39 540 ?

Regards Huttey

[hotwire 352]

Well I got a new brake light switch, installed it and now my brake lights are on all the time when the key is on (whether my foot is on the brake pedal or not), the brake lights are not on when the key is off.

Can someone advise where the brake light relay is on an Auto 1999 E39 540 ?

Regards Huttey

Maybe obvious question but is the switch adjusted correctly? Particually if this is all you have done since the lights were no go. Not sure on exact layout of switch on the pedal assy - never had to go there on an E39. Make sure that the switch is being compressed when pedal is released though. To confirm switch operating - use a volt meter or test light to confirm power switching on & off on one side with brake application - as previously mentioned.

System on these cars a bit more complex - lights are controlled by the LKM unit - dont think there is a relay involved in the circuit

[jochen 4]

Can someone advise where the brake light relay is on an Auto 1999 E39 540 ?

There is no relay:

"Brake light switch The brake light switch is permanently monitored by the LCM when terminal R is activated.

It is designed as a Hall sensor and features 3 lines (terminal R, terminal 31, signal line). The test switch is not necessary."

Additonally...

Brake-light switch cable

<h2 class="chapter">Possible cause(s) of fault:</h2>

• Brake-light switch incorrectly set
• Cables to the brake-light switch defective
• Brake-light switch defective

<h2 class="chapter">Storage condition(s):</h2> If the control unit for traction control detects that the brake-light switch has been activated for longer than 60 seconds, but at the same time braking appears illogical, this fault is set. Simultaneous operation of the accelerator pedal module, for example, is an argument against braking.

<h2 class="chapter">Troubleshooting procedure:</h2> If the fault is currently not present, there is no change in status when the brake-light switch is actuated, i.e. the success of the repair can be seen from the renewed change in status of the brake-light switch. The change in status can be read out via the diagnosis queries.

• Adjust brake-light switch
• Check the cables between the brake-light switch and control unit for traction control. Signal name (depending on model series): S_BLS or BLON or 54
• Replace brake-light switch

Pin 4 of the brake light switch (S_BLS signal, blue/red wire) is connected to pin 25 of the light control module (LCM)

The LCM switches on the brake lights electronically

That's why you know where the fault is without even looking: if all 3 x bulbs were blown, the LCM would tell you 'CHECK BRAKE LIGHTS', and you'd know you had blown bulbs.

As the LCM controls all 3 brake lights separately, - each has its own transistor driving it - a fault that affects all 3 lamps at the same time cannot plausibly be the LCM.

Your issue now is simply the switch adjustment.

Note that the switch also feeds the DME engine control unit, so the engine can also provide braking power when braking.

J

[jochen 4]

brakelight switch will be around your brakepedal somewhere. not sure on exact location. u should be able to figure out where when you look.

heres what u need to do.

<snip>

Sorry sydwayz, but the old E30 way of diagnosing lights and relays doesn't work on the E39

The brake light switch is a hall-effect sensor, you cannot measure it with an ohmmeter.

There is no brake light relay.

The LCM switches the lamps on and off using short-circuit proof transistors, and monitors the bulbs individually, and hot and cold.

Regards, Jochen

[Huttey 0]

Thanks Jochen, this is the complete story so far, in case something in the process may have caused this:-

- Initially the "check brake light" started showing but the brake lights were working.

- then all of the brake lights stopped working, "check brake light" message still showing

- changed 2 lower bulbs and fuse 41 no change

- fitted new switch, no change, lights still not working

- went to an auto electrician, he played around with switch with an amp/volt meter and the lights started working with key on, didn't work with key off, depressing brake pedal made no difference with key on or off

- unfortunately he had no BMW manuals, so advised me to go to BMW and suggest to them the brake relay may be stuck, they weren't interested in having a quick look and suggested I book it in for a service

- swapped old switch back in, same results as with the new switch, so something has changed

- went for a drive, tried cruise control, turns on but won't set and hold speed, probably because it thinks the brake is being pressed

PS - wife ran the battery flat about a month ago, not sure if this is relevant.

Regards Huttey

[Huttey 0]

bump

[Huttey 0]

Interesting

All fixed - it was a problem with the rear window stop light socket which needed to be replaced. The interesting part is (as I was advised by the experts) if there is a severe problem with stop lights which will result in them not working, the cars electronics defaults the status to on-always as a safety precaution.

We live and learn