R class brake lights on with ignition

[SkegVegas]

2006 R500 with brake lights on all the time the ignition is on. The ABS module is recognising the brake pedal position.
We have disconnected the trailer wiring from the car. Replaced the rear SAM with a new one from Mercedes. Still the same. It only has a single filament bulb for tail and stop. The rear SAM increases the voltage. All stop lights including the high level are on. Checked the rear wiring loom from SAM to rear lights. It is only showing one code in rear SAM, The lnput of circuit 54 has a short to positive. 909A this is a hard and stored code.
Any help especially on circuit 54 or on how the rear SAM gets its command to operate the stop lamps gratefully received.

 

[ALFAitalia]

I don't know these cars.....but I assume you have checked the switch on the brake pedal itself......most cars have them.

 

[richard]

The rear SAM has a circuit 54 input (connector C pin 2) which is stuck positive (and causing it to switch on the brake lights),. This is fed from the ESP module. Look at actual values in the rear SAM, ESP and front sam to see if you can see where it reads the brake light switch and check that is not stuck on. If you can see brake light switch actual value operating OK (when someone presses the brake pedal), then Check the voltage on connector C pin 2 (yellow/white or pink/white) with the plug disconnected from the SAM and see if it is at +12. If it is not, then you have a SAM issue. If it is at 12, go look at the ESP module and see if you can see why it is outputting +12 on that line.

Hope that helps
R

 

[SkegVegas]

The rear SAM has a circuit 54 input (connector C pin 2) which is stuck positive (and causing it to switch on the brake lights),. This is fed from the ESP module. Look at actual values in the rear SAM, ESP and front sam to see if you can see where it reads the brake light switch and check that is not stuck on. If you can see brake light switch actual value operating OK (when someone presses the brake pedal), then Check the voltage on connector C pin 2 (yellow/white or pink/white) with the plug disconnected from the SAM and see if it is at +12. If it is not, then you have a SAM issue. If it is at 12, go look at the ESP module and see if you can see why it is outputting +12 on that line.

Hope that helps
R

Thanks for that. The ABS module reads the brake pressure switch and is not constantly on. I will check the wiring on circuit 54 and let you know what we find. Thanks again. If the rear Sam lost the signal wire from the ABS module could this bring the brake light on as a safety feature. Like the cooling fan going balls out on some cars.

 

[richard]

Probably - also it may be that circuit 54 is how the SAM knows to put the brake lights on (rather than it being on the CAN) - theres never any info as to exactly how things work ...

 

[SkegVegas]

The rear SAM has a circuit 54 input (connector C pin 2) which is stuck positive (and causing it to switch on the brake lights),. This is fed from the ESP module. Look at actual values in the rear SAM, ESP and front sam to see if you can see where it reads the brake light switch and check that is not stuck on. If you can see brake light switch actual value operating OK (when someone presses the brake pedal), then Check the voltage on connector C pin 2 (yellow/white or pink/white) with the plug disconnected from the SAM and see if it is at +12. If it is not, then you have a SAM issue. If it is at 12, go look at the ESP module and see if you can see why it is outputting +12 on that line.

Hope that helps
R

Bit of an update. Checked wiring on pin 2 connector c. Has voltage when brake pedal pushed. No voltage when not pressed. Strangely with connector c unplugged the brake lights work perfectly. So it must have another way of knowing the pedal has been pressed. I am considering cutting pin 2 wire to see if it still works. Before I do it do you have a wiring diagram for connector c.
Thanks for the help

 

[SkegVegas]

Probably - also it may be that circuit 54 is how the SAM knows to put the brake lights on (rather than it being on the CAN) - theres never any info as to exactly how things work ...

Another update, we now have a constant 12 volts on pin 2. Do you have a wiring diagram showing where that wire comes from. Just scanned the abs module and the brake switch is operating correctly

 

[SkegVegas]

Another update, we now have a constant 12 volts on pin 2. Do you have a wiring diagram showing where that wire comes from. Just scanned the abs module and the brake switch is operating correctly

Just disconnect the abs module connector and now have lost the 12 volts on pin 2, it looks like the abs module to me. Weirdly the module is reading the brake switch correctly but still powering pin 2

 

[SkegVegas]

Just disconnect the abs module connector and now have lost the 12 volts on pin 2, it looks like the abs module to me. Weirdly the module is reading the brake switch correctly but still powering pin 2

We have cut the wire to pin 2 and all seems good. Still don’t understand why it needs 2 brake light inputs to the rear Sam.