Involuntary Brake Lights

[Q-car]

At the moment I am trying to diagnose a problem which involves the brake lights coming on when the ignition is switched off and the car locked and secure. The brake pedal switch has been looked at and would not appear to be the problem. I am now trying to establish whether it has something to do with the locking system. ie do they only come on when the vehicle is locked. Somebody has suggested that it may be the control box malfunctioning. Was just wondering if this problem had been recorded or if anyone else has had a similar experience. Many thanks. Mike.

[Tim]

First time I've heard this one Mike. Wonder if its arcing across something like the security system which is one of the sub systems that stays live.

[Q-car]

Thanks for replying Tim. I must say it is rather frustrating, not being able to isolate the problem by simple means. I have now established that the brake lights come on - sometimes immediately, other times after a number of hours - whether the car is locked or not. I suppose at the end of the day it's going to be down to one of the systems controlled by an electronic-cum-computerised circuit. We'll get there in the end!! Maybe I should update the post when resolved?

First time I've heard this one Mike. Wonder if its arcing across something like the security system which is one of the sub systems that stays live.

[Nich]

Although it may well be an electronic fault in one of the 'black boxes' it might well be something simpler. I would work on the basis that the current to power the brake lights has to come from somewhere with the ignition switched off and trace the loom back to the brake light switch checking for any signs of wear and tear on the cables and then check the switch itself. There are a number of connectors and headers in the circuit which may well be 'shorting' out if the terminals have corroded. Do the brake lights work normally when the car is running or is it just when the ignition is off that they illuminate by themselves?
Have you got the wiring diagrams Mike?

Nich.

[Herman]

Brake lights are driven from the light module. There is no direct connection between brake pedal switch and lights themselves.
Brake switch feeds into the LSM and the LSM controls the lights.

• How is your battery
• Check for fault ground connections
• If you pull the brake switch plug off, does it still happen
• Do the other light function normally

My guess is either LSM or a bad connection

[SteveW]

Had a similar issue appear on mine, but not quite the same.
If my wiper stalk was left in any other position then off, the wipers would start to operate at random intervals, even when the car was locked & alarmed.
I found a small amount of water on the RHS of the boot, and the Sat Nav DVD unit had signs of corrsion on the casing.
I removed the DVD player and the odd wiper problem went away.

[Q-car]

Thanks Nich for your advice. Not being too technical myself, I will pass on this information to a man who is. The brake lights however function correctly when the ignition is switched on and vehicle being used. The problem is when the car is stationary and ignition off (locked or not). Oh and no I don't have a wiring diagram - maybe I should!

Although it may well be an electronic fault in one of the 'black boxes' it might well be something simpler. I would work on the basis that the current to power the brake lights has to come from somewhere with the ignition switched off and trace the loom back to the brake light switch checking for any signs of wear and tear on the cables and then check the switch itself. There are a number of connectors and headers in the circuit which may well be 'shorting' out if the terminals have corroded. Do the brake lights work normally when the car is running or is it just when the ignition is off that they illuminate by themselves?
Have you got the wiring diagrams Mike?

Nich.

[Q-car]

Thanks Herman - 1. Battery appears to be A1 although I have to periodically trickle charge when flat.
2. OK - will do.
3. Have tampered with brake switch plug but no change to problem.
4. All other lights function correctly

• How is your battery
• Check for fault ground connections
• If you pull the brake switch plug off, does it still happen
• Do the other light function normally

My guess is either LSM or a bad connection

[Herman]

Thanks Nich for your advice. Not being too technical myself, I will pass on this information to a man who is. The brake lights however function correctly when the ignition is switched on and vehicle being used. The problem is when the car is stationary and ignition off (locked or not). Oh and no I don't have a wiring diagram - maybe I should!

Maybe this helps.
ewd_260_04MY_EN.pdf

[Q-car]

Interesting one Steve. I have had issues with damp but my boot is currently dry since replacing the rear light seals. However damp does effect the driver's footwell and should be considered along with the other suggestions. Thanks very much to all.

Had a similar issue appear on mine, but not quite the same.
If my wiper stalk was left in any other position then off, the wipers would start to operate at random intervals, even when the car was locked & alarmed.
I found a small amount of water on the RHS of the boot, and the Sat Nav DVD unit had signs of corrsion on the casing.
I removed the DVD player and the odd wiper problem went away.