Brake lights

[spudd]

Have a 2003 xc90 that has brake light failure. The upper tailgate light works but not the others. replaced bulbs no difference, Can hear relay clicking but no difference. I replaced the relay (what fun)but still no brake lights. I assume as I can hear the relay the pedal switch is ok.
Any ideas please.

[SwissXC90]

If I recall correctly, the brake lights operate over 2 independent circuits (for reliability reasons):

One circuit for the center high-mount LED brake lamp

Another for the left and right taillamp brake lamps

Maybe they are on 2 circuits on the brake pedal switch.
VIDA has full details, if I remember I'll look it up tonight....

[SwissXC90]

Here you go, extracted from VIDA

The brake lamp switch is a single pole switch positioned at the brake pedal lever. The brake lamp switch self-adjusts during installation.

The brake control module (BCM) receives information about the status of the brake lamp switch via the Engine Control Module (ECM). The brake lamp switch signal is used by the brake control module (BCM) to diagnose faults in the pedal position sensor.
The brake lamp switch is supplied with power from the battery. When the switch is closed, the signal is grounded through the Engine Control Module (ECM). The Engine Control Module (ECM) then transmits a signal on the Control area network (CAN) indicating that the brake lamp switch has been activated.
There are diagnostics for the brake lamp switch.

Pedal position sensor

The pedal position sensor is mounted on the servo unit in the engine compartment. The sensor has several functions. It is used to:

register rapid pedal movements for emergency brake assistance
shut off the traction control during braking
to diagnose the function of the brake lamp switch
it is used by the Engine Control Module (ECM) for cruise control. The signal is transmitted via the control area network (CAN).

The sensor, which responds to the push rod for the pedal, is a sliding potentiometer. 0 mm corresponds to 0 V and 38.5 mm movement corresponds to approximately 95 % of battery voltage (Ubat).

Brake pressure sensor (only DSTC/ESC)

Two brake pressure sensors are positioned on the master cylinder. One sensor is located on the primary circuit and one on the secondary circuit. The information is transmitted via two analogue signals (measurement range 0-5 V) and is used by the brake control module (BCM) to calculate the brake pressure during active yaw control, and when the driver applies the brakes.
The sensors are supplied with voltage via the brake control module (BCM). There are diagnostics for the sensors.
When replacing the brake pressure sensor, the brake control module (BCM) must be calibrated according to the information in VIDA vehicle communication or manually according to the test drive.

So the brake lamps are controlled by CAN bus signals from the ECM through the CEM to the REM.
On the REM the brake lamps are on pin B3 (Yellow wire)

The center mount brake lamp is on a separate wire.

[spudd]

Sorted was a broken wire to the relay.

[Harley Dave]

Thanks for close out.

It's always good to have the issue sorted and the fix recorded.

Cheers

Dave