Brake lights not working

PVSPete · Jul 15th, 2021, 00:01

XC90 2011 brakelights in main light units not working but high level brakelight is. 2011 model so REM fusebox module is transistorised with no relays to replace. Bulbs / fuses fine. I've replaced the REM with a second hand one and no change. More testing has shown that the REM resets itself and the brake lights do actually work . . . until the sidelights are either switched on or come on with the ignition . . . at which point the REM shuts down the brakelights. So there must be a fault, but where? So I unplugged the main light clusters, one at a time and the tailgate wiring but everytime I activated the sidelight circuit, the brakelights stopped working. My guess is some sort of earthing fault or perhaps damaged wiring, but where? Any thoughts anyone?

SwissXC90 · Jul 15th, 2021, 11:52

At this stage you plug in VIDA and test each tail light circuit independently, and you should test the brake light switch as well.

VIDA can issue a command to switch circuits on and off, and you can thus test the entire switching very easily and reliably.

PVSPete · Jul 16th, 2021, 10:19

Thanks for the thought, however I don't have VIDA at this time. I'm happy that the brake light switch is working as it works the high level light ok and is only a single circuit switch with power in and switched power out. Individually, everything works, its just when both sidelight and brakelights are used at the same time that the fault occurs. More probing needed.

SwissXC90 · Jul 16th, 2021, 11:37

Do a thorough check of your ground connections

Measure the voltage drop from the ground of the lamp itself (measure at the lamp) to a solid piece of bodywork.

Measure ground voltage drop also at the REM.

Check the lamp wiring harness grounding point, and look for any corrosion damage.

PVSPete · Oct 14th, 2021, 21:26

The fault has turned out to be the CEM under the dashboard on the driver's side. No faults found with the wiring or lights themselves. Testing the brakelight output at the CEM (a green/yellow wire) I found that the output was at fault. I removed the CEM and sent it away for repair. There was no water damage (common problem) but the brakelight driver had failed. This was fixed and once reinstalled, brakelights worked again. The repairer said that a short circuit could have caused the problem - there is evidence of the brakelight wiring having been previously trapped and damaged by one of the studs that secures the light unit in place so perhaps this caused the failure.

Harley Dave · Oct 14th, 2021, 22:50

Good to have the problem fixed and even better to find the original (probable) cause

Cheers

Dave

Jul 15th, 2021, 00:01	#1
PVSPete Junior Member Last Online: Jan 5th, 2024 15:14 Join Date: Nov 2019 Location: Peterborough	Brake lights not working XC90 2011 brakelights in main light units not working but high level brakelight is. 2011 model so REM fusebox module is transistorised with no relays to replace. Bulbs / fuses fine. I've replaced the REM with a second hand one and no change. More testing has shown that the REM resets itself and the brake lights do actually work . . . until the sidelights are either switched on or come on with the ignition . . . at which point the REM shuts down the brakelights. So there must be a fault, but where? So I unplugged the main light clusters, one at a time and the tailgate wiring but everytime I activated the sidelight circuit, the brakelights stopped working. My guess is some sort of earthing fault or perhaps damaged wiring, but where? Any thoughts anyone?

Jul 15th, 2021, 11:52	#2
SwissXC90 Premier Member Last Online: Mar 20th, 2024 18:26 Join Date: Jan 2016 Location: Cross Country	At this stage you plug in VIDA and test each tail light circuit independently, and you should test the brake light switch as well. VIDA can issue a command to switch circuits on and off, and you can thus test the entire switching very easily and reliably. __________________ XC90 R-Design MY2009, Black Sapphire Metallic. HP-Sound, RSE, Nav, Tel, ParkingCam, BLIS, ParkingHeater, RestHeat, Removable Towbar, Summer: CRATUS 20x8 on Pirelli Scorpion Zeros 255/45, Winter: NEPTUNE 17x7 on Continental WinterContact 4x4 235/65.

Jul 16th, 2021, 11:37	#4
SwissXC90 Premier Member Last Online: Mar 20th, 2024 18:26 Join Date: Jan 2016 Location: Cross Country	Do a thorough check of your ground connections Measure the voltage drop from the ground of the lamp itself (measure at the lamp) to a solid piece of bodywork. Measure ground voltage drop also at the REM. Check the lamp wiring harness grounding point, and look for any corrosion damage. __________________ XC90 R-Design MY2009, Black Sapphire Metallic. HP-Sound, RSE, Nav, Tel, ParkingCam, BLIS, ParkingHeater, RestHeat, Removable Towbar, Summer: CRATUS 20x8 on Pirelli Scorpion Zeros 255/45, Winter: NEPTUNE 17x7 on Continental WinterContact 4x4 235/65.

Jul 16th, 2021, 10:19	#3
PVSPete Junior Member Last Online: Jan 5th, 2024 15:14 Join Date: Nov 2019 Location: Peterborough	Thanks for the thought, however I don't have VIDA at this time. I'm happy that the brake light switch is working as it works the high level light ok and is only a single circuit switch with power in and switched power out. Individually, everything works, its just when both sidelight and brakelights are used at the same time that the fault occurs. More probing needed.

Oct 14th, 2021, 21:26	#5
PVSPete Junior Member Last Online: Jan 5th, 2024 15:14 Join Date: Nov 2019 Location: Peterborough	The fault has turned out to be the CEM under the dashboard on the driver's side. No faults found with the wiring or lights themselves. Testing the brakelight output at the CEM (a green/yellow wire) I found that the output was at fault. I removed the CEM and sent it away for repair. There was no water damage (common problem) but the brakelight driver had failed. This was fixed and once reinstalled, brakelights worked again. The repairer said that a short circuit could have caused the problem - there is evidence of the brakelight wiring having been previously trapped and damaged by one of the studs that secures the light unit in place so perhaps this caused the failure.

Oct 14th, 2021, 22:50	#6
Harley Dave Volvo Driver Last Online: Today 09:48 Join Date: Mar 2016 Location: Basingstoke	Good to have the problem fixed and even better to find the original (probable) cause Cheers Dave

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