My 3rd Brake light stops working after I start my 2003 V70

padzilla · Post by **padzilla** » 27 Nov 2017, 22:30

Hi Everyone!

I was hoping I could get some help diagnosing the 3rd brake light in my 2003 Volvo v70. I got this car as a Christmas gift from my aunt last year (love it so far!) and would love to be able to do this myself as I’m pretty good at working on my cars. I’m no expert but I’ve managed to do a fair bit of work on them over the years.

Here’s what’s going on. The rear 3rd brake light is not functioning properly. Ironically it works if the keys are out of the ignition or the keys are in the ignition and turned to the 1 position. The moment I turn the keys to the 2 or 3 (starting the car) the 3rd brake light no longer works. A computer issue perhaps? This has been very consistent the last couple of months, however I can remember at least 1 time where I went on a drive and saw the 3rd brake light working when I returned home and backed into my spot. To my knowledge that was the only time it’s worked properly in a couple months. The left and right brake lights work just fine however.

Any ideas on what’s causing this or how to start diagnosing it? I’m not very knowledgeable when it comes to car electronics and would love to know more about how that brake light is controlled.

Thanks for reading!

Alex

Post by **RickHaleParker** » 27 Nov 2017, 23:39

The third brake light is controlled and grounded by the Rear Electronic Module (REM). The REM is located behind the rear most panel on the left side of the car. Drop the tailgate, put your left hand against the wall, that is where the REM is. My best guess is there is a defect in the REM or in the Data that the REM uses to decide when the third brake light comes on.

Before you tear in to it, go for a drive and have somebody follow you. There is a chance the third brake light only comes on under certain conditions.

Post by **jimmy57** » 28 Nov 2017, 07:04

This still can be a bad wire in the harness. It could be grounding through something and when that item is powered up when switch is on then it loses ground. LED lamps need such little current so they can ground through things that would make a regular filament bulb very dim. Put something between front of seat and brake pedal and adjust seat forward to depress brake pedal so brake lights are on. Then go slowly raise tailgate to see if the brake lights come on during the travel or when it gets open.

Georgeandkira · Post by **Georgeandkira** » 28 Nov 2017, 08:09

Wow, a 9 hour old membership! I'd welcome you aboard but I'm not on the welcoming committee.
Cheers, G&K

padzilla · Post by **padzilla** » 30 Nov 2017, 21:59

Thanks for all the help everyone. Really appreciate. Gonna start with the REM and go from there.

Cheers!

Alex

Post by **RickHaleParker** » 01 Dec 2017, 03:02

Gonna start with the REM and go from there.

You might want to take jimmy57's suggestion first. The wiring in the D pillar near the hinge sometimes fails due to years of movement and it is a lot easier to troubleshoot then the REM. The REM is a microcomputer control module ...a lot more complicated. The harness in the D pillar is just wiring.

Post by **chrism** » 01 Dec 2017, 20:00

If you decide to go after the tailgate wiring, which is probably a good choice, this might help:

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