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My car is an old (2001) Volvo V70.

Last year I experienced intermittent failures of the brake lights, and it led my mechanic to exchange the break pedal switch. This seemed to work fine for some months.

But, now my brake lights have completely failed.
Obviously we checked all the involved fuses and the issue is not here.

Now my mechanic suspects it might (or "should", he says) come from the big black box located in the trunk, near the rear left wheel. It contains a number of fuses, but also a lot of electronic components (relays?) which are not described at all in the vehicle documentation.

Is there some way to identify which of these components might be responsible for the brake lights failing?

  • I have the same year, make, model, and problem. I also checked the pedal switch, fuses, and there are relay's in the rear fuse box. It is hard to find any info on these relay's but I am fairly sure it is shown in this picture. – user50153 Jul 5 at 21:00
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My suggestion would be to chase the wiring directly rather than trying to identify all the parts of a system your mechanic did not even identify for you.

If possible, find an electrical diagram for the car online. A Google search for 2001 volvo v70 wiring diagram turned up many images of wiring diagrams that should be very helpful.

To trace this back, start at the lights themselves. I suspect the bulbs are fine, but best to be thorough and methodical, you never know. With a rod pressed against the seat and the brake pedal to keep it depressed, see if there is voltage at the bulb bases. (You will probably want to protect the seat with a wood 2x4 or something similar to spread the load rather than press it directly into the seat)

If there isn't any voltage present, use the diagram to tell you where to look next. I'm guessing here but I suspect the fuse for this circuit is as close to the battery as they designers could make it, so it's probably the foot switch. The idea is to divide and conquer. The flow of power to the brake lights is a chain, and any break in the chain will cause them to fail. You are looking for that break. The diagram is your map to the entire chain, from the battery to the brake lights.

If perhaps this box in the trunk is actually in that chain somewhere, the diagram will tell you where power comes into that box, and where it comes out to go to the brake lights. Check the input and output to see if it is the box. If it is, then you will need to identify what it is so you can determine how it works. There is almost certainly a part number molded into the plastic case that can be Googled, or, your local Volvo dealer may be able to identify it for you from the part number.

I hope that helps!

  • Wow, what a rich answer! And exactly what I looked for. Explanation: I have no skill at all about engine electrics, and my mechanic have no skill at all about internet :) But now, thanks to your guidance, I can Google data though I don't understand them, and will give them to my mechanics. Thanks a lot. – cFreed May 23 '16 at 21:11
  • You're very welcome. One other possible source for a wiring diagram is the Chiltons repair manual. They have an online version that might be helpful if they have the repair manual for your car. Just Google chilton 2001 volvo v70 – cdunn May 23 '16 at 21:16
  • Yes, they have it! And it seems very complete. Thanks again. – cFreed May 23 '16 at 21:43
  • You're very welcome :) – cdunn May 23 '16 at 21:51

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