Brake Lights Inoperative

Post by **chrism** » 15 Feb 2022, 17:03

2003 V70 Base model, 250,000 miles.

It all started with a stop light failure message. I found the RH brake light bulb was burned out. LH brake light bulb was working fine. When replacing the socket with a new bulb inside the tail light assembly the plastic crumbled away where the socket locks in due to old age/heat damage. So I purchased a new OE upper RH tail light assembly. Put the bulbs in the new tail light and connected the wires. Now NEITHER the LH or RH brake lights work. I have brake light power at least as far as the 15A fuse in the REM, PLUS the center tailgate brake light still works OK! I don't have a copy of the actual wiring diagram other than a Haynes version. Apparently the differences between the way the RH and LH brake light wiring differs from the center brake light wiring is that the center light is powered by a yellow wire straight from the REM whereas the RH and LH bulbs are fed power via a relay and a shunt on the REM. I'm starting to suspect one, or both, of those components.

So I'm wondering if anyone has real a Volvo wiring diagram for the brake light system. Also, can anyone tell me what locations on the REM I would find the relay and shunt for the brake lights? Is it OK to pull out either component and run jumper(s) in order to determine which piece isn't working right?

Thanks for any help!

Post by **firstv70volvo** » 15 Feb 2022, 19:18

This wiring should be correct up to 2004 MY cars. I would check for power at the sockets with brake pedal applied. The power to the left/right brake (10/50 and 10/43) lights goes through the RMI5 2/79 relay (see pdf) and fuse 3.

brake lights wiring.pdf: (236.14 KiB) Downloaded 322 times

Post by **chrism** » 15 Feb 2022, 21:19

firstv70, thank you! That's just what I needed.

I'm having trouble reading the small text on the pdf. 20/31, which I assume to be a shunt, also looks to be a possible suspect as the yellow wire from the relay (2/79) runs through it on the way to the tail lights. Can you tell me in what position 20/31 is located? The text is a bit too fuzzy.

Post by **firstv70volvo** » 15 Feb 2022, 23:05

chrism wrote: ↑15 Feb 2022, 21:19 firstv70, thank you! That's just what I needed.

I'm having trouble reading the small text on the pdf. 20/31, which I assume to be a shunt, also looks to be a possible suspect as the yellow wire from the relay (2/79) runs through it on the way to the tail lights. Can you tell me in what position 20/31 is located? The text is a bit too fuzzy.

20/31 RSH6

REM relays and shunts.pdf: (205.5 KiB) Downloaded 227 times

Post by **chrism** » 16 Feb 2022, 05:39

Perfect!! Thank you very much!!

Post by **Krons** » 16 Feb 2022, 07:51

chrism wrote: ↑15 Feb 2022, 17:03 Now NEITHER the LH or RH brake lights work. I have brake light power at least as far as the 15A fuse in the REM, PLUS the center tailgate brake light still works OK!

I had this symptom (no left or right taillight but functional center brake light) and it was a bad relay for the brake circuit. On this 02 S60 the relay was in the trunk on left hand side. I don't recall which one it was...

Post by **chrism** » 16 Feb 2022, 09:06

Thanks Kron. I appreciate the response. Firstv70 posted the wire diagram and REM map so I know which relay, and possibly shunt??, that I need to focus on. I’m not at the car right now but my plan is to swap the relay with a neighboring one to see if gets the brake lights working and kills some other function.

I’m curious as to what function the shunt provides, and how likely it is to fail.

Post by **firstv70volvo** » 16 Feb 2022, 10:14

chrism wrote: ↑16 Feb 2022, 09:06 Thanks Kron. I appreciate the response. Firstv70 posted the wire diagram and REM map so I know which relay, and possibly shunt??, that I need to focus on. I’m not at the car right now but my plan is to swap the relay with a neighboring one to see if gets the brake lights working and kills some other function.

I’m curious as to what function the shunt provides, and how likely it is to fail.

The shunt is used to detect a bulb failure, not likely to fail, just a low resistance resistor, more likely the relay.

Post by **chrism** » 19 Feb 2022, 13:02

I ran over to Voluparts and picked up a new relay and still no LH or RH brake lights. So I swung the REM down and out so that I could follow the circuit in and out of the relay and then in and out of the shunt which is downstream from the relay. I had power all the way through and out of the shunt but it never reached the tail light connectors. I found I could jump 12v directly to the LH tail light and it would illuminate BOTH RH and LH bulbs. So apparently the single yellow power wire, that leaves the shunt, splits into two wires somewhere between the shunt and tail lights. I suspect that split, wherever it may be, has opened in a way that both RH and LH brake lights are still tied together but are no longer energized by the wire out of the shunt. The schematic I was using didn’t show any disconnects or junctions between the shunt and the tail lights, other than the disconnects right at the tail light housings. The schematic shows that there are separate RH and LH yellow wires joined at the same pin on the shunt. That isn’t in agreement with how my car is really wired. I also read somewhere that there is another disconnect between the shunt the and tail lights and maybe an additional fuse??? I finally said the heck with this and I ran a piggy back wire from the shunt to the LH tail light disconnect which energizes both LH and RH brake lights. And it does so without the “Check Stop Light” error message. Good enough for a 19 year old car. Thank you to first v70 and Krons for the documentation and personal experience.