2003 V70 High mount brake light

Googs1080 · Post by **Googs1080** » 07 Mar 2015, 20:04

Hi all,

The high mount brake comes on every 10 seconds. it lasts for just a second then will go back off until 10 secs later. The high mount light will not come on when the brake is pressed. The top rectangle thing in the rear fuse compartment clicks with the light. I swapped out that one with another one. The other one clicks every 10 seconds as well. Anyone have any ideas where to look or what is happening here? The left and right brake lights work fine.

Googs1080 · Post by **Googs1080** » 07 Mar 2015, 20:16

I should have said, when I swapped out the rectangle clicking with another one, the new one clicked as well. So I am thinking I have some problem upstream of the rear electrical maybe??I failed inspections so my days are limited before paying a dealer big money to find the problem. I feel and hear a click in the emergency brake when I press the brake pedal. I don't recall this before. Maybe unrelated.

Post by **chrism** » 07 Mar 2015, 22:43

"...So I am thinking I have some problem upstream of the rear electrical maybe..."

Follow the link below to a diagram of the brake light wiring:

http://www.volvowiringdiagrams.com/volv ... agrams.pdf

Scroll to page 71. You might check for voltage in the wire between the CEM and the REM to see if the signal is pulsing on the 10 second frequency. If so, the issue might be with the CEM.

Googs1080 · Post by **Googs1080** » 08 Mar 2015, 13:03

Sorry I couldn't follow/understand the diagram. should it be pg 68 or 71? 71 was just xc90 so wasn't sure. I cant really follow this to what wire I should test. Looking at pg 68 it looks like relay 4, shunt 4 and relay 7 are the ones I should check if I can get the CEM exposed? Would it make sense to switch around those relays or see if any of them are clicking? I definitely know it is relay 5 in the REM that is cycling if I read the schems correct. Truly appreciate any thoughts and advice. I guess I am a hammer mechanic: if my one hammer doesn't fix something, then grab a bigger hammer and try try again. lol

Sardine · Post by **Sardine** » 08 Mar 2015, 18:28

Since you don't read schematic, I took a look for you.
On page 71 (Wiring Diagram V70/XC70/V70R/XC90, 2004, TP397102), the break switch 3/9 sends a +12V signal via a yellow wire to connector 54/3LG pin 12 and then via a yellow wire to CEM pin B:4. CEM output is pin B:6 which is then sent to connector 54/43 pin 13 via a GN-Y (green/yellow) wire. From connector 54/43, a GN-Y wire sends the signal to REM pin A:4. One output from REM is pin D:4, a Y-R wire, goes to connector 54/78 pin 3. For V70/XC70, a Y wire connects the signal (+12V?) to the the "High Mount Light" relay. This is the signal path that you should concentrate on. The other output from REM is relay 2/79 that operates the left and right brake light. Since the Right and Left break light operates normally, the output from CEM is probably OK. The fault could only be either the REM output D:4 or the High Break Light module 10/19.
I'm not really sure why Volvo needs the flexibility of two separate outputs for the High and Left/Right. It looks like all three lights could be operated by relay 2/79. You could try disconnecting 10/19 pin 1 from 54/78 pin 3 and connecting 10/19 pin 1 to relay 2/79 or 54/51 pin 4 to determine if 10/19 is faulty.
The wiring diagram does tell you the locations for all modules, connectors etc.

Post by **chrism** » 10 Mar 2015, 14:41

"...I'm not really sure why Volvo needs the flexibility of two separate outputs for the High and Left/Right. ..."

I'm guessing that if they didn't break the stop light wiring into two separate circuits there would be no way to compare resistances for the bulb failure warning.