2003 v70 dim high mount brake light

Googs1080 · Post by **Googs1080** » 12 Mar 2015, 11:06

Thought I would start a new thread on my high mount. Just repaired frayed and broken wire bundle in the tailgate to restore the high mount brake light. The light comes on now when brake pressed but it is dim. I measured the voltage at the light and at the repaired section. Same voltage at both places. Voltage fluctuates from 7 to 9 volts. Think should be 12. you can watch the light brightness change as the voltage changes. Anyone have ideas on what might be happening and where I should be looking? the regular rear brakes are nice and bright and normal. wondering if there is another kink in those wires somewhere down the line or something else is causing the issue.

Post by **chrism** » 12 Mar 2015, 12:43

I wonder if the chronic shorting/grounding may have damaged a relay. Does the voltage fluctuate on a repetitive cycle?

Googs1080 · Post by **Googs1080** » 12 Mar 2015, 14:40

i cant tell if there is a repetitive cycle. I will watch them longer and see if I can see a pattern. Still cant find any relay that is for the high mount light. maybe a relay is damaged as it went really really bright when I had it hooked up at first and now this dim wavering thing

Googs1080 · Post by **Googs1080** » 15 Mar 2015, 21:40

I tore the back end apart to get to everything. At the connection harness on the driver side d-frame I found and measured the voltage. First I measures the voltage on the female side of the harness sticking my probe into the back end of the harness at the pins I identified for the high mount. When brake is pressed I still only got 7 volts. When I disconnected the harness and measured voltage on the proper male ends, I got 13.3. Could it be that the female end of the harness has damage due to the short circuits and cut wires that ran to the tailgate? Or maybe the male end isn't long enough anymore for mailing to female ends? I am open for any further suggestions on what to test or do next. I tried jumping wires from the male ends to the high mount but couldn't get the light to come on when brake pressed. Looking at the harness it is the 2nd from bottom left and third from bottom right which I identified as the high (mount pins?) Can I remove the pins from the harness itself and test further? I didn't see a way to do so.

Sardine · Post by **Sardine** » 16 Mar 2015, 00:20

Hi Googs1080. I see you still haven't got to the bottom of your problem yet.
Explain clearly what you are doing and may be I can help.
Are you saying that with the male and female ends of 54/78 connected, the voltage at pin 3 (relative to ground) is 7 volts. With the male and female ends of 54/78 disconnected, the voltage at pin 3 on the male end (relative to ground) is 13.3 volts? If that is what you are saying then there may be there is nothing wrong with that. The current through the High Mount Brake Light is responsible for the drop in voltage.
Here is what I would do to isolate the problem:
1) To make sure that you did not connect the wires incorrectly: a) Jump the wire from pin 3 on the male end to the High Mount Brake Light pin 1 (like you did before.); and b) Connect a ground to the ground of the High Mount Brake Light, i.e. pin 5 of the High Mount Brake Light (10/19). The High Mount Brake Light needs a +ve voltage at pin 1 and a ground at pin 5 to work. I assume here that you have somehow disconnected the High Mount Brake Light from its harness, including its ground. If the light is still dim then either the High Mount Brake Light has failed or the output from REM (REM pin D:4) has failed. When you say dim, make sure that it is actually abnormally dim. The High Mount Brake Light may be naturally dim, I know mine is not that bright.
2) If the light is now OK then measure the voltage to know what the correct voltage should be, then redo your your wiring, correctly this time.

Googs1080 · Post by **Googs1080** » 16 Mar 2015, 08:49

Thank you sardine. Dim meaning you can only see it during a dark night. I tested the voltage on the male end of harness by applying my probes to pin 3 and the other pin for the high mount. Should i get same voltage if i use any ither ground pin on the male side of the harness? Or what should i use as ground to measure pin 3 voltage? I tried the jump from male pins to the high mount but i dont think my jump was done well. I just put my jump wires into the pin 1 and 5 space in the high mount harness presuming the contact would be enough to show me something of a connection. If i test the female of the harness with the high mount disconnected then i should receive the same 13 volts to prove that the high mount is making the drain to 7 volts. Maybe i need to make better jumping wire arrangements to test proper but i thought i had good contacts for my testing

Googs1080 · Post by **Googs1080** » 16 Mar 2015, 09:36

Here is update, i spliced the light into the correct wires before the harness. Measured without high mount and got 13.3 volts. Then i spliced the high mount harness into my jump wires. LED was barely visible and voltage dropped to 7 volts measured at the jump splice. It waivered some and got to 8 volts then 9 volts for nanoseconds and went back down to 7 volts. I think the REM is bad based on the fluctuations. It should receive load and then spool up to provide the proper voltage i would reckon as the light says 12v in plastic. Make sense? Going to buy new rem and see. I tested and got same results with second high mount light i have

Googs1080 · Post by **Googs1080** » 16 Mar 2015, 11:40

ugh, I switched REM 5 with the one below it. Still only get 7 volts to the high mount. I tried swapping #5 and #6 shunts to see if they were causing an issue but no success. Unfortunately, when I put the swapped REMs back into their spots, none of the rear brake lights now work. Did I damage a REM or a shunt? I checked the #3 fuse and it was still good.

Sardine · Post by **Sardine** » 16 Mar 2015, 13:59

I don't follow what you are saying here at all. What are "REM 5" and " #5 and #6 shunts." The word "REM" should only be used to mean "Rear Electronic Module", that is the whole module in the picture you posted a while back. You seem to use the word REM to mean a relay. As I explain in one of the posting a while back, relay 2/79 (and shunt 20/31) operates the Left and Right brake lights. None of the relay or shunt has anything to do with the High Mount Brake Light. The High Light Brake Light is driven directly from REM output D:4.
If none of your Left and Right brake lights now work then the entire signal path for these two lights should be checked. These are: fuse 11D/3, relay RMI5 (or 2/79), shunt RSH6 (or 20/31) and fuse 11D/14. You just have to get back to where you were when just the High Mount Brake Light was faulty.
If you read my previous post explaining the schematic, you will see that none of the relay or shunt has anything to do with the High Mount Brake Light. It is a very simple module with pin 1 driven directly from the REM, and a ground connect to pin 5.

Googs1080 · Post by **Googs1080** » 16 Mar 2015, 19:26

Sorry yes relay 5 and relay 6 is what I was swapping just to see. That killed both back brake lights. I put a new correct relay in for 5. That got the passenger side back brake light to work. The drivers side is still dead. Bulb is good. I measured the voltage at its harness and had 13.3 volts when brake was activated. I measured the voltage at the metal plate in the driverside light assembly (where you turn the bulb into) and got 13.3 volts, but the driverside brake light wont come on. So I know I have power all the way to the bulb socket assemblage. I even switched bulb holders from passenger side and driverside to see but still no driverside brake light. I did a test based on a previous response and jumped the highmount to the driverside harness and viola the highmount was working perfectly bright and lovely when the brake was pressed. I am going to try and find REM output D:4 . Looking at the actual REM, I am not clear where to look to figure out which one on the backside is D:4. If I find low voltage at D:4, then what would be next to test or address?