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Discussion Starter · #1 · (Edited)
Lamp failure indicator light has been faulty for years and I finally decided to try my hand at fixing it without bypassing it. In my case all the rear lights work fine it was just the detection that was not working properly.

First attempt was to look for any obvious burnt components to replace but those all looked fine. I cleaned things up with alcohol and a toothbrush but no change. So I took it back out again and on the back I noticed that there were some dark areas on some of the traces and my scrubbing with the toothbrush earlier had started to expose the copper underneath. I did some some continuity testing on the solder spots in that trace and sure enough I found that it was not contiguous across the whole area. Using the EWD it looks like this trace is connected to pin 9 on the plug which is ground, so a fairly important trace.

Examples of what the corrosion looked like.
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I ended up identifying 3 "zones" of continuity in my testing.
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Next I took a tiny flat head screwdriver and started scraping away at the corroded areas to get rid of it best I could around the back of the board. The layer of copper is very thin and after removing the corrosion I was able to see where the copper had corroded entirely away creating the break in continuity.

Examples of the copper being corroded completely away and braking continuity. The first couple images are a part of the trace that is very thin and I wouldn't be surprised if it's the first to go for most of us.
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After hitting all the corroded spot that I could find I wanted to test if my issue is just that broken ground trace so I taped on some wires to connect the 3 zones together and gave it a trial run.
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Success!
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Testing that the sensor is actually functioning correctly and can tell when a tail light goes out again. Success!
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Now on to the fix in the next post.
 

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Discussion Starter · #2 · (Edited)
The fix!

I took one more pass to scratch/scrub off any visible corrosion and cleaned things up with alcohol. Then I soldered in my first wire to connect 2 of the zones together. I used two existing solder points and added just a tiny bit more solder, careful not to accicentally connect it with another trace.
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Using clothespins to help hold the wire down to connect the last "zone" and complete the full trace circuit.
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Finished result, pretty happy how it turned out :)
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Didn't want to leave the open trace copper exposed so I put a couple coats of liquid electrical tape over those areas.
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Going in a plastic baggy this time
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Final success!
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Nice. I've fixed mine a couple of times just by reflowing all the solder joints, but corrosion wasn't the issue.
 

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To further protect the board from heat/humidity and future corrosion you might consider using a layer of conformal coating over the whole circuit board. You can find this stuff on Amazon in spray cans- an acrylic version will do the job nicely and will seal off all of the traces and solder joints from future damage.
 

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As an aside, the box can be modified to work with LEDs properly. I show how here
 
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