Hi everyone,

The LEDs in one of the tail lights of my car went dim and I am trying to fix it.

There is a small board that I am sure is not working properly since I have verified it with the board from other tail light.
As you can see in the picture, left one is the problematic one.

I have checked resistance readings for every single resistor and all look alright. When I supplied 12V to the board and was doing some continuity check on the board, I accidentally touched one of the transistors and all the LEDs started to work properly! The place that I touched with the cable can be seen in the video. After some research I found out that I simply connected Pin number 1 and 2 of a MJD31 (https://www.onsemi.com/pdf/datasheet/mjd31-d.pdf), which were BASE and COLLECTOR.

As my circuit knowledge is very limited, I am pretty lost and I don't want to just connect these pins to make it work. I would really appreciate it if anyone could help with identifying the problem on the board.

Thanks in advance!

 

Welcome to AAC!

There is a small board that I am sure is not working properly since I have verified it with the board from other tail light.

Does this mean that if you swap the boards, the defect moves with the board you've identified as bad?

I have checked resistance readings for every single resistor and all look alright.

In properly designed boards, resistors are unlikely to fail (unless they're being used as fuses to protect other circuitry). In-circuit measurements don't always give the expected results.

The place that I touched with the cable can be seen in the video.

Post a picture so we don't have to look at a bouncy 45 second video.

 

Does this mean that if you swap the boards, the defect moves with the board you've identified as bad?

Exactly.

In properly designed boards, resistors are unlikely to fail (unless they're being used as fuses to protect other circuitry). In-circuit measurements don't always give the expected results.

Is there a better/more reliable way to check?

Post a picture so we don't have to look at a bouncy 45 second video.

Here is the picture of the transistor pins that I connected.

 

Is there a better/more reliable way to check?

Without having a schematic for the board to see what else is connected to the resistors, one terminal would need to be disconnected.

Here is the picture of the transistor pins that I connected.

Are you able to verify that the emitter of the transistor is connected to ground, trace the components connected to the collector to the positive supply, and determine what's connected to the base?

 

What I see that looks like either water damage or overheating on the back of one of the boards, while the other board looks undamaged. Certainly water between an energized section of board copper and any grounded material will cause damage. So the problem might possibly be damage to that discolored section on the bottom side of the one board. That can be repaired with wire and solder, but some real soldering skill is required.

 

I checked the copper trace on the board and found the problematic part!

As @MisterBill2 guessed, the discolored part was the problem. One of the copper lines has a break on it, probably due to the water damage. (This specific tail light actually had a recall from the factory due the bad seal gaskets. So it is very likely that some water leaked inside the tail light.)



After some cleaning I connected the two ends with a cable to test. And Voila!

I am not sure if it is any safe to connect the ends this way though. Would you suggest anything to make it more reliable for the future? Would epoxy help? I will anyways use silicone on the gaskets to stop water leakage.

Here is my solution:


Thanks for the suggestions!

 

I would suggest spraying the bottom of the pc board with a clear epoxy paint after the repairs and cleaning.

 

@sghioto would it be okay if I use a hot glue on that part?

 

@sghioto would it be okay if I use a hot glue on that part?

If you mean to hold that wire down OK.

 

While I was putting everything back, the light flickered. So I am guessing that the connection is not that good.

Do you guys think that connecting BASE and COLLECTOR pins on one of the transistors is safe? (I read that it makes it a diode) Because it is easier for me to solder and it works really well.

 

Do you guys think that connecting BASE and COLLECTOR pins on one of the transistors is safe? (I read that it makes it a diode)

Would doing that affect other functionality?

Because it is easier for me to solder and it works really well.

Improve your soldering skills. The wire you used was overkill. You could have laid a #30 AWG solid wire over the trace that had corroded. If you couldn't solder a bare piece across the part that opened, you could use insulated wire between the two points where you tried to solder the big wire.

 

Would doing that affect other functionality?

The board has only one function. It lights the LEDs up for parking lights. The rest of the bulbs (brakes, reverse and turn indicator) are on a different electrical plate and powered up separately. So if it fails it will just fail the parking lights which is already dimmed at the moment.

Improve your soldering skills. The wire you used was overkill. You could have laid a #30 AWG solid wire over the trace that had corroded. If you couldn't solder a bare piece across the part that opened, you could use insulated wire between the two points where you tried to solder the big wire.

The problem is that I don't have wires and necessary tools. But I will try to supply.