I purchased a new circuit board for the rear tail lights as the one in my car was butchered. I had no tail light or brake light and thought this must be the cause. I was somewhat right after installing the new board then bending,cleaning and using dielectric grease on all contacts. Much to my surprise was after seeing the lights go on I notice the other side brake light now does not work. Wow one step forward one back. I now understand why everyone wrote about the tail light nightmare. This seems to be some kind of wiring issue as I did check the bulb. Just thought I would ask if this problem came up with someone else or can anyone help lead me to my solution. thanks in advance

 

Not sure what kind of car you have but I'm guessing a 240 sedan. If this is the case, then the wiring for both right & left side tail lights go through the left side tail light circuit board edge connector.

Just out of curiosity, which tail light circuit board did you replace? Left or right?

With the parking lights on, I would wiggle the connector on the left side tail light and see if you can get the right side light on. If not, try the right side connector. More than likely, it is one of these two connectors that is not making good contact.

If you have a 940 or 740, these comments don't apply.

 

I forget this forum includes other models but yes it is a 1991 240. I did replace the right, passenger side, circuit board. I did notice when installing the connection to the board the yellow wire inside the plastic piece looked a little black. I tried to scrap it to clean it but I do not have a wiring diagram to let me know if that wire, right/passenger side effects the left. To add to the story after the install I had the daughter apply the brakes and she told me they both worked fine, but when I turned on the headlights that side, with new parts, the tail light did not come on. I went and shook/ turned the bulb and both lights went on, brake and tail light, but caused the other side, left/drivers brake light not to work.

 

So either one side works or the other?

How is your ground connections. I'm not all electronics savvy, but I would think you only have one side grounded and only one side can operate because of that? Really weird the the right side would work and not the left seeing the supply is from the left.

 

Just like Burn Stains said, looks like a grounding problem. I would really go through checking the wiring diagram and the traces on the circuit board. Like I said in my earlier posts, all the wires that feed the right side tail light go through the edge connector of the left side tail light circuit board.

Not sure of the wiring colors. Don't have my Haynes manual handy at the moment. When I get home I'll take a look at the wire colors.

 

This is for a 1991 240.

 

Thanks everyone, checked it out last night and found a burnt spot on the left circuit board, (Melted). When I changed the right circuit board and everything worked I noticed the left brake light bulb not coming on so I first rotated it then pulled it to check if the bulb went out. I did this all with the power to the battery, negative not pulled. It appears that a lesson I learned is not to work on any electrical issue without pulling the ground of the battery. So back to square one and I am ordering a new circuit board.

 

Yes, very important. Tell that to my instrument cluster.