201 and 124 Chassis Tail Light Malfunction

It is not always the bulb or the wiring.

Today I ended up checking everyones tail lights out in the parking lot. It all started when Kristin ask for help in repairing her inoperative brake likes. After fixing those we checked out all the lights on Kaias wagon and then my wife, Linda, drove in in her 1993 300D, so I decided to check those as well.

Problem & Solution

Common among these chassis:


I immediately noticed something was not quite right with the left brake light. It was coming on but was not getting as bright as the right side. I removed the bulb holder to find that the running light was coming on with the brake light when the brake pedal was pushed and the the headlight switch was off - and both bulb would only glow dim as you can see in the picture below.  Now what do you think would cause both bulbs to glow when hitting the brake pedal.  I had seen similar bulb weirdness before so I had a pretty good idea what the problem was. I showed the condition to my wife Linda and she said I should grab the camera and make repairing this problem a subject for a technical tip. You know I married smart!

I started by installing two new bulbs and coating the tips and the grounds with dielectric grease. Always use this grease when replacing bulbs. It helps to prevent corrosion and improve electrical conductivity. Next, I took the plug and put a small amount of the same grease into each of the contact pin holes. I plugged it back together and retested. 

This did not fix the problem. The two bulbs were still coming on dimly at the same time when pressing the brake pedal.  Now I should turn this into a contest. You electrical wizards out there may already have a good guess.  In all my years of working on all types of cars - almost every time I have run across a problem with front and rear lights doing weird things - and by that I mean not glowing brightly, cross feeding, coming on at the wrong time, intermittent operations, etc, etc, it has almost always turned out to be an electrical grounding problem.  And it was no different here.  I took a close look at the grounding strap connecting points and I could see corrosion.  I applied some dielectric grease and squeezed the contact points together with some needle nose pliers and Wallaah - the running bulb went off and the brake light bulb came on super bright.  A good day and another one fixed.

That thin metal strap running between both metal bulb holders completes the ground circuit!

What is the moral of the story and this tip?

  • Always check all the lights every 30 days.  Have a light testing day with friends and co-workers
  • Always use dielectric grease on all light bulb contact and plug points
  • Suspect a grounding problem when you run into weirdness....