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1981 300SD
133 Posts
Discussion Starter #1
Hello All,

Thought you might enjoy and interesting trouble shooting story.

I have been experimenting with LED bulbs in my car. Discovered the usual stuff like the original turn signal flasher doesn't like the reduced load (flash rate doubles), but they work well for dash, marker lights, etc.

I replaced my brake lights with a 1156 LED replacement for the brighter light and slightly quicker response time (compared to an incandescent). However, not long afterward I noticed that the brake lights glowed very dimly whenever the engine was running. I concluded that they were responding to a small amount of leakage in the electrical system but didn't take the problem as being serious (I've checked and redid most of the ground connections in the car, so I knew it wasn't an open ground causing a feedback).

I recently did a highway trip and decided to use the cruise control - it didn't work which surprised me as it had been flawless on the previous trip. Pulled out the MB Service Manual upon my return home to look at the electrical circuit for the cruise. Due to the fact that MB uses an electronic control box for the cruise it wants to see a ground connection through the brake light circuit. If the ground is open it assumes the brake lights are on.

It dawned on me that the glowing LED brake lights may be fooling the cruise control circuit into thinking the brakes are on. I replaced the LEB bulbs with the originals, went for a teat drive and guess what - cruise works again!

I decided to compare the LED bulb with an incandescent using my Ohm meter. The LED had an infinite resistance reading whereas the incandescent has around 1 Ohm. Interesting!

So, beware when substituting LED lamps in circuits, they can have some unusual side effects. I now only use them for marker, interior and dash lights.

Hope it helps.

Take care,
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