PeachParts Mercedes-Benz Forum - View Single Post - '09 R320 Bluetec- no tail/brake loghts after installing repair harness

jay_bob · #7 08-01-2019, 01:12 AM

MOSFET is an acronym for a Metal Oxide Semiconductor Field Effect Transistor

It is a highly efficient device that switches dc voltage on and off with a small dc triggering signal. Thus making it perfect for switching the several amps of dc needed to light several tail light bulbs with only a few thousandths of an amp from a logic chip.

Never had a SIM apart before but if you pop the top off you will probably see some devices that have a black case, bolted to a heat sink, about the size of your thumb nail. With a little sleuthing (look at the board traces) you should be able to find the one that has a connection with the tail light pin on the body harness connector. Then with a meter you should be able to probe the pins on the device and find one that has 12 V (the ‘source’), one going to the bulb (the ‘drain’) and one that is leading from the logic chip on the SAM (the ‘gate’). When the gate is energized it allows current to flow from source to drain and out to the load (light bulbs).

Look at the markings on the front of the device and punch into Google. May have to search for ‘semiconductor trademark images’ to get the manufacturer’s name. Once you’ve figured this out you should be able to go to Digikey or Mouser and order a new one. They have minimum orders so you’ll probably have to order multiple to meet minimum.

Then it’s just a matter of being handy with a soldering iron and swapping the component.