K6JRF at the mic K6JRF's Page
formerly W6FZC


My Mercedes Benz
S500 Coupe

K6JRF's MB S500 Cpe
(New: Sept 20, 2005)

Analyze and Troubleshoot "Check Engine" MIL and Electronic Control Units (ECU)!

SRS Light: Seat Belt Switch Faulty. . .
How many MB owners have seen the SRS light come on and after reading the DTCs, find it to be a 'faulty seat belt switch'?   Personally, I've have NEVER had the problem but have seen it a number of times in other cars. Well, this week after having the car washed (only hand wash!!), the SRS light came on.

After reading the codes with a scanner (AST Retreiver) in the "SRS" section, gave the single code of "driver's seat belt switch (SBS)". Since it just came on suddenly, I figured that some dust/dirt had gotten into the switch at the car wash. It worked perfectly when I drove in, why should it not work now??

Problem Diagnosis
The SBS fault reacts in a set way when starting the car. It goes like this; When the key is turned to position 2, the SRS light comes on along with all of the IC lower cluster lamps (CE, ASR, etc). This is normal. When the car is started, the SRS will flash indicating that it has completed its power up diagnostics and will momentarily, go out.

If all is 'ok', the SRS light will stay off. If something is amiss, then it will come on again and stay on indicating that there is a problem someplace in the system. If it's the SBS, the SRS light may go in a few seconds to a few minutes. In my case, it would go out in apx 3 minutes while driving. Some stay on much longer, some much shorter.

In any case, a scanner is required and it must be a "digital" unit since the home brew LED/switch combo will not work properly because it can't read the digital codes from the SRS system module. Once verified, it's good idea to use the scanner's "reset" feature to see if the code goes away. In my case, it would not.

Trial Fix
Since the SBS is really nothing more than a 'switch', it's certainly probable that some dirt/dust has caused the 'micro-switch' to not close causing the SRS failure. So, using my 'trusty' cleaner, I sprayed a liberal amount into the switch from the top opening. Then using the seat buckle as a 'tool', repeatedly engaged it until I thought it must be good by now. But before testing it, I use my shop compressor to blow out whatever accumulated in 12+ years of use. Actually, I lot of 'stuff' came out. After that, I tested it and it was fixed.

It's been a few weeks now, and it still working perfectly. There's many 'cleaners' that you can use but a good source is Radio Shack's "TV tuner/control cleaner and lubricant", PN "64-4315". Cost is about $10. Price of new SBS is $100+ so it's worth while to try this first.

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