Near the firewall, on the driver's side of the valve cover, on top of the intake manifold. There's a picture in this thread:

sensor identification on 103.983 motor

I'm not convinced thats your problem though. Are there any other symptoms, like does your ABS light randomly come on?

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Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino