Speed sensor fault

[vince47]

Well, having read the forum for the last 2 years since I got my 07 170SEL, it was about time I got my first fault.:(
While driving along yesterday at about 45mph I suddenly got a steady amber symbol that looks like 2 loops of a coil spring. The display flashed up something about `go to garage.` then rather annoyingly disappeared never to return. The car went into limp home mode, limiting me to 40mph and a feeling that I was 165bhp short of power! I stopped in a layby and turned the ignition off/on and that was that ..fault gone. I continued to work and back, giving it a bit of welly but it was fine. As its got another years warranty I got it checked over today and it apparently had a fault code that suggested the `engine speed sensor` needs replacing. They didnt have one but tomorrow they will fit it and the car will be returned. The car didnt leave me stranded so that was good but the technician said it would be best to leave it with them overnight as it might do next time!
So apart from describing this fault to add to the forums info base, can any techy tell me what this `engine speed sensor` does and do they aften fail? My guess is it will feed the ecu to control something or other. Anyone else had one go?

[Quatrelle]

If I may, it's yet another reminder to carry a 10mm spanner to disconnect the battery temporarily as a get you home fix just in case turning the ignition off/on doesn't do it.

So, if in 'limp-home mode' try the ignition, if that doesn't work and it's safe to do so, try the battery.

No, I haven't had one go.....

[gamichea]

A very worthwhile reminder. I would add that some faults although not cleared by switching on/off immediately will have disappeared if you wait a few minutes and try on/off again before resorting to the 10mm spanner.

This was the case when I had this warning light appear and the cause was turbo variable vanes not responding as expected. It happened twice 2k+ miles apart and patience was rewarded on both occasions. A new turbo was the cure as the tech.said disappointedly "we're not allowed to disassemble these on pain of not getting the warranty claim money".