engine speed sensor, AAN

[gavs20v]

Ello all,
Dropped the fault codes on my 93 s4 this pm, only one it came up with was engine speed sensor. Erased the code, took it for a run then checked it again. Same fault code. What does this sensor actually do? what symptoms would this fault create?
Cheers in advance
Gav

[imsatim]

gav .........

check it running,if the ign is on un running it will come up with this............tim

[gavs20v]

cheers for that Tim, ****** not a simple answer to my running probs then, och well!
Nice one
Gav