In addition to what Stu said, the VSS also is used to determine spark strategy at idle (spark is sometimes used to control rpm) in the COSY ecu. How did you find out the VSS wasn't working, did you get error 452? Incidentally are you running a chipped ecu? I know of one tuning company whose 270 chip lets the ecu think there is no VSS present, if youv'e got one of their chips a faulty VSS wouldn't be the cause of your problem.