sensors like this are actually minature electric generators so u need a multimeter capable of mesuring ac millivolts.remove the speedo connection set meter for ac millivolts connect to sensor if it registers voltage increasing with speed then sensors ok faults in wiring/speedometer.if it dosnot register voltsthen remove sensor clean or replace
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.