Chevrolet Repair: 91 s10 blazer electronic speedometer

Question
Tom
My blazer's speedometer comes on and goes off at its own whim. I got one from a salvage yard and it does the same thing. How can I check to see if it is getting power. I have no fancy test equipment. I sent my old speedometer off to be repaired and am waiting for its return, want to make sure its not an electrical problem before I reinstall it.
Thanks
Ron

Answer
HI RON, DO YOU KNOW THAT THIS HAS A VSS. SPEED SENSOR ON TAIL SHAFT OF TRANSMISSION..WE HAVE HAD PROBLEMS WITH THIS..CK FOR VOLTAGE ON THE PURPLE AND WHITE WIRE OUT OF THE SENSOR WHEN THE TRANS IS IN GEAR YOU SHOULD SEE A CHANGE IN THE VOLTAGE..IF NO VOLTAGE CHANGE THE VSS IS BAD,PROVIDING YOU DO HAVE POWER DOWN ON ONE OF THESE WIRES.IF NO VOLTAGE AT ALL TO THIS SENSOR CK FOR A OPEN IN  THE WIRING.. BUT IF YOU MEAN THE LIGHT ON THE SPEED GOES OUT MORE THEN LIKELY THE SPEEDO HEAD CIRCUIT BOARD IS BAD..THIS IS WHAT YOU ARE SENDING OUT FOR REPAIR I WILL ASSUME...THE OTHER PROBLEM IS MAKE SURE THERE IS POWER TO THE DASH ASSEMBLY WHERE YOU REMOVED IT FROM THE DASH FACE OR HEAD...IF I DID NOT ANSWER YOUR QUESTION,,GET BACK TO ME...GOOD LUCK TO YOU,,,TOM...