My 305 1988 chevy van has a re-accuring VSS error code, when i am coasting down hill 45mph. When i turn off the van it goes away, and dont come on till I am cruising down hill again. When this happens the idle gets a little higher and stays that till I turn off the van. Then it runs normal when restarted. I cant seem to find the VSS speed sensor, or control unit on this model. G20 series. 1/2 ton.