Have a 1999 American Dream by Fleetwood with Allison 6-Speed, Cummins 330. Inoperative VDO Tachometer has local mechanics stumped. First trip out, Tachometer functioned normally, but Speedometer was inoperative. Return trip from re-fueling, the Speedometer and tachometer were normal. Tachometer failed shortly thereafter. Mechanics replaced Allison sending unit = no Tach. Installed new VDO like Tachometer = no Tach. Have schematic from Spartan, but mechanics are stumped. Help!