Inop VDO Tachometer

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.