[Chris Burkard]

I agree that both gauges failing at the same time is likely a grounding/wiring issue. I would do as suggested and go through all of the grounds. Capt. Georges suggestion of Corrosion Block is an excellent one! I dose all unpainted metal parts and electrical connections with the clear spray Corrosion Block. If you have the mechanical Glendinning engine synchronization system, I would definitely check the connections there.

Good Luck!