• Some 3-wire valves require an R8239A1052 Isolating Relay. The thermostat will not be damaged unless it is used to directly control a line voltage system. A Honeywell R841 or R8239D1015 Isolating Relay must be added to the thermostat control circuit for proper system operation.Handy Wiring Diagram that shows a Paper Trail of how the Electrical System Works for the 7.3L Powerstroke Engines, all Trucks, Excursions, Vans. It was a Copy I had to PhotoShop to make it Readable & Printable. it's very detailed and shows how every wire for each component routes to the...