There are two "switches" inside sender. One for hot and one for cold. When starting, I believe red hot should come on as a bulb test, then go off. Green on to about 100 deg and red comes on about 200 deg. No lights in between 100 and 200.
I believe a hot wire goes to the bulb(s), out each bulb to the sender (two wires on sender). The sender grounds the wires for hot and for cold and lights the appropriate bulb. So take off wires from sender, ground each wire to see if red and green bulbs light up with ign on. You must have good working bulbs and wiring to continue testing. Be sure thread sealing tape is not keeping sender from grounding thru threads. Report results.
