To the OP, you don't indicate what year truck, and your profile only references a van. I found an old post referencing a "slantnose"; so you have an '80-'86? If so, '85 EVTM diagram is here:
http://userpages.chorus.net/elephant/FuelTankSelector1985.pdf . Terminal 4 goes to the gauge head, terminal 3 goes to the front sender, terminal 5 goes to the rear sender. For the gauge, the selector is a simple double throw switch that either connects 3 and 4, or 4 and 5, depending on which tank's valves are open.
The gauge head reads both voltage and resistance, since the two are inverses of one another in this case. But it's useless to think of it that way. Better to test each component in isolation. To test the front sender, put an ohmmeter between terminal 3 and ground. Should read somewhere between 8 (full) and 85 (empty) ohms. If it reads open (infinite resistance or "OL"), then there's an open in the circuit to the sender, or the sender isn't grounded. Same for the rear sender, except ohm between terminal 5 and ground. To test the gauge head, jumper terminal 4 to ground. Needle should peg past Full. If not, gauge head is faulty, or fuse, or open in that circuit.
To IDIoit, I always forget whether the "logic" of the bricknose is the same as the slantnose (high resistance empty), or "flipped" like the OBS (high resistance full). EVTM diagram for '95 is here:
http://userpages.chorus.net/elephant/FuelTankSelector.pdf . Terminal numbers change, but their relative position is the same, and wire colors for the sending units / gauge are the same, so they're probably the same for the bricknose. So to read just the rear sender, put a jumper wire between the "last" (yellow/light-blue) and "second last" (yellow/white) terminals on the connector.
If anyone comes across available new replacements, please let me know!
