[4x4k30]

Originally Posted by ray_mcavoy

Yes, the tan wires going to pins 7 and 18 are for the fuel gauge. If you have a gauge cluster without a factory tachometer it only makes use of the wire at pin 18 and leaves the one at pin 7 unused. Similarly, if you have the factory tach cluster with the small fuel gauge down in the lower left corner, that only makes use of the wire at pin 7 and leaves the one at 18 unused. I believe they were connected like this to make the wiring compatible with both styles of gauge clusters (with & without tach).

If you look closely, the tan wires for the brake warning light should have a white stripe on them. These wires were also routed to 2 different pins on the connector to make the wiring compatible with all gauge clusters (with or without tach or clock) as well.

And just so you are aware, the 2nd diagram you posted for the dual tank wiring sort of abbreviates the "production fuel gauge" wiring. That section of tan wire runs from the cluster plug, out through the firewall bulkhead connector (located behind the fuse box). Then it runs along side the wires going down to the starter. From there, it runs back along the frame and on a single tank truck it would have plugged directly into the sending unit on the passenger side fuel tank. But with dual tanks, it instead plugs into the separate dual tank harness that is detailed in that diagram. That dual tank harness actually routes the tan wire back up inside the cab again to connect it to the tank selector switch.

I changed the cluster with a tach from a 79k20 the 77 dash fuel switch has 5 wires to it the new dash switch has only 4 I am running dual tanks with efi fuel pumps all the wires on my 77 are tan and white no solid tan also look at last pic there is a tan and white wire that goes to 17 and nowhere else ??