Ford Repair: Instument Panel Gauge, gas gauge, ground path

Question
Hello Clay,

 I own a 1986 F150 4x4 w/ a 302ci engine.  Not too long ago my gas gauge started alternate between working ant not working, depends on the weather I guess.  My question is, in most cases of the gauges failure where does it stem from?  I want to be hunting in the most common spot.  Is it the gauge itself, the sender or the wiring?
FYI--It is a two tank system and the switch over valve has never worked since I'ved owned it, the gas is always fed from the front tank, I'd like to replace the valve but when I have looked for it, I'm never sure what it looks like even after looking in a Haynes repair book, I cant tell which is the pump and which is the switch.

Thank you,

Dar  

Answer
Dar, those two-tank switching valves are difficult to diagnose sometimes, but usually you'll find the black plastic cannister mounted on the frame rail under the driver seat.  It has six hoses attached: two for the front tank, two for the rear tank, and two for the engine (a feed and a return for each).  
The fuel tank sender is common to fail with age, as rust and corrosion takes a toll on the electrical terminals and on the brass float.  If you disconnect the wire harness near the fuel tank, look for the four wires in the harness connector - the two larger wires are for the fuel pump, andt he two smaller wires for the sending unit.  Jumper a test light beween the two sending unit wires on the harness side and the test lamp should come on dimly - this tells you the instrument cluster side is providing power and the ground path is intact... and if this works OK, the gauge should read something, as the resistance of the test light is sufficient to command the guage to a reading between E and F.
Hope this helps...
Clay