I found the wires from cluster to rear and its either a faded pink or tan color
It's pink -or should be anyway. And there should only be one pink wire (not wires) leading from the instrument cluster and heading back to the area of the tank(s).
when I connect it to the purple wire from sending unit the gauge moves to tank's level position
The purple lead is the ground wire for the sender. The sender end of the ground wire is soldered or crimped to a terminal:
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and the other end should be secured to the frame with a bolt:
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I don't know why you are connecting the pink to the purple - maybe for a test?
But anyway, if the ground wire has a tight/clean connection to the frame and you short the pink wire to it...the gas gauge should peg straight to the "E" mark.
On the other hand if the pink wire is not connected to anything - and is not allowed to contact the frame - the gas gauge should peg to the "F" mark and then some.
Now I need to know what the gray wire coming from sender unit is for .
What "gray wire coming from the sender"? There are no gray wires in that circuit - only pinks. Is it possible that the gray wire is actually the original pink and has faded to a gray color?
Here is the wiring diagram for an 85 with duals:
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Notice that the lead from the gas gauge to the switching valve was pink. The leads from the Pollak valve to the production production & auxiliary tanks were pink/white & pink/black respectively. When the switching valve was shuttled, the sensing leg was also changed to whichever tank was selected.
The same wiring harness was used for trucks with dualies as with a single tank - up to to a point. Because the lead used with a single tank LH (driver's) side was to short to reach the switching valve (which was located on the RH side), an extension was added to allow the common pink to reach the valve. The splice is under the cab floor almost to the back wall:
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To do an initial test of the gas gauge (and the wiring between the dash and tank),
With key in RUN, find the common pink wire that runs along the inside of the LH side frame rail. Make a good clean surface on the frame and short the pink to it. The gas gauge on the dash should bang low.
With the key still in RUN, isolate the pink from the frame and again check the gas gauge. It should over the "F" mark.
Let me know what you find. I'll try to check everyday or so.
