Fuel (gas) gauge

[cffisher]

My gas gauge goes all the way to the full side way past full. I have disconnected the wire from the tanks. I does not return when key is off. I have removed the clear bezel and can move the dial back. but as soon as I turn on the key it heads back past full. My gauge cluster doesn't want to come out or I'd check more. What can be causing this? I think I'd have to grind some of the dash out of the way to remove the cluster. Guess I'll have to do that if changing the gauge is my answer. 1977 C20 dual tanks

[bd]

The gauge pegs to ~3:00 o'clock because the sender wire is open somewhere between the gauge, sender and sender ground.  Temporarily grounding the sender wire should cause the gauge to drop to empty.  You need to find the open in the circuit.  For example, it could be a very poor connection, a severed wire, a faulty sender, or a poor sender ground connection to frame.

[cffisher]

OK Thanks I'll continue checking

[cffisher]

I am replacing the one wire unit with a 5 wire and changing the switch. Which wire from the old switch goes to the gauge? I'm a bit confused in this. there are 4 wires on the left side of fram green blue tan and tan with white... there is a tan wire going down the right side of frame going to front. The green went to the old valve, the blue goes to the left tank and the tan goes to the right. I can't get the gauge cluster to come out without breaking it. where do I find the wire that goes to the gauge  from the 5 wire valve?

[cffisher]

I asked a question yesterday got an answer but have more on same topic. I have a 77 C20 with dual tanks. the 1 wire valve went bad and is no longer available . I intend to replace it with a 5 wire valve. I have to replace the switch in the dash also. On the left frame rail there are 4 wires tan / white / green /tan / blue. Blue is sending unit for left tank. Tan/white is sending unit on right tank. Green used to go to 1 wire valve, not in use now. On the right frame rail is a tan wire I don't know where it goes I think it goes to the gauge. I can not get the gauge cluster out of the dash without breaking something. Is the tan wire on the right frame rail going to the gauge in the dash can it be used as  the B wire for the Pollak valve. I have attached the drawing I have.
Thanks Charlie

[bd]

The tan wire running the right frame rail runs to the dash fuel gauge, labeled "PRODUCTION FUEL GAUGE" on the schematic you posted.  Yes, it should connect to the "B" terminal of the Pollak valve.

You won't need to cut or trim metal to remove the instrument cluster.  The following sequence should outline the major steps:

• Disconnect the battery ground cable.
  
  
• If you have an original style radio, remove the radio knobs by pulling straight outward.
  
  
• Remove the headlamp switch knob (see Diagnosing dash instrument lights for a how to).
  
  
• Remove the instrument cluster bezel retaining screws.
  
  
• Remove the sheet metal panel below the steering column.
  
  
• If you have AC, remove the duct that crosses under the steering column.
  
  
• If you have an automatic transmission, rotate the gear selector lever downward and disengage the gear indicator cable from the steering column shroud.
  
  
• Unbolt and lower the steering column onto the seat.
  
  
• Remove the instrument cluster bezel.
  
  
• Reach up behind the instrument cluster and release the speedometer cable from the back of the speedometer (hint: push the spring steel locking tab that retains the cable toward the backside of the cluster with your thumb while simultaneously grasping and pulling the cable in the opposite direction, away from the cluster toward the front of the truck).
  
  
• Pinch the locking tabs of the IP harness connector together with your left hand and pull the connector out of the back of the cluster.
  
  
• Remove the screws that retain the cluster housing to the dash; the cluster should be noticeably loose in the dash if trapped behind sheet metal infrastructure.
  
  
• Grasp the cluster with both hands and rotate the top of the cluster toward you as you lift upward and very slightly toward the left.  Finding the exact combination of moves can be a jigsaw puzzle.
  
  
• Use an appropriate wrench and backup wrench to disconnect the oil pressure gauge capillary tube from the back of the mechanical gauge.  Wrap the line in a rag to capture any oil drips.  DON'T CRANK OR START THE ENGINE!
  
  
• Remove the cluster the rest of the way out.

The first time through will test your patience.  Rest assured that each time it becomes easier.  Installation is the reverse of removal.  Be careful that you do not overtighten fasteners during reassembly.  While you have the cluster out, take advantage to remove and lubricate the speedometer cable.  Inspect the PC board for oxidation and damage.

[cffisher]

Thank you for your in depth reply. I will proceed with caution.

[cffisher]

Well it's all back together and working. Don't let anyone tell you its easy. The old valve was 3/8 and 1/4 hose the new one is 3/8 and 5/16. You will not get the 1/4 "hose on the 5/16 fitting. Other than that the valve is straight forward replacement. The other thing is the switch is not a direct replacement either, it's bigger. So open the hole before pulling wires through. The wiring is as shown on the drawing, it's a shame the valve manufacturer doesn't supply a drawing for the wiring.
Thanks for all the help