Gas Gauge Stays On Full

seawolf18 · April 27, 2009

I just bought a 1980 chevy 1/2 ton for cheep. I can not figure out the gas gauge. The gauge stays on over full. The truck has dual tanks, and selecting the tanks does nothing. I checked the grounds on both sending units and they are good. On the passenger side tank I noticed three wires: green one goes to the tank switcher valve, white one goes to the tank sending unit, and a red one with a male connector on it, that is not connected to anything. On the driver's side there is the standard ground and gauge wire on the unit, and both are connected.

I took a resistance reading of both sending units and came up with 53.5Ohms passenger tank and 23.5Ohms driver's tank. I believe the readings are within specs for the sending units. I'm lost to what could be wrong.

I have included a pic of the mystery wire.

txab · April 27, 2009

According to my info if the tanks are full each tank unit should read between 88 to 92 ohm.

I just looked at my '80 Silverado. The loose wire you have is connected to another white wire lead on mine. I'm at a disadvantage since my bed is still installed. Look around and see if you can find another loose wire between the tank and frame maybe. It has a rubber connector much like the lead that hooks to the tank sending unit. Do you have blue wires for your grounds at each tank?

seawolf18 · April 27, 2009

yes, both grounds are blue. I cannot find a disconnected wire for the connector. I will keep looking.

txab · April 27, 2009

I believe that loose wire you have is from the driver's side tank sender. get that hooked up and see what happens. Be sure and look closely at any wiring bundles to see if a wire was cut off or broken off inside the bundle. Tomorrow I can get a better look at mine from underneath. It's raining currently

seawolf18 · April 27, 2009

I hunted around the frame rail some more, and still didn't find a wiring hanging around. I did notice On the tank selector valve it looks like two wires should go to it. One is green and it looks like the other broke off and someone electrical taped it. I am wondering if this is where the mystery wire goes. If it does go there, would it effect the operation of the gas gauge? At the minimum, it looks like I need a new selector valve.

txab · April 27, 2009

If the tank unit is disconnected or the sending wire is grounded the gauge will read full

seawolf18 · April 27, 2009

OK, I will look on the driver's side too, and see if it goes there. there is a wire hooked to the tank sender, but maybee it is a two piece deal.

seawolf18 · April 27, 2009

No dice, The driver's side tank sending unit wire is connected to the sender, and the wire goes directly into a bundle that goes along the drivers side frame rail. I guess I will have to open the wiring harness up and follow each one along, and check for opens. Also, how many wires does the tank selector valve have on it?

I will also see If I can get a wiring diagram for the fuel system. That would also make the task much easier.

txab · April 27, 2009

Is there a wiring bundle that crosses over right behind the cab that might tie into that other bundle you see? If you want to know if that disconnected wire is the unit wire for the driver tank, just check for continuity between both ends with a volt/ohm meter.

seawolf18 · April 27, 2009

OK, I checked continuity between the two, and that wire is not for the driver's side tank. I would think the wiring for both sending units would have to merge somewhere. I followed the driver's side sending unit wire, up the frame rail on the driver's side. I am also trying to figure out if the tank selector valve is hooked up correctly.

They guy who had the pickup before me, really did a doosy on it.

txab · April 28, 2009

No dice, The driver's side tank sending unit wire is connected to the sender, and the wire goes directly into a bundle that goes along the drivers side frame rail. I guess I will have to open the wiring harness up and follow each one along, and check for opens. Also, how many wires does the tank selector valve have on it?

I will also see If I can get a wiring diagram for the fuel system. That would also make the task much easier.

My selector valve has 1 connection, a stud that sticks out, with a simple push-on fitting on the wire. Wire looks to be red and comes out thru a hole in the frame rail right at the pump. There is a white wire that runs from this area all the way forward to the wiring bundle for the starter. This white wire should connect to your loose wire you have pictured. Start your search around the starter bundle. Look for a broken or cut wire.

seawolf18 · April 29, 2009

Thank you very much. I will check for the white wire at the starter wiring and work my way back. I also got the factory service manual for the truck on the way. Between the two I should get this sucker whipped.

seawolf18 · May 3, 2009

I got it fixed. There was a broken wire next to the starter. Ran a new wire and connected it to the free one by the fuel tanks. Wala I now have a functioning gas gauge. Thank you for the help!!

txab · May 3, 2009

I got it fixed. There was a broken wire next to the starter. Ran a new wire and connected it to the free one by the fuel tanks. Wala I now have a functioning gas gauge. Thank you for the help!!

Congrats!