Gas gauge/tank problem ('74 C20)

[MWS1994]

I have a 1974 C20 and the gas gauge has acted weird since I got it a couple months ago. It's not road legal so I have just started it up to tinker with it and drive it around the house. It has run out of gas a couple times and I only put in about a gal at a time.

Now for the weird part. I only put gas in the left (drivers side) tank but the truck will only run with the switch flipped to the RH side (that does mean the right side, right?). When I switch it to the LH side, it runs for a little then sputters and dies and the needle goes to the full position and then slowly goes to about 3/4 full. When I switch it back to the RH side, it starts up and the needle sometimes stays where it was (3/4) but most of the time it either read 1/4 tank or fully empty. I have searched around but couldn't find any answers.

Anyone have any ideas?

[willett]

Check your grounds at the split valve and at your senders. I would start there.

[Cabnchassis]

X2 the ground on the senders on mine was to the frame and gets gunk under it kinda easy. If gauge sits all the way past full and stays there, the ground is bad for sure. Also if no one has ever changed it, the sending unit in that truck is housed in metal and will probably be pretty rusted, the ones in my 91 were flat out nasty. The new part store ones have a plastic housing around the sender. My bed is off right now so I was able to replace both sides in about an hour and half. I think doing it from below it's probably hard than that, but hopefully it's just ground.

[MWS1994]

This is my first project so I'm still learning, where is the split valve?

[Cabnchassis]

Originally Posted by MWS1994

This is my first project so I'm still learning, where is the split valve?

The lines head toward each other from the tanks and run parallel to each other toward the engine. Follow them until you find the point where both lines go into the same object. It should have some wiring coming off it into the harness. I've never messed with one on a chevy only fords, so that may not be super helpful. There shouldn't really be anything else around there to confuse it with I can think of any other wires under that part of the truck.

[Cabnchassis]

I'm rained out of work I'll go crawl under mine and see if I can get a pic for you

[Cabnchassis]

Mines a 91 crew cab cab and chassis so yours could a little different but you'll see a road shield under the back of the cab on the passenger side. Just a thin sheet metal shield and under is the diverter is bolted to the frame. You'll see the 4 tank hoses going into a small "valve" and two coming out. I couldn't see where it was grounded so you'll probably have to take that shield off and get cozy under there looking. It's possible the ground is in the harness. If that is the case I'd try to jumper it to a clean spot on the frame to check ground. You shouldn't have a hard time finding it I couldn't get a picture to come out clear enough though.

[MWS1994]

Thanks! I'll take a look when I have the time.......and when it isn't raining.

[hatzie]

Originally Posted by MWS1994

I have a 1974 C20 and the gas gauge has acted weird since I got it a couple months ago. It's not road legal so I have just started it up to tinker with it and drive it around the house. It has run out of gas a couple times and I only put in about a gal at a time.

Now for the weird part. I only put gas in the left (drivers side) tank but the truck will only run with the switch flipped to the RH side (that does mean the right side, right?). When I switch it to the LH side, it runs for a little then sputters and dies

That's correct. LH and RH are from the seated driver's perspective. In North America typically driver's side is LH and passengers side is RH.
This sounds like the LH & RH hoses on the input side of the valve are mixed up. Easy to correct. The valve is on the RH frame rail and there probably isn't a sheet metal guard.
It'll either have 3 or 6 hoses. 6 ports switch the fuel return lines... if your truck is equipped with a return.

This is the Standard Motor Products FV1 3 port solenoid valve for a visual.
http://www.amazon.com/Standard-Motor-Products-FV1-...
This is the 6 port AC Delco 467513 solenoid valve for a visual.
http://www.amazon.com/ACDelco-467513-Fuel-Selector...

Originally Posted by MWS1994

the needle goes to the full position and then slowly goes to about 3/4 full. When I switch it back to the RH side, it starts up and the needle sometimes stays where it was (3/4) but most of the time it either read 1/4 tank or fully empty. I have searched around but couldn't find any answers.

Anyone have any ideas?

This one is a little tougher.
With only a gallon in one tank and the other one empty neither should read over E.

Go to Radio shack and get a 50Ω wire wound resistor.
The stock 73-80 dual tank system uses the dash switch to select the sender feed to the gauge. This gives you a convenient test point without crawling under the truck.
Pull the switch out of the dash and unplug it. See the diagram below for the plug terminal positions.
Turn the ignition on. The fuel gauge needle should go to 3:00 or pretty close. If not you have a calibration resistor, ground, or power issue at the fuel gauge in the instrument cluster.
Ground position 4 (from the wiring diagram below) of the plug with the ignition on. If the gauge goes to E the gauge works and the wiring between the switch and gauge are OK. If not fix the wiring or gauge.
Attach the 50Ω resistor between position 4 on the switch plug and a good ground. The fuel gauge should read somewhere around 1/2 tank more or less. If not fix the wiring or gauge.

The gauge works but with the switch plugged in you get odd readings...
Change the dash switch. Standard Motor Products DS455 is a top shelf replacement for the obsolete AC Delco 329854 and it's less than $15.

If it's still screwy...
Set your meter to the 100Ω scale and hook the COM probe to a ground. Probe position #3 then position #5. The tanks are close to empty so the meter should read pretty close to 0Ω no more than 10Ω. If not you have a sender problem. Either a bad sender or damaged wiring. Check the ground from the tank to the frame. Check the sender connection to the blue or tan wire.
Check that the blue or tan wires are not damaged somewhere between the tanks and the firewall plug.
Check the molded tank harness plug at the firewall for corroded terminals.

This is the 1978 RPO NL2 dual tank wiring diagram. In 1974 AUX tank is LH and Production is RH.

[MWS1994]

Wow, that's a lot for me to process. Thanks! Now I have to go learn what that means and do some testing.