fuel gauge and fuel tank selector

[paintslinga2010]

Are the two of these related at all? I put a new sending unit in my tank a while back and the fuel gauge was working. Then my fuel tank selector went bad and I unplugged and bypassed it with some fuel hose. I'm pretty sure around that time my fuel gauge quit working. So does the selector have to be plugged in for the gauge to get signal?
Thanks
jake

[hatzie]

Yes and no. Depends on the year.
The early single wire valve uses the dash switch to select the sender.
Later 5 wire motorized valve uses the valve to select the sender.

[J Knight]

Yes, you have two sending units(one in each tank) and one gauge, IIRC the selector valve is where the conection is changed from one sending unit to the other.

[paintslinga2010]

Thanks fellas. Looks like I will be hooking that back up.

[Desert1957]

If you had an 87 TBI , It would not even start or run.
This might be your setup with a Carb.

Desert

[paintslinga2010]

Hooked it up and the gauge works! It went straight to the E side though...dangit

[hatzie]

Originally Posted by Desert1957

If you had an 87 TBI , It would not even start or run.
Desert

No. That's incorrect. It'll run just fine with the valve unplugged as long as you don't change the cab switch position, shutting off the fuel pump that has open pipes, after you unplug the valve.

Why? The TBI fuel pump power is spliced into the wires between the cab switch and the valve.
It's a pretty clever arrangement on both carburetor and TBI trucks. The polarity of the two wires from the cab switch depend on the cab switch position. These wires reverse polarity to drive the DC valve motor right or left.

The only differences between TBI and carb wiring is the colors and the fuel pump splices.

TBI wiring colors are different from earlier years. Carbs have light and dark green wires from the cab switch to the valve. TBI uses TAN and Gray valve wires.

On a TBI truck selecting the left tank makes the TAN wire hot and the Grey wire Ground. This gives the LH fuel pump power and the RH fuel pump two grounds. It also drives the valve motor to the LH open position. Two grounds will not power the pump so the RH pump is off.

Flipping the switch to select the RH tank reverses the TAN and Grey wires polarity. This grounds the LH pump and powers the RH pump. It also reverses the valve motor to the RH open position. The fuel pumps get power with or without the valve plugged in. It will run without the valve plugged in unless you flip the cab switch while the valve is unplugged.

[Desert1957]

I stand corrected, Here is a diagram showing 87 wiring.
Your are correct fuel pumps ARE spliced in before the plug at the switch.

You guys keep me sharp.....LOL

Desert