Help with reading wiring diagram

[mrk5]

In my crew cab build thread I had posted about a problem with the tank selector switch. I was planning to do my own custom Carling switch, but I'm having a little trouble understanding the wiring diagram.



The gray and tan wires for each tank is pretty straight forward. Then you have a single wire power in to the switch which makes sense. What I'm a little baffled about is the ground wire.

It looks like in the diagram, the left tank is selected. What I don't understand is why the gray wire for the right tank is connected to ground. Part of what makes it hard for me to understand is I don't get what's going on inside the valve on the right side of the diagram.

I think when power is applied to the tan wire thru the tank selector switch, the gray wires becomes a ground that runs thru the tank selector switch. Then when the right tank is selected the gray carries power and the tan wire acts as a ground.

If this is true, any normal DPDT switch isn't going to work like I'd hoped.

Looking to see if anyone can confirm my suspension.

 

[Blue85]

Power for the fuel pumps has to come in on circuit 120 (does that go to the fuel pump relay?). The selector switch is swapping circuits 920 and 921 between hot and ground, which serves to have 12V across one pump and 0V across the other. It is also swapping the polarity seen by the selector valve, which throws it between the R and L tank inputs. Also part of the selector valve is the fuel sending unit switch. The pink is the fuel gauge (circuit 30).

A regular DPDT switch should work, you just wire two pairs of terminals together to make it work like a typical polarity reversing switch, like in this picture:

Look at the selector switch diagram and notice that 120 and 150 each appear at 2 terminals of the switch. It's the same thing, except the selector switch probably has those two crossover connections internally.

I'm just going off your diagram - I've never worked on one of these setups before.

 

[longbedder]

This is off of a different site (linked image), but should help - it's a simplified version. Swap out the green wires for tan and grey. Hope it helps.

 

[mrk5]

Power for the fuel pumps has to come in on circuit 120 (does that go to the fuel pump relay?). The selector switch is swapping circuits 920 and 921 between hot and ground, which serves to have 12V across one pump and 0V across the other. It is also swapping the polarity seen by the selector valve, which throws it between the R and L tank inputs. Also part of the selector valve is the fuel sending unit switch. The pink is the fuel gauge (circuit 30).

A regular DPDT switch should work, you just wire two pairs of terminals together to make it work like a typical polarity reversing switch, like in this picture:

Look at the selector switch diagram and notice that 120 and 150 each appear at 2 terminals of the switch. It's the same thing, except the selector switch probably has those two crossover connections internally.

I'm just going off your diagram - I've never worked on one of these setups before.

I did occur to me that the DPDT should work because of the DP part. I'm working on wiring it up now so I'll see if it works.

The circuit 120 did go thru the fuel pump relay from the ECM. Now I have it being fed by the fuel pump power in the Sniper harness.

This is off of a different site (linked image), but should help - it's a simplified version. Swap out the green wires for tan and grey. Hope it helps.

I didn't clarify in the post, but mine is an 89 TBI truck so the wiring is a little different I think.

 

[mrk5]

Okay, here's how I wired up the switch and it worked.



This is the Carling Rotary switch I used.

 

[6872xtc]

I am surprised that you found a switch so quickly, apparently in stock! And nice work!

 

[Blue85]

Okay, here's how I wired up the switch and it worked.

You're welcome.

 

[1-ton]

I is kind of weird that one wire sends the hot single to selector switch to select one tank, then the other wire for the other tank becomes the ground. I guess that means the electron polarity flow goes the opposite way as well if you are using the Gray and Tan wires to be both hot and ground.

I think once the fuel pump solenoid on the firewall takes over it uses the BAT/ALT circuit to power the fuel pumps while the engine is running, and that the dash board rocker tank switch is taken out of the circuit loop, unless you change tanks, or turn the engine off.

 

[Blue85]

The valve is a solenoid. Apply power in one direction to push it one way and reverse to push the other. Now the valve may well be latching and you could unplug it while running, but the fuel gauge wouldn't work, of course. If it's not latching you would dead-head a pump or possibly transfer fuel from one tank to the other. I don't know the internals.

Look at the diagram. The power for the fuel pumps is always passing through the dash selector switch. It's the power to the selector switch (circuit 120) that would go to the fuel pump relay and maybe oil pressure switch. It's designed so you can't have both fuel pumps running at the same time.

 

[1-ton]

The valve is a solenoid. Apply power in one direction to push it one way and reverse to push the other. Now the valve may well be latching and you could unplug it while running, but the fuel gauge wouldn't work, of course. If it's not latching you would dead-head a pump or possibly transfer fuel from one tank to the other. I don't know the internals.

Look at the diagram. The power for the fuel pumps is always passing through the dash selector switch. It's the power to the selector switch (circuit 120) that would go to the fuel pump relay and maybe oil pressure switch. It's designed so you can't have both fuel pumps running at the same time.

I have a 91 Chevy pick up with the same system in the wiring diagram, and the reason I think the rocker dash panel is out of the circuit loop once the firewall solenoid kicks in is because I have taken a test light and touched it to the TAN/WHITE wire, and it lights up the test light when the key is turned on, but as soon as the firewall solenoid is heard energizing the test light goes out.

 

[mrk5]

If you don't actually start the truck the light will go off once the fuel pump is done priming. That's what I noticed with the indicator light on the new switch I installed. Once the truck is running the light stays on.

 

[1-ton]

If you don't actually start the truck the light will go off once the fuel pump is done priming. That's what I noticed with the indicator light on the new switch I installed. Once the truck is running the light stays on.

That may be the case because I never started my truck when I was messing around with all the rocker switch wiring because all the wiring was taken apart too much to want to start my truck.