Fuel transfer valve with no 12v power

[aolitesnsirens]

Hey fellas, need some help with my 77 K10 stepside. I'm trying to trace down the power for my fuel transfer relay switch under the truck. It had power at one point and was working fine until I noticed I was not using fuel from one tank. I first thought it was the new transfer relay switch I installed, so I returned it and got a new one. That one worked for a while until I had the same problem. So I decided to check if it was a power issue and sure enough it was. I can't seem to find out where the power is coming from so I thought I'd come here for some advice. I already checked the fuses under the dash and none of them are blown. Any help would be appreciated.

 

[Smtty58]

To avoid confusion, the part under the truck is called a valve. Did you replace the switch on the dash, or the valve under the truck? When you change switch positions, does the fuel gauge change? I don't have a 77 wiring diagram, but the wiring should pass through the firewall at the bulkhead connector under the master cylinder, a common place for problems on our older trucks. I would start by downloading a factory wiring diagram.

 

[Radiohead]

Plus, the original factory system used a wire that when power was applied, the solenoid would pull to one side. Switch the other way and power dropped which changed tank draw. The newer replacement (like Standard brand) uses polarity switch to shuttle the valve guts between tanks.

The original valve is unobtanium to my knowledge, so a bit of wiring change is necessary to accommodate the newer system.

 

[75gmck25]

Power for the fuel valve is from a wire running directly from ignition-switched power in the fuse box. Then there is a wire bundle from the switch that runs through the firewall, down the driver side frame rail, and across under the truck to the solenoid.

The switch has two “halves” (DPDT) so it can switch both power and the sending unit wires. The power half of the switch has fused power on the middle terminal, solenoid power wire on one side, and the other side is open.

- main tank position; switch connects power to the open terminal (no power to solenoid)
- aux tank position; switch connects power to the line running to the solenoid (solenoid is powered)

Also remember that on a ‘77 the main tank is the passenger side and the aux tank is the driver side. GM switched this setup in later years (not sure when).

I have not checked in a while, but I believe the original 3 port solenoid/valve is still available. The one you cannot find is the single terminal 6 port valve (6 ports are needed if you have a fuel return line), which is no longer available.

There are wiring diagrams on the net that explain how to use the newer valve 6 port (with multiple electrical terminals) on an old truck. However, I’ve never tried it.

 

[Broken85]

I think this is for an 82’ but bottom line. “Pink is Power” here.

 

[Radiohead]

Power for the fuel valve is from a wire running directly from ignition-switched power in the fuse box. Then there is a wire bundle from the switch that runs through the firewall, down the driver side frame rail, and across under the truck to the solenoid.

The switch has two “halves” (DPDT) so it can switch both power and the sending unit wires. The power half of the switch has fused power on the middle terminal, solenoid power wire on one side, and the other side is open.

- main tank position; switch connects power to the open terminal (no power to solenoid)
- aux tank position; switch connects power to the line running to the solenoid (solenoid is powered)

Also remember that on a ‘77 the main tank is the passenger side and the aux tank is the driver side. GM switched this setup in later years (not sure when).

I have not checked in a while, but I believe the original 3 port solenoid/valve is still available. The one you cannot find is the single terminal 6 port valve (6 ports are needed if you have a fuel return line), which is no longer available.

There are wiring diagrams on the net that explain how to use the newer valve 6 port (with multiple electrical terminals) on an old truck. However, I’ve never tried it.

I knew someone would explain it better than I did. Thanks for that.

 

[aolitesnsirens]

Thanks for all the input. Just so we're clear, I replaced the valve under the truck. It is a 3 port valve with an electrical terminal at the top. I've tried multiple times to get power to that with no luck. I did talk to someone today at the Auto parts store who said that the power is coming from the switch inside the cab. I think that might by my problem. There might be a bad switch that changes from the RH to LH tanks. As someone said in an earlier post,

"The switch has two “halves” (DPDT) so it can switch both power and the sending unit wires. The power half of the switch has fused power on the middle terminal, solenoid power wire on one side, and the other side is open.

- main tank position; switch connects power to the open terminal (no power to solenoid)
- aux tank position; switch connects power to the line running to the solenoid (solenoid is powered)"

I ordered a new switch and it'll be here tomorrow. I'll keep you posted on the status.

Thanks for all the help.