I have an 85 F350 with twin fuel tanks, diesel. Trying to figure out the wiring on the fuel selector valve which are not correct. The front tank is all it will run on. When I attach my volt meter I get one pin on the harness that shows constant voltage and one that is intermittent. Anyone know which what that means? I think the constant volt tells me that the switch is on the front or rear tank but I am not sure about that. Not sure what the intermittent volt is all about. Anyone know what all that means? I know the switch is good on the dash and the wiring to the selector valve is also.

 

The intermittent voltage is the fuel gauge. The valve on the frame does the switching for the fuel gauge sending units to read the correct tank. The pulsating you are seeing is the gauge regulator voltage coming from the cluster. So most of the wiring you see if the sending unit wiring.

There are two other wires that operate the valve. You will not see a regular voltage on these wires. You will see a voltage that reverses polarity. The valve operates by a little motor inside, much like a electric window works. The motor physically moves the valving for the fuel lines, and it also moves a switch inside for the sending units. When you want to go to the other tank, the switch in the dash switches the polarity of the two motor wires and makes the motor move the other direction. Here is a diagram of your fuel switching unit.

 

Ok
that makes sense, so the constant voltage is the front or rear tank...which ever the switch is on correct?
And thanks for the clarification.

 

If you put your meter to a good ground, and then probe the motor wires, that is what it will look like to the meter. If you put the negative of the meter on the opposite motor wire, you will notice it changes polarity.

 

So the intermittent/pulsating wire should be the orange wire or the BR/W?

 

You will get that voltage all the time on the yellow/white which goes up front to the fuel gauge. Depending on what tank is selected, you are going to get that pulsing voltage on either the darkblue/yellow(front tank) or the yellow/lightblue(rear tank).

 

Ok, that's my problem, the constant voltage is on the wire for the tank selected and the pulsing is on some other wire.
the problem is the wires to the selector valve are all black because someone rewired it with a selector from a 90s truck.

 

So I tested it again and the yellow/white pulsate and when I flip the tank switch either the orange or brown are constant. What does this mean? Wiring switched somewhere?

 

The system has previously been converted with a newer style valve? And now that valve has failed? Can you just purchase another valve of that same newer design and simply plug it in?

 

I am testing the wires to the valve, not the valve its self.
The valve is fine.

 

If your system is wired per the diagram previously posted, here is what you should see at each of the 5 wires. If I understand correctly, they have been spliced to all black wires at the valve connector, so you will have to mark each one for identification. Maybe fold a piece of masking tape over each wire and you can write on it. All tests are done with the connector removed from the selector valve, and the black meter lead connected to a good ground.


Pin 1 at valve connector, wire 974 Orange: Turn the key to run. Set dash switch to rear tank. Set meter to DC voltage. Should read battery power. Change meter to resistance. Select front tank. Should show good continuity to ground.

Pin 2 at valve connector, wire 674 Brown/White: Turn the key to run. Set dash switch to front tank. Set meter to DC voltage. Should read battery power. Change meter to resistance. Select rear tank. Should show good continuity to ground.

Pin 3 at valve connector, wire 673 Dark Blue/Yellow: Set meter to resistance. Resistance to ground should read between 8 ohms (full) to 85 ohms (empty) for the front tank.*

Pin 4 at valve connector, wire 29 Yellow/White: Should read 5V, pulsing on and off.

Pin 5 at valve connector, wire 675 Yellow/Light Blue: Set meter to resistance. Resistance to ground should read between 8 ohms (full) to 85 ohms (empty) for the rear tank.*


*For pins 3 and 5, do you know the quantity of fuel in each tank? If so, the resistance value can help you can determine which wire goes to which tank. If not, you'll have to guess and make a temporary connection. If not reading correctly, switch the two wires.

 

I filled the tanks because it stranded me when I switched to rear tank. It is only using the front tank, cant get it to switch thus the wiring problems from previous 10 owners over the past 40 years.

It sucks from the front tank fine but if I hit the switch for the rear tank it sucks all the fuel out of the line and wont switch. It sounds like the wiring upstream from the selector has been altered. I installed a new selector and it does the same thing. Switch is good unless I mixed up the wires to the switch on the dash but that would only make the front and rear backwards right?

 

Is the switch wired incorrectly? The voltage and resistance checks I gave in post #11 should help sort out if the switch is sending the correct commands to the valve. It will also test the wiring between the switch and the valve, too.

 

With the engine off, key on, when you switch the dash switch if the valve is working you should hear a clunk. If you hear clunk, but it won't pick up fuel from the rear tank there are several common problems that cause that;

1. The tank has a 1/4 of a tank of fuel, but the shower head on the end of the pickup inside the tank is broken off. Very common problem. It will work ok if you have 1/2 to a full tank

2. The metal lines on the sending unit are rusted and have a pin hole. So it is sucking air. Another very common problem, the outside part of the sending unit likes to rust.

 

Excellent point. The valve may indeed be responding properly and the problem is with the supply line from the dead tank.

Have you tried temporarily bypassing the valve completely with a short length of hose? This will take the valve completely out of the picture. If you still can’t draw fuel from the dead tank, you’d know the problem is with the supply line, most likely in the tank.