OK, I have an 88 f150 2WD all stock with a 351/C6. I maybe driver her 1/2 times a month just to the dump or garden store etc. Anyhow, things are starting to break on her now (after 110K I expect some things to break), like the trip meter no longer works apparently and the gas tank switch doesn't either.

I had the front one selected-pushed in- and I was pulling from the rear tank. I was getting nervous since the gas gauge is famous for going from full to 1/4 in a blink of an eye, so I tried to fill the front tank and it was totally full. So I put about 12 gal in the rear tank. I then switched to the rear tank and am just waiting for the gas gauge to register that fuel is going down. I am going to assume that either the switch on the dash is bad or the canister pump switch (the one attached to the frame) is bad. What is the best way to check as to which one it is? The trip meter is another question down the road, I will fix that when I swap the tach out.

Thank you,

Mike

 

What you wrote is a little unclear, to me. So does the gauge usually read correctly for the selected tank? That is a separate problem from the fuel delivery issue. The sending units in both of my tanks are very accurate. When you select a tank does the pump in that tank turn on? That style of fuel system the Dual Function Reservoir (DFR) controls the flow of fuel to and from the selected fuel tank. Fuel pressure from the in-tank pump forces the valve in the DFR to move allowing fuel to flow to and from the tank and closes off the ports for the other tank.

What you are calling a “canister pump switch” is a fuel reservoir for the high pressure pump on the frame rail. There is no pump and no switch per se but an internal mechanical valve.

 

Sounds like you have multiple problems. I suspect your fuel tank selector switch is just fine. But you can pull it out and test it with a multimeter. This diagram is for the 9th gen truck, but I think the 8th gen truck switch is wired the same.

I suspect your sending unit in the fuel tank is the real problem. When they begin to die, they can't create enough pressure to activate the dual function reservoir valve.

 

Thank you for the information on the fuel system. To clarify things up a bit:

1) My fuel gauge is a bit 'wonky'. In that I will fill the tank up and then it will bounce around and over time it will stay there. Then at some point it just drops to about 1/4 full, skipping the 3/4 and 1/2 marks. So to me that says the sender is a bit wonky in its float capabilities.
2) The tank switch works or worked up until recently. I had it pushed in, so that I thought I had the front tank selected-F pushed in. (I will see if I can test it with a multi meter. I assume it should read 12v).
3) I did not realize that is how the switch on the frame worked, thank you for that. So if the rear or front tank sending unit is going bad: A) Is there a way to test it and B) if it is bad how do I rebuild it? Are the replacements any good? I know I have to drop the tank to get into it-ugh.
4) Is there a way to manually switch to the front tank so that I can empty it? Trying to pump out 18 gal of gas will not be fun so that I can replace the pump/sending unit there if it is bad?

Hope this helps. If not let me know and I will try again to clarify things as I am trying to describe things well.

Mike

 

1) It is most likely a bad sending unit if it only does that on one tank and not both tanks.

2) You had it “pushed in?” You still haven’t said if the pump in the front tank turns on.

3) I’m sure there is probably something on the forum showing what the resistance should be for full and empty. How do you rebuild the sending unit? I have no idea where or how you would do that. Replace the pump and sending unit assembly.

4) You could disconnect the feed line at the DFR and use the pump to empty the tank but that’s only if the pump is working. Other than that you’d have to find some other way to get a small hose down the filler neck and use a transfer pump.

 

I will check the pump this weekend. But if it takes the pump in the tank to push the switch over to the other tank when selected I am going to assume it is bad and not working.

Thank you for the help.
Mike

 

The sender on these are an exposed variable resistor with a sweep.

After time, rust deposits & corrosion on the exposed contacts make the smooth action of the float arm impossible. (and a smooth reading)

You COULD pull it out & clean it up & have a better than average of it working after testing with a VOM. (done it, save the flame throwers)

They're pretty resilient if not rusted to moon scape.

 

I see the sender floats getting heavy (sink) quite regularly and affecting the fuel gauge
I see the in tank fuel pump wiring plug / connector being faulty on those early ones like his 88 (TSB to replace the tank connector)
In this case, I think his selector switch is faulty and would replace that first
Test away on the switch and the valve
Here is some testing for the crossover valve

 

OK, I turned the truck on and never a peep from inside the tanks, no matter which tank was selected. Now, I just saw the pics and diagrams as for some reason they did not show up either on my phone or at the office computers. So back to square one for testing. I will start with the switch and see what happens. If it is one or both tanks, I will have to run one dry and then transfer gas manually-fun.

Mike

 

Update:
I drove her last weekend on the 'R' tank and the gas gauge started to move and started going down and holding roughly where the fuel level should be after X amount of miles. So that tells me the switch is good since I flip to the F tank and it went back to pegged at full. AND, the trip meter is now working. So I will run the rear tank dry and see if she switches over to the front. If not I will siphon the front into the back and swap out the pump unit.

Mike