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Posted

Need some help fellas.....86 F150 4x4 351 Windsor HO with dual tanks and mechanical fuel pump.

My fuel gauge has not worked for nearly a decade. I can however switch tanks. In fact, switching tanks is how I keep from running out. Anyway, I just put a new gauge cluster assembly in my truck to acquire a tachometer and still no fuel gauge. As such, I do not think it is the gauge or anything in the cluster.

About five or six years ago I had the bed off and changed both sending units. As I recall, I checked to see if they worked by moving the float and watching the gauge move. Thinking they worked because the gauge seemingly worked, I buttoned everything up and put the bed back on to later realize, I still did not have a working fuel gauge. I suspect I probably have some kind of ground related issue, but not sure how and where things are wired in. I am not the best at reading the electrical schematics but I don't see a schematic here specific for my carb equipped truck. Your input and diagnostic advise would be most appreciated.

Thanks!!!

 

Posted

John, you may want to look over this post as I think it will help you test & figure out what could be going on with yours.

http://forum.garysgaragemahal.com/Another-dual-tank-guage-problem-td74590.html

In short I would check with a test light at each sender if you get a blinking light.

If you do then check the ground at the same plugs, test light between the 2 wires it should blink and if so all is good to the senders.

When checking make sure you have the dash switch to the tank you are testing.

If no blinking light you need to work forward. There is a harness plug under the brake booster by the frame rail & floor board.

Unplug that and test for the blinking light, you should have 2 wires that blink, 1 for each tanks when switched.

If still no blinking light you need to get to the tank switching switch and check there.

You should have a blinking wire going into the switch and 2 coming out when the switch is switched.

That is if it is set up like my 81 is and I think yours is with the pump on the motor.

It get a little harder to test if you have in tank pumps and that crazy switching valve but you should still have blinking test light at the senders if everything is good but the senders.

Dave ----

Posted

John, you may want to look over this post as I think it will help you test & figure out what could be going on with yours.

http://forum.garysgaragemahal.com/Another-dual-tank-guage-problem-td74590.html

In short I would check with a test light at each sender if you get a blinking light.

If you do then check the ground at the same plugs, test light between the 2 wires it should blink and if so all is good to the senders.

When checking make sure you have the dash switch to the tank you are testing.

If no blinking light you need to work forward. There is a harness plug under the brake booster by the frame rail & floor board.

Unplug that and test for the blinking light, you should have 2 wires that blink, 1 for each tanks when switched.

If still no blinking light you need to get to the tank switching switch and check there.

You should have a blinking wire going into the switch and 2 coming out when the switch is switched.

That is if it is set up like my 81 is and I think yours is with the pump on the motor.

It get a little harder to test if you have in tank pumps and that crazy switching valve but you should still have blinking test light at the senders if everything is good but the senders.

Dave ----

Dave, I had looked over that link before I posted. The wiring diagram and very valuable ground location that both Jim and Gary left there are not specific for my truck. At least, as far as I can tell. I am certain I have a ground problem and knowing where the location of these grounds are is most important. From what I have read too, a faulty ground is a common problem in these trucks in regards to the fuel gauge not working.

Anyway, I will attempt to locate the problem as explained by you using a test probe. I must admit, I'm a little nervous in testing a live circuit for fear that I will fry the circuitry. What can go wrong, always goes wrong for me. I have terrible luck. Can you confirm that I will not fry anything with that test probe....lol! Thanks for helping out!

 

Posted

Dave, I had looked over that link before I posted. The wiring diagram and very valuable ground location that both Jim and Gary left there are not specific for my truck. At least, as far as I can tell. I am certain I have a ground problem and knowing where the location of these grounds are is most important. From what I have read too, a faulty ground is a common problem in these trucks in regards to the fuel gauge not working.

Anyway, I will attempt to locate the problem as explained by you using a test probe. I must admit, I'm a little nervous in testing a live circuit for fear that I will fry the circuitry. What can go wrong, always goes wrong for me. I have terrible luck. Can you confirm that I will not fry anything with that test probe....lol! Thanks for helping out!

As long as you are playing with the DB/Y and Y/LB wires you can't hurt anything. You can ground one of them and, assuming the switch is pointing to it, all that can happen is the gauge goes to Full. Or put your test light on and, as Dave said, it should blink.

And Dave is right that your truck should have the ground for everything in the rear going to G701. You can see that on the schematic below for the fuel system, and on the page below that you can see that G701 is behind the instrument panel near the right side of the radio.

2630970_orig.thumb.jpg.04e7ca461cc6ef0c9c12be7f96b31566.jpg

3393280.thumb.jpg.dd260ae8e2d89c3afcee0343954593d3.jpg

 

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