i have a 91 302 with single tank. Fuel gauge is pegged past empty with key on pegged past full when the sending unit is unplugged and floats with the key off. It has a newer sending unit and I just replaced the float assembly to make double sure even though it was fairly new. So is my next step to start tracing the wire looking for a pinch? Thanks

 

I would pull the sending unit and test the resistance with an ohm meter (multi-meter) first. You say it's newer, but I've had a bad part out of the box before.
It's easy enough to test, ground lead to the body of the unit and positive on the float lead. Moving the float up and down will change the resistance. If that doesn't happen the unit is bad.

 

Thanks I will do that first. I was going to when I had it out but assumed it was good since it's only a year or two old. That's what I get for assuming.

 

I've never seen what kind of fuel level sensor Ford uses in these trucks but for YEARS the auto companies used a brass or plastic float on a swinging arm to measure the fuel level and it was common for the float to develop leaks and to sink to the bottom of the tank. I would guess that that's what is going on here.

 

That is not what it is. It has maybe a year old sending unit and I also just replaced just the float assembly to make double sure. While I had it out I didn't check the ohms. I have been on vacation and haven't had time to mess with it since I got back.

 

Did you check the float to be sure that it's actually floating and did you check the arm to be sure that it's moving? You should be able to connect the sensor with it out of the tank and move the arm manually and see that change on the fuel gauge. You'll probably need to ground the sensor (body not the output) since they may not have a ground wire going to it.

PS If the float body is brass, I'd put it into some HOT water and see if any air bubbles come out of it. I've seen a lot of them get tiny pin holes that were hard to see and they would SLOWLY sink and throw the gauge off. If that body is plastic you can probably use some what hot water but I wouldn't use any that is too hot to put a finger into.

And most of the "new" floats that I've had, sunk the first time that I used them so I now check them before I install them.

 

If there is a break in the wire going to the float it will register pegged full. I replaced a front fuel pump in my 96 f150 and a few weeks later it pegged the gauge. when I raised the bed and pulled the pump and hangar assembly, I found the sender wire had slipped off the terminal and upon replacing the wire to the terminal, it reacted normally ever since.