The fuel gauge on my 89 F-150 stayes pegged until about half tank then reads accurately??

 

They all seem to do it. You can go to a lot of trouble and replace the sending unit, but I am not sure that will fix it. My 89 does the same thing. I don't know if it's half a tank, but I do notice when I fill it, it stays pegged for a long time, and when it does start moving, it doesn't take long for it to make it to empty. As long as empty is correct is all I worry about.

 

It just started doing it about a month ago. But at least it does still function.
Thanks

 

Well, replacing the sending unit might help it out then.

 

With fuel gauges, the more resistance that is in the path, the farther toward empty the gauge reads. Ideally you want all of the resistance to appear through the rheostat in the sending unit, but if resistance shows up anywhere else along the path, it will provide a false offset from full. A good example of this is ground shift - because of rust, road grime, corrosion, etc - the ground seen at the sending unit moves away from true ground. My suggestion is to disconnect the sending unit lead and ground connection, and measure the resistance of the sending unit rheostat. Then reconnect the sending unit, and measure the resistance seen from as close to the fuel gauge as possible to ground (with the fuel gauge disconnected). On my '79 there is a connector at the firewall in the engine bay that I can probe against the engine block. It essentially disconnects the sending unit from the back of the instrument panel. Any additional resistance seen in the second measurement is due to the wiring to the gauge, or ground shift between the sending unit and true ground. On my '79 at least, the sending unit is grounded to the frame, which is grounded to the cab with a strap, which itself is grounded to the engine block with another strap. I can't imagine your '89 would be all that different since this is a very standard setup. Of course, the sending unit itself could be bad as well.

Fuel gauges suck regardless. Mine didn't work when I got my truck; it permanently read empty because the float punctured and filled with gas. I found this out because the resistance would show empty, yet I'd know the tank was full. I pulled the tank to replace the gauge, and the new gauge lasted about a week, until the same thing happened (thank you China). Now I write my mileage on the dash with a piece of chalk. You've got to pick your battles!

 

That is true for the Fords from the early 60's to the late 80's/early 90's. I am not sure what year they did it, but when the changed the gauge cluster(maybe 87?) and got rid of the gauge voltage regulator system, they reversed the way the fuel sending unit works, and low resistance is full. GM always used this method in their early cars also.

We will get a post now and then of someone complaining their gauge reads backwards after they swapped a later tank system with a fuel injected engine into their earlier 80's truck with the original early dash unit.

 

Gotcha, thanks for that information. That's good to know. Did you mean low resistance is empty on the newer models? On my '79 (with a gauge voltage regulator), low resistance is full and high resistance is empty; grounding out the sending unit pegs the needle to full and open-circuiting the sending unit pegs the needle to empty.

 

I can't ever keep it straight, I just know it's reversed. That sounds correct.