I just bought a 1979 f-150 custom and when i was driving the beast home i realized that the speedometer was reading way to high and then half way hom i ran out of gas and the guage reads full. i have no idea what the problem is with either of these. Thanks for your help.

 

Is the fuel gauge *Pinned* to the Far right?

Has the speedo been acting up lately or just now?

 

the gas guage was pinned but then when i went to drive it 2 nights ago it was down and never came back up it reads far bellow and empty. The speedo i just don't know about because i just bought the truck and the person i bought it from wasn't the biggest soure of info but i asume it has been like that for a while.

 

The Fuel Tank sending unit is the problem.
Sometime cleaning & testing it resolves the problem.
Possibly the new grounding surface makes it work better.


The speedo probably has the wrong gear in the tranny to match the rear end ratio.

A quick pop-out of the speedo cable at the transmission will clarify which one is in there.
They are coloured for easy ID.

 

You test the fuel guage by momentarily grounding the wire at the Fuel tank sending unit.
If you pop the wire off the sending unit, and ground it, the fuel guage should start to rise. (You'll need a buddy for this one)

That wire goes into a connector just in front of the frame member that hides the tank sending unit. Pop it apart there and test.

If you insert a light bulb in between the sending unit wire and ground, you will see the frequency of the Dash Voltage regulator. The light will blink on/off. That will show you the circuit is good from the tank sending unit up to your fuel guage. No light? Equals an open circuit!
This works for the Temp, Oil and fuel guages. Ignition switch on for that test.

When your fuel guage was pegged, it probably meant the wire going to the Tank sending unit was shorted to ground.
Now you have no fuel guage input, probably means the wire is dis-connected(Broken). Maybe disconnected right where it was shorted to the frame.

 

thanks for the help i will check that out when it comes back i have it in now for a pre safety. I am anxious to see how much work this crate is going to need. thanks agian bob

 

On my truck, the front tank reads perfectly fine, while the back tank "peggs" to the right, no matter how little gas is in there. I have tried to find the wiring for it, and trace it, with no success. I know on the Supercab, some of the wiring is "split" down the passenger and driver side. I believe the wiring for the rear tank, is ran down the passenger side, until they meet up in the front, correct!?

Also, in the last thread I just read, you mentioned something about testing the wires to ground, etc. This should also work for me too, correct!? As well as the whole light buld test theory!? If it *is* the sender, then the tank needs dropping, correct!?

Also, my speedo is approx. 10-15 mph higher, after 35 or 40, all the way to the end of the spectrum.

A few years ago, I went from 33x12.50s to 35x12.50s. This made it about 6 or 7 mph higher. I recently put a locker with 4.10's in the rear, and that's when the speedo went so high.

I know there is a "formula" for figuring out which colored bullet you need. Does either anyone know the formula, or anyone know, off the top of their head, which bullet I need, and a suggestion as to where to get it!? I have contacted a couple speedo shops and tranny shops here in the South Bay area or SoCal, and every one of them says they don't know either what I am talking about, or where to get them!

--- Charles!

 

Well the "bullets" are speedometer gears. Any speedo shop can recalibrate your speedo. Of course being out in so Cal.... (insert Twilight Zone song here)

The wiring can be traced from the tank selector valve which if I remember right is on the driver's side frame rail.

Your wiring to your rear tank sensor is shorted to ground somewhere beyond the tank selector valve.

 

This site might help you with your speedo .

http://www.seansa4page.com/resource/autotech.html