I'm having problems with my 1978 F100 fuel gauge after replacing the following items:

new printed circuit board - the old one was bad on the fuel gauge circuit
new circuit board voltage regualtor
new Spectra fuel sending unit (from AutoZone)
replaced the fuel gauge - the original was bad at one of the posts
new wiring from the factory quick connect to the sending unit pig tail

the symptoms:
1) with the fuel sending unit out of the tank and in my hand the fuel gauge registers full when the sending unit is at full, mid when at mid, and empty at empty.

2) with the fuel sending unit installed in the tank, and the tank mounted in the truck, the gauge reads full when the tank is full, but after driving approx 100 miles (half a tank) the gauge gradually reads empty and when I fill up I'm only down about 9 gallons on the tank.

3) I have dropped the tank twice now to adjust the float thinking that it was improperly adjusted, but the second drop I adjusted the float pretty accurately and reinstalled.

I can't seem to figure out where the problem is. Everything seems to work properly when it's not installed in the tank, but once the sending unit is in the tank the issue arises. Help???

 

When you have the sending unit out of the tank are you using the same ground as when the sending unit is in the tank?

 

I am using the factory connect in both cases.

 

Thats odd. One thing you can try, take an ohm meter and check the resistance of the sending unit. Fill the tank up and check the resistance, it should be between 8-12 ohms. Then drive the truck untill the gauge reads empty and check the resistance again. It should be between 60-86 ohms. If the resistance is somewhere inbetween but the guage is reading empty I would suspect the fuel gauge or the IPV even tough you've replaced both.
Also, just a thought, check the float on the sending unit for cracks or see if it feels heavy.

 

thanks for the tips. I will run the check tomorrow.

What seems odd to me is that the sending unit works when it's not installed in the tank. With the float arm up the gauge reads full, with the arm at half, the gauge reads half, and the the arm at bottom the gauge reads full. The gauge registers full when the tank is full, which leads me to believe that the float isn't the issue. The only thing that changes is that the unit is in the tank. Strange.

 

Yeah, thats strange. I really don't understand why it would act like that.

 

you have the wrong sending unit for size of fuel tank

 

Spectra part number: FG73A - http://ecat.spectrapremium.com/apps/...A&splash=false

Although the float on the one I installed (and a second one I examined at the auto parts store) is slightly different. It's a black rectangular flatter style float that is attached at 90 degrees to the arm and actually spins.

 

I checked the resistance on the sending unit this morning, it is reading 60 ohms and the fuel gauge is at E. I bought another sending unit, same model, and tested it's position at 60 ohms, and the position on the float arm indicates that the gauge should read about 1/3 of a tank. My next step is to test the new sending unit out of the truck with the gauge at the same position to see where the gauge registers.

 

My troubleshoot seems to indicate that it's a bad gauge. Both sending units register E on this gauge. So I will begin the hunt for a NOS gauge.

 

finally got a NOS gauge from a seller on ebay. the issue is not the gauge. any ideas???

 

thegringo, i am having the same issue with my guage. I put in a new sending unit and now i can put 10 gallon in it and it will show full but within 100 miles it goes to empty.

 

I didn't know if you got this issue resolved and what was the remedy. I am having the same issue.

 

Welcome to FTE. In the future, please start a new thread for your question. I almost responded to the OP instead of you. I fear other folks may do the same.

Can you please describe the problem from the beginning? Are you actually having the exact same problem as the OP, or does your fuel gauge simply not work? The OP's problem was a little more complicated and it's unlikely you went through the exact same steps that he had to.

 

yes I am having the same issue verbatim and hoping he found a fix for his that he just didn't respond back with.