Fuel Gauge Stuck at Full

[JimsHot66]

The gauge used to work. I was messing with mounting my spare tire correctly (it was bungee corded in place before) and now the fuel gauge is stuck at full. I was careful not to mess with the yellow wire and assumed I must’ve damaged it and it is now open. After some checking I’m not so sure.
What I’ve done:
- The yellow wire is not grounded (open to ground)
- Disconnect yellow wire from sending unit and gauge goes to E
- Ground yellow wire and gauge goes to F (tells me wire is intact)
- Measured sending unit at the tank to Ground = 15.4 ohms with about a half a tank of gas.

Tomorrow I’m going to put about 40 miles on the car and will measure sending unit to ground to see if/how it changes. Also, the tank and sending unit are fairly new and Mustang parts. Any ideas or suggestions? I’ve heard the sending units are problematic but I don’t think it’s a coincidence the fuel gauge breaks at the same time I was messing near it’s wiring. Photo of trunk, with yellow wire near back seat, for your viewing pleasure. Also, I have the shop manual, with wiring diagram...it's pretty straight forward...on paper.
Thanks,
Jim

[A/FX]

Fuel level gauges have the ability to test your patients. I have a stock tank in my Comet and a new old stock original sending unit, the tank reads full for a long period of time then drops to 3/4 full. After that it seems to go towards empty at a normal rate. I removed the sending unit after the initial driving season and everything looked good, I tested it with an ohm meter and it worked as it should so I reinstalled everything. With the tank drained (20 gallon capacity) I poured in five gallons and watched the gauge go to just over 1/4 full, which is close to what it should read. I then cut out a small red vinyl dot and installed it on the gauge face even with the needle so that I know when I have 5 gallons left in the tank.(nothing worse than running out of fuel)
Also stock sending units are set up for a 5/16” fuel line, I modified mine to fit 3/8”.
I just saw this unit which is priced reasonable and has a 3/8” tube already installed:
https://www.oerparts.com/product/9275a.html
Jim

[Joe Travers]

I have a stock tank in my Comet and a new old stock original sending unit, the tank reads full for a long period of time then drops to 3/4 full. After that it seems to go towards empty at a normal rate.

Mine does exactly the same, Jim.
I make a point to fill the tank when I get below a quarter full because it is bone dry @ the 'E' mark.

Joe

[fryedaddy]

my 66 always ran out at 1/4 tank,but it worked above that.i never did fix it.

[JimsHot66]

Jim, Thanks for the link and info. I’ve got a 3/8” outlet with a 16 gallon tank.

I’m pretty sure I need a new sending unit. After 40 miles of driving the ohm reading from sender to ground measured 17.4 (was 15.4). The gauge fluctuates between ¾ and full. I should have burned about ¼ tank and the gauge should be around ¼.

Talking to folks at Cars and Coffee yesterday some indicated the replacement sending units are garbage and replacing these is tricky without damaging the seal.

Are there any tricks to replacing these? Looks pretty easy to me, once I have all the gas drained.

Local part stores (Napa and O’Reilly) have these for $87.00 versus $30 online. Anyone have experience, good or bad, with replacement units? The one at O'Reilly, made by Precision, looks pretty cheaply made based on the photo. Buying local makes it easy to return if I have any issues.

Thanks,
Jim

[Lou's Comet]

Are there any tricks to replacing these? Looks pretty easy to me, once I have all the gas drained.

Test the new one to make sure it works right before taking the old one out.

Put a thin coating of grease on the O ring gasket to hold in in place and protect it during installation

Lou

[A/FX]

Jim, Thanks for the link and info. I’ve got a 3/8” outlet with a 16 gallon tank.

I’m pretty sure I need a new sending unit. After 40 miles of driving the ohm reading from sender to ground measured 17.4 (was 15.4). The gauge fluctuates between ¾ and full. I should have burned about ¼ tank and the gauge should be around ¼.

Talking to folks at Cars and Coffee yesterday some indicated the replacement sending units are garbage and replacing these is tricky without damaging the seal.

Are there any tricks to replacing these? Looks pretty easy to me, once I have all the gas drained.

Local part stores (Napa and O’Reilly) have these for $87.00 versus $30 online. Anyone have experience, good or bad, with replacement units? The one at O'Reilly, made by Precision, looks pretty cheaply made based on the photo. Buying local makes it easy to return if I have any issues.

Thanks,
Jim

I Have no experience with aftermarket sending units so not much help there. The installation is straight forward, just be careful to have the O ring in the groove and not slip while turning the locking ring. As Lou mentioned a slight film of lube helps with the install.
When I initially installed the sending unit during the build I had the tank temporarily in the car at ride height, I measured the angle of the sending unit mounting surface and set the sending unit at the same angle but in a vise. I then tested the unit with an ohm meter throughout the range to make sure the readings were correct. I then hooked up the guage to the temporary mounting of the sending unit to confirm that it read correctly. All that and it still sits at full then suddenly drops to the correct level. Yes, I did make sure the float does not stick when it is in the full location, I may pull it again this winter to try and remedy the issue.
Jim

[JimsHot66]

This is embarrassing…apparently my car gets better gas mileage than I thought. What I expected to be about 2 gallons of gas in the tank turned out to be closer to 7 gallons, which jived with gas gauge before I started draining. After draining, gauge close to E and sender measured 64 ohms. There was still gas in it because I ran out of empty containers . Put 2 gallons of gas in it, gauge closer to ¼ and sender measures 60 ohms.

The few times I’ve checked mileage I’ve gotten between 8.8 and 10 miles to the gallon (really varies depending on how you drive it ). Tomorrow I’ll fill it and start checking mileage more diligently. Still possible the sending unit is flaky so I’ll just hang onto the new sending unit.
Note on the sending unit: Got it from Napa for $71. It has brass float and cloth filter. Apparently many other replacements are plastic float with foam filter.

Thanks for all the advice on installing a new sending unit…probably come in handy someday.

Jim