fuel gage

[flathead 53]

1965 comet fuel gage, seams to work but when you reach 1/2 tank mark it doesn't go down . when you shut off car the gage goes to empty like it should . turn power on and gage will go to half way mark. turn key to acc and gage shows 1/4 tank which is correct

 

[Daddio]

Sending units are potentiometers that vary according to fuel level.
Ford sending units (and Chrysler for that matter ... GM is exactly the opposite) display their highest resistance (around 74 ohms) when the tank is empty and lowest (around 5 ohms) when full.
If your Comet is like my Tbird, there's an access cover in the trunk floor to the sending unit. There should be 2 wires running from it, a ground and a gas gauge wire. Disconnect the gas gauge wire and see what your dash displays ... it should show an empty tank (ie; max resistance). Then touch this wire to ground (briefly, so you don't harm the gauge) and it should read full (ie: min resistance).
This will confirm the dash gauge and wiring are good and the problem is in the sending unit.
I suspect there's a short in your sending unit that only allows a maximum of say 35 ohms (1/2 of max resistance), hence the 1/2 full tank on the dash gauge.
Now why it makes a difference in acc. versus run, I don't know, this is the extent of my knowledge, based on the work I did on my Massey and Caddy fuel sending units.
Hope it helps.

 

[dmsfrr]

Sending units are potentiometers that vary according to fuel level. ......

It depends on the year the car was built and if the gauge & sender are original or newer reproductions.
Here's a diagram of the oem setup Ford used until the late 50's.
The gauge reading is controlled by bi-metal contacts in the sender & a heating coil in the gauge.
Not sure what year Ford changed to the newer version, I'm guessing somewhere between '57 and the early 60's.

 

[Bob-93021]

By your description it seems the electrical part of your fuel gauge circuit is in order. I would suspect the float arm somehow hanging up around the 1/2 tank mark. I have also seen gauge pointers that were rubbing against the gauge face and not returning to "0". I actually had that happen on a speedometer. In that case it was a plastic pointer and bent itself from heat? - Bob