I've done extensive work on my own 55 Crown Vic engine temp & fuel guage senders which both work on the same principle.

Both sender AND guage have bi-metallic heat-sensitive 'motors' in them along with resistance wire wound around the 'motor' elements.  The engine temperature 'influences' the sender bimetal 'motor' position, which influences the long-term-average current through both the sender and guage, which then heats the guage 'motor' to the appropriate calibrated position.  The fuel guage works similarly in that the fuel tank sender position 'influences' the sender bimetal 'motor' position, which acts identically on the fuel guage as the engine temperature guage explained above.

These senders & guages are matched-pairs, and cannot be used with any other senders or guages!  They will not work seperately!

IF YOU KNOW WHAT YOU'RE DOING, both the senders and guages can be re-calibrated; I have done just that; but, they must be matched, and you must completely understand the operating principles involved (I have an electrical engineering background - not working int he field now, am a 'Ham Radio operator', previous TV broadcast transmitter engineer, etc. who has been involved with this 'stuff' since about age 6 or 7.

Both the heat sender and the fuel tank sender have calibration screws; and, both guages have two calibration screws each: One for 'zero' adjustment (which is extremely sensitive and very painstaking to 'get right', and one for 'full-scale', which is easier to adjust; but, each is interactive with the other! So, you must go back & forth betweent he two many times.

If you should try any adjustments, PLEASE scribe the initial starting points before making ANY adjustment trials so that you can at least get it back to where it was.

These sender/guage systems are a full 6-volts for original 1955 units; and, are connected in series between the battery posts (use a 6-volt motorcycle battery for 'shop' adjustments).

I hope that this will help.

Thanks,   JLB

Thanks,   JLB

Yep, It was a simple very basic drawing to illustrate only.  You're absolutely correct, as my 55 is wired as you say.

In each case, the actual 'second wire' is vehicle chassis ground.

In the case of the original question in this thread, with both guages at their 'top', there is probably a problem with power getting to the guages from the ignition switch; had it been only one problematic guage (especially the fuel guage), I would suspect the ground connection (engine temp. sender would be difficult to imagine having 'lost' its ground; but, fuel tank sending unit losing ground is quite common - either between the tank and chassis, or between the sending unit and tank itself).

Thanks,   JLB

JLB.  Welcome to the site.  Here’s a link that gives more instructions in regards to picture posting. 

Picture posting instructions

Pay particular attention to the parts about picture sizes.  And all members on the YBF site have the capability of going back into their previous posts and editing which would include adding real time pictures to posts that they originally authored.  Or add pictures to a new post, your choice.

My sincere appologies if my first drawing confused you (or others!).

I do believe in the K. I. S. S. (to me, NOT you folks: "Keep It Simple, Stupid!") principle,; but, tend to go-on, sometimes (engineering background -- yes, I know!).  Sort of like the reminder to ask one's self: "Is it plugged-in, is it turned-on, is the fuse (or circuit breaker) blown, is there power at the wall outlet?", before "digging-in".

I've attached a modification of the initial drawing with the guage & sender shown in the correct 'as-wired' order for the '55 Ford. I hope that it helps 'unconfuse' anyone who looked at the first one.

Both guages read 'high' ('hot' or 'full') without power; this guage system is designed that way: Definitely "fail-safe" for the engine temperature; but, definitely NOT failsafe for the fuel guage.

Thanks,   JLB

As for why M/T's guages would go to 'empty'/'cold' at power off, I don't know; maybe his guage system has been changed to more 'normal' innards. Some folks have don't just that - and don't necessarily tell the new owner.

By the way, Old Carw Weekly just had some comments on the Ford King-Seeley guage system in their Q & A section last week. That information serves to verify my independent findings.

Thanks,   JLB

I meant to say 'your' guages, NOT 'M/T's' guages.

JLB

That's still strange: Your fuel guage should go to above 'full' with the key off.  Something else is still not right.  I'm really wondering whether your guage system hasn't already been replaced with a different system than OEM-style King-Seeley.

For the OEM King-Seeley system ONLY & per the Ford Manual: To test the guage portion, you momentarily jumper/ground the fuel sender lead from under the dash to a clean piece of dash metal to establish a 'solid' ground connection (with the ignition switch 'off') - be very careful to ground ONLY the lead coming from the sending unit and NOT the side from the ignition switch! - Or you can burn-up your under-dash wiring harness -- maybe worse!

Then sit up on the seat, and, while watching the guage, turn-on the ignition switch. If the guage is O. K., the fuel guage will quickly go to below 'empty'; do NOT leave the ignition switch 'on' any longer than it takes to see whether the guage goes below 'empty', or you WILL burn-out the guage!

DISCONNECT the temporary grounding jumper after turning-off the ignition switch first.

You can 'test' the sender wire between the gas tank & dash guage by simply repeating the grounding procedure AT THE SENDING UNIT CONNECTION under the ~4"-round cover located in the trunk.

Finally, after and only after verifying the above tests first, you can test your gas tank 'ground' by installing an actual wire from a clean metal area on the sender base (not the gas tank) directly to your vehicle frame to another clean metal area (NO RUST!!!!).

Please let us know what you find.

Thanks,   JLB

The fuel gauge in my ’55 Customline reads empty when the key is turned off.  I’ve switched the electrical system over to 12 volts but have simply wired in voltage reducers for the gauges.

I've had a '54, & two '55 Fords, and worked on '56's (12 volt, but same basic system), all of which have had the King-Seeley system; which works exactly as I, the original '55 Ford Shop Manual, & the recent Old Cars Weekly all verify, as I first put forward.

Is there another OEM system out there?  Any CVA members out there with the wiring diagrams, or other information.

My curiosity is really piqued now.  More please!!

Thanks,   JLB

I forgot about this!

I stand corrected, the fuel guage reads EMPTY with key off.

The TEMP guage reads HOT with key off.

Regards,   JLB