Fuel gauge stays full

[tartan18]

I am at my wits end. The tank and sender are fully grounded. I did have a loose connection between the gauge and the speedo wiring harness connections but that is now corrected. I believe the problem is at the gauge connections to the wiring harness and not the tank or sending unit. Any suggestions if where I need to test the proper connections? I have a brand new sender and new fuel tank. I also have an older fuel gauge (that worked) and sender that work the same as my new one so I am confident that neither of these is the source of the problem. If the problem is the sender .. then how do I confirm this?

[spider2081]

The fuel warning light and the gauge needle both test correctly when you ground the individual wires. This test comes from Fiats own 1980-81 electrical diagnostics Manual. For testing the gauge and cars wiring for proper operation.

If I ground the other wire to the sender's metal ring the gauge indicates the proper fuel level in the tank.

I am not sure of how you are making your ground connections for these tests. If you touch the senders mounting ring with the gauge wire and it reads less that full deflection that is a bad ground.

Grounding the wire at the sender pegs the fuel level fully

By grounding the gauge wire for this quote is it also by touching the senders mounting ring?? Did you use the same ground point both times??? Pegging the fuel level fully is the correct thing that should be happening when the wire is grounded.

[tartan18]

I have tested the ground for the fuel gauge sender unit that is installed in the fuel tank by touching the two wires to the metal top of the sender unit. In both cases either the low fuel light illuminates or the fuel level needle has full deflection. To test for proper grounding I am using an ohm meter between the sender's metal top and the electronic fuel pump mounting bracket (to test both connections). However neither wire when connected to the spade terminals on the sender unit generate any signal. My car is a 1975 and the battery and fuel pump are located in the trunk. All readings indicate zero ohms of resistance between the sending unit's metal top and the car's body. Today I will re-install a brand new sending unit and see what results. However I do not think the wiring in the trunk is the problem but rather the wiring to the gauge itself is the problem.

[spider2081]

I have tested a number of gauge systems in various year cars and the test you state indicate a defective sender, Although I have not seen a sender have both the lite switch and the gauge section fail, usually its just the gauge section is intermittent.

[tartan18]

While I was wiggling the various connectors for the gauge cluster in the dashboard the fuel gauge momentarily sprung to life (partial deflection of the fuel level). I will try to trace all the wires in each connection block to isolate the problem. Electrical issues make me nuts.

[tartan18]

Problem solved. It turned out to be a loose connection to the gauge itself. Gauge now works as designed. Thanks for the help.

[spider2081]

Great job. Intermittent conditions are the most difficult to find. From some of you tests I truly believed the problem was at the sender. Sorry for adding to your frustration.

[Bennie]

Just for the record I replaced my fuel sender but it didn't change my pegged fuel gauge. Pulled the sender back out, acquired a better multi meter and verified the sender potentiometer/resister readings to verify it was working properly. Reinstalled, checked grounds still no change: anybody know what BN means on the back off the fuel gauge? Because, after testing the continuity at the gauge I moved the 12v feed and its jumper and the White Black ground wire and bingo. Working. Now on the temperature sender . Love Fiats

[spider2081]

anybody know what BN means on the back off the fuel gauge?

I have a spare gauge from a 1979 and I don't see a "BN"stamped into the gauge The gauge does have a terminal with the letter "B" stamped near it. This terminal has a white/black wire on it and is the gauge ground. it is located on the plastic for the low fuel lamp in the gauge.
I don't believe the cars wire colors match the gauges colors at the 6 cavity connector.

[18Fiatsandcounting]

I also don't know what "BN" stands for, although I have seen it on some gauges. Along with "GN".

My understanding: The Yellow/Black wire is the 12V supply for the gauge and the low fuel light, the White/Black is the ground for the light for the gauge housing, the Pink goes to the fuel tank sender unit, and the Grey/Red is the grounding on the sender unit to turn on the low fuel light. White is the positive for the light for the gauge housing.

-Bryan

[DieselSpider]

Body Neutral/Body Negative and Battery Negative come to mind as what BN meant on wiring diagrams from other cars and devices I've worked on in the past.

[YoukaiMori]

Problem solved. It turned out to be a loose connection to the gauge itself. Gauge now works as designed. Thanks for the help.

I'm having the same problem I will definitely check this first thing!