Fuel Gauge - how it works

[IlludiumQ36]

I have seen a number of post in here on how to troubleshoot the fuel gauge/sender combination but I'm always a fan of figuring out how it works so here it goes.

First of all, a shout out to VetteVet for posting the schematic:


There are just a few details missing. This one tells the same story a bit better:


Ignore the external resistor for now, it is used to control the current ratios between the Empty and the Full coils, it does not really change the theory of operation for this type of gauge.

As a current passes through a coil a magnetic field is produced. The more current and/or the more windings, the stronger the magnetic field.
The needle itself is attached to the armature which is pushed or pulled around by the opposing E & F coil magnetic fields.

For the case of an Empty fuel tank, the sender is at 0 ohms, the current passes only through the Empty coil & then to GND -> the needle motion heads in the Empty direction:


For the case of a Full Fuel Tank, the sender is at maximum (30ohms for early models, 90 ohms for later) and current now passes through the Empty Coil, the Full Coil in parallel with the sender resistance. The effect is that the Empty coil sees less current than it did at empty AND the Full coil sees as much current as it would ever expect to see with a functioning sender.



The coils have to be properly sized, the external resistor properly sized, and the armature location correct for this balancing act to work. Very simple but complex at the same time. Clever folks. It is, in effect, a very very weak motor that is capable of moving an indicator needle.

The takeaways:

1) Missing or bad ground at the gauge/dashboard location - all the current runs though only the Empty coil -> Sender & the gauge reads hard E.

2) Missing or bad ground at the tank/sender, or failed open sender - all of the current runs through the Empty -> Full coil path & the gauge reads hard F.

3) Missing or bad 12V - no current & the gauge just sits there where it was.


Some folks had some reads on the coil resistances - I'd expect that they would probably vary by sender ranges. I have a 57 Chevy gauge and a 70 C-10 gauge, I'll have to check that some day.

The external resistor from IGN to Sender shunts some of the current around the the Empty coil, making its magnetic field weaker than if it had all of the current - seems to be a "select at test" type of item.