<VV> Fuel Gauge/sender questions

[Dale Dewald]

At 21:45 2/09/2011 -0500, Bob Gilbert wrote:

>Is that true of our older Corvair gauges or is the variable current caused
>by the variable resistor (the sender) changing the magnetic strength of a
>coil - and thereby moving the needle?

This is exactly how the Corvair fuel gauge operates, except that there are 
two coils working against each other.  Both coils are attached to the 
ignition circuit and receive battery voltage whenever the switch is 
ON.  The coil that pulls the needle towards F (call this the F coil) is 
grounded at the instrument panel and thus applies a constant magnetic 
force.  The coil that pulls the needle towards E (call this the E coil) has 
more windings for greater magnetic force and is grounded through the 
variable resistor attached to the fuel tank float.  When the fuel tank is 
full, the resistance in the sender is high (either 30 or 90 ohms), which 
reduces current and magnetic strength in the E coil; allowing the F coil to 
pull the needle to F.  When the fuel float is at the bottom of the tank the 
sender is at 0 ohms, the E coil develops full current and enough magnetic 
force to overcome the force of the F coil; pulling the needle over to 
E.  When the ignition switch is turned off both coils lose power, magnetic 
force drops, and the needle remains in the position it is at.

If there is a break in the wiring to the tank sender or failure of the tank 
sender that creates an open circuit, the F coil will not be balanced by any 
magnetic force from the E coil and the needle will swing to the right way 
beyond the F mark on the gauge.  If there is a loss of ground on the dash, 
the needle may swing way to the left or operate erratically with the 
operation of the dash lights, turn indicators, etc.

The instrument clusters on our 2001 Dodge, 2004 Jeep and my wife's 2003 
Audi are all of the digital electronic type.  Apparently they use small 
stepper motors to position the gauge needles based on digital input from 
one of the vehicle's onboard computer(s).

Dale Dewald
Hancock, MI