Denny Graham,
I have enclosed a copy & paste description of how the fuel gauge functions. Stu and Bill have studied it and know it to be true, as well as the engineer in Detroit who designed it.
After you study it, see if it seems logical to you, then let the engineer in Detroit know you have put the "Sandwich Stamp of Approval" on it so he will rest easy tonight.
"The fuel gauges in our old trucks use two coils (electromagnets) to position the needle.
One coil (called the limiting coil) pulls the needle to the left or empty position. The other coil (called the operating coil) is positioned at 90 degrees to the limiting coil and pulls the needle towards the full side.
Current from the ignition switch, enters the limiting coil when the switch is in the 'on' position. The other end of the limiting coil is connected to both the operating coil and the tank sending unit. The other end of the operating coil is connected to ground.
If the gas tank is empty, then the resistance of the sending unit is near zero ohms and all of the current from the limiting coil goes to ground through the sending unit. With all of the current flowing through the limiting (empty side) coil then needle is pulled to that side.
If the gas tank is full, then the sending unit has about 30 ohms of resistance and most of the current from the limiting coil will pass through the control coil to ground. This current will cause the needle to be pulled away from the limiting coil (empty side) and towards the operating coil (full side). The operating coil has more windings than the limiting coil so it takes less current to generate enough magnetic force to overcome the limiting coil and pull the needle to the full side."
Stuart
