It’s a fairly simple circuit and I think if I explain how its suppose to work, it won’t be hard for you to figure out what’s happening with your fuel gauge.
When the ignition is turned on 12 volts are sent to the CVR (constant voltage regulator). The regulator drops the voltage down to 5 too 7 volts; this is done in an attempt to keep the voltage at not so much a lower voltage but constant voltage, for all the gauges; regardless of electrical demand on the rest of the system. Remember back then the car likely only had a 28 or 32 Amp alternator; with the lights, heater and wipers on. (Yes I mean all the accessories. LOL) the battery voltage would be fairly low and could affect the gauge accuracy (another joke back then).
Voltage is then applied to the electromagnet of the dash unit, through the dash unit too the fuel tank-sending unit’s variable resistor, too ground.
Depending on the amount of fuel in the tank and the position of the float will determine the amount of current (amperage) flow through the circuit. The more current flow the stronger the electromagnet will become; the stronger the magnet the more the needle will be pulled towards empty.
On the other hand if there is no current flow in the circuit the permanent magnet will overcome the Electro and send the needle towards the full mark.
Why the needle in yours moves differently with the ignition on, then when it’s running is likely some stray magnetic fields being produced from another electrical device.
One quick check would be to ground the wire going to the tank unit; if the circuit from the battery through the CVR, sending unit to the tank unit is ok, the gauge’s needle should drop to or below the empty mark.
Back then almost every GM had the same colour of fuel gauge wire leading back to the tank and that would be a tan wire. You will likely find it back by the filler neck of the tank
Dan.
OK I CAN"T SPELL BRUSH! LOL
Canadian "EH"
(This post was edited by DanD on Dec 11, 2007, 4:23 PM)
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