Fyi
This might help too: From
CarJunky.com
If he unplugs the fuel tank sender the gauge will (Or should) go to the full mark (high resistance) but if he would ground out the purple wire at the sender the gauge should go to the empty mark.
This is with the ignition in the run position.
A word of caution though if you are going to try and ground the purple wire, do it only long enough for the gauge to reach empty. It should only take a couple of seconds to get there any longer you could cause damage to the cluster due to the high current flow.
Dan.
Below is taken from a GM service manual.
Gauge Description
Fuel Gauge
The fuel gauge is an electrical instrument that measures an electrical current from a variable resistor in the fuel tank. A float controls the variable resistance.
When the fuel tank is full, the resistance is high. Then, the fuel gauge moves to the maximum position (Full) on the gauge face. For diagnostic information, refer to Instrument Cluster System Check .
Fuel Gauge Sender
The fuel gauge sender attaches to the top of the fuel tank. A cam lock ring retains the sender. The tank and the sender uses a seal between them.
The sender has two or three pipes attached to hoses. One pipe is for the fuel feed line. The second pipe connects to the vapor canister. The third pipe is a fuel return line to the fuel tank.
Some senders use a short pigtail connector lead. On other senders, the connector attaches directly to the sender. For diagnostic information, refer to Instrument Cluster System Check