Normally there's two coolant temperature sending units. The sensor for the Engine Computer will always have two wires. The sensor for the gauge almost always has a single wire.
I can't find any reference to the single-wire sensor for the gauge, (I looked for the 3.4L engine), so it is likely there is just the one. By 2002 most instrument clusters were their own computers and they shared information with the Body Computer and Engine Computer. If that's the case you'll only find the one sensor. It is close to the thermostat housing at the end of the upper radiator hose.
Before we get wrapped around the axle chasing a problem that might not exist, feel the heater and radiator hoses after driving a while. They should be too hot to hold onto for very long. If they are not, you may have a thermostat that's sticking open. That would result in the gauge staying low. You could verify that by connecting a scanner to view live sensor data to see what the actual temperature is.
The radiator fan turning on when you unplugged the sending unit is a safety feature and will occur regardless of engine temperature. That is because with an open circuit, (sensor unplugged), the computer can't know coolant temperature so it runs the fan in case the engine starts to overheat. Unplugging the sensor is a quick way to verify all the rest of the fan circuit is working. When the sensor is properly connected the computer goes by its voltage to determine when to turn on the fan. That can be as high as 220 degrees. On most other car brands the fan turns on at around 210 degrees and it turns off at around 198 degrees.
Tuesday, June 11th, 2013 AT 1:17 AM