Key on/off just means the sensor is powered or not. It does not include the blue override, so the blue wire does not need to be connected to +12V for this test. This test is done with the white wire from the fan connected to the white wire of the sensor, you are just probing the wire for voltage. The fan has an internal pull up resistor between the red wire of the fan and the white wire of the fan. So when the fan is connected to constant battery voltage, you will measure battery voltage on the white wire. The fan provides the ‘high’ +12V portion of the square wave PWM signal, the sensor supplies the ‘ground’ or ‘low’ portion of the square wave PWM signal.
This test is conducted with the white wire from the fan connected to the white wire of the sensor, you are just monitoring the voltage on the white wire:
- No +12V on the red sensor wire (key off): The fan provides battery voltage on the white wire. You should see ~12.5V
- Sensor powered on red sensor wire with +12V (key on/cold engine): The sensor will wake up and pull the voltage down to zero volts upon being powered. This shows that the electronics in the sensor are activated. You should measure zero Volts.
- Sensor outputting (key on/warm engine): When the sensor is outputting you will measure a DC voltage that is proportional to the speed request going to the fan. At the minimum coolant temp you will see 20% of supply voltage. At full speed or override you will see 90% of supply voltage. (Example: If the sensor is overridden with +12V on the blue wire and the key on/engine is off, assuming 12.5V is a fully charged battery, as the sensor overrides to full speed, you will measure 90% * 12.5V = 11.25V on the PWM wire.)
