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.)
I asked for a little more explanation last night and here's what I got this morning. But I am still not getting any of this to happen.
Won't get to work on the Jeep today but I'll be at it tomorrow. Hopefully get this figured out.
But you get 12V on the white when the sensor is unplugged?
Yes but that isn't part of their test. I just happened to have the sensor unplugged and tested it. As soon as I plugged the sensor in voltage went to 0. In my mind this indicates that there is something wrong with the sensor too.
yes something would seem wrong there. If you’re sending 12v to the red input of the sensor then you should see it drop to zero but not if it’s not seeing power.
If you’re not seeing voltage on the white wire with the key off, either the fan (red wire) isn’t connected to constant power, or the fan’s pull up resistor inside is damaged. If you can give me the part number for the fan, I could tell you the value of the resistance for the pull up resistor and you can measure it between the red/white wires of the fan. This would confirm if the resistance is correct, or if the fan is damaged.
If the pull up resistor inside the motor is damaged, the fan may still run when you override the fan. (White to ground, yellow to +12V) But the fan would not work with PWM control.
Now the plot thickens....
So based on this now it's the FAN and not the temp sensor that is bad?? Fuck this is getting more expensive... If I have to replace the fan then I'll upgrade to the 500 watt model instead of the 300 watt version.
But you said you had 12V at the white when the sensor was unplugged.
But that doesn't have anything to do with the tests they are having me do..... As I said before I just happened to check this.
I DO NOT have 12V on the White wire with the Key Off and everything connected as it's supposed to be.
Do what they tell you and see where you end up.
For this fan the pull up resistance is 2.7 kOhm. This is measured by measuring the resistance between the fan red wire and the fan white wire.