aux fan failure confirmation

[oldskewel]

Originally Posted by DTM_Theory

...One question though; is the aux fan triggered by the temperature read by the CTS on the lower radiator hose?

I tested the hell out of my bad fan before replacing it several years ago. Toughest part as usual was separating internet fact from fiction.

Based on tests on my 2001 3.0i, with an at-the-time dead aux fan ...

Short answer is that yes, it uses that temp, but the engine must be running for it to turn on the fan due to a high temp there, and also just pulling the connector off the sensor does not work since it simulates -54*C. Putting a 100 Ohm resistor across the connector simulates hot enough to turn the fan on when the engine is running.

My notes:
Experiments with the temp sensor on the lower radiator hose
This has nothing to do with the temp gauge in the instrument panel (which uses a different sending unit). No warnings result when the temp varies to extremes.
Tests done with ignition ON, engine not running, battery charger maintaining voltage.
I used the Foxwell to monitor live data on that temperature. Measured about 71*F (ambient temp) when starting.
Disconnected the connector, simulating infinite resistance ==> -54*F
Measured resistance of the sender = 2.8 kOhms
Put the following resistors across the connector to simulate different temperatures:
temp [F] resistance [kOhms]
-54 infinity (unplugged)
71 2.8 (actual sensor)
94 1.48
122 0.80
159 0.40
213 0.151
241 0.100

These were done to confirm how I could simulate an overheated radiator hose, to hopefully trigger the fan to turn. Interweb lore says that you can just unplug the sensor, but that does not seem to work, which makes sense. The 100 Ohm resistor simulates 241F, and successfully triggers the fan to come on (the command comes, even though the fan does not spin).

Monitoring voltage on the fan control wire from the ECU:
Ignition ON, engine stopped, battery charger maintaining voltage at ~14V

Normal state - 14V (It is a square wave, but the DC Voltage meter showed this solid value; switching to AC gave numbers bouncing between almost that and zero; implying the DC reading shown is an integrated average)

Foxwell active test of the fan - almost instant drop to 1.6V, holding for 20 seconds. So 14V means fan is commanded off, 1.6V means it is commanded full speed on.

Putting the 100 Ohm resistor to simulate 241F at the lower radiator hose - no effect on fan control voltage. (but will have effect when engine is running)

Now started the engine, AC off.

100 Ohm resistor to simulate 241F at the lower radiator hose - control voltage gradually dropped to 2.6V over a period of about 30 seconds. This would gradually ramp up the speed of the fan to almost full speed. Repeating, this response did not happen with the engine stopped.