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Overheating - what triggers the cooling fans to be turned on and at what temp?

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My 08 MKX is overheating. I tested both fans direct to 12V and they both work.


So I was pretty sure it was the Fan Control Module that was bad, but after a little more testing, I am not so sure anymore.


Please feel free to chime in if you have the answer to any of the below questions.


1- To help further test, does anyone know at what temp the fans are suppose to come on?
2- And what triggers - or sends power to the Fan Control Module to turn on the fans? (is it the coolant temp sensor)?


The main connector that plugs into the FCM has 3 wires (12v + Ground + a small white wire). I tested the small white wire for power (with a test light) and nothing, I tested it for power all the way up to 206 degrees F (then I shut the car off as I didn’t want it to overheat)


3- Does anyone know when and how that small white wire from the FCM should have power - if that’s what triggers the fans to turn on?

4- With my OBDII scanner, I can monitor the ect (engine coolant temp) and the temp seems accurate, would this confirm that my ect sensor is not the fault here?

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Sorry to hear your MKX is having problems.  usually overheating is due to failed fans, failed water pump, and failed thermostat, in that order.  Very rarely, the CHT sensor will be involved.


210F is when the fans come on in the 2007-10 MYs.  Between 192-210F is passive cooling through the radiator.

The PCM uses data from the CHT sensor to turn on the fan.  If the A/C is activated, both fans will run, regardless of coolant temperature.  It is one way to bring down temps even tho it hurts mpg.

How do you know the ECT temp is accurate?  Are you comparing OBD data to actual infrared temp sensor data, or ...?

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The white with blue strip wire runs from the Powertrain Control Module (PCM) and generates a fan Control Variable signal for the Cooling Fan Module .


The PCM monitors certain parameters (such as engine coolant temperature, vehicle speed, A/C on/off status, A/C pressure) to determine engine cooling fan needs. 

For variable speed electric fan(s): 


The PCM controls the fan speed and operation using a duty cycle output on the fan control variable (FCV) circuit. The fan controller (located at or integral to the engine cooling fan assembly) receives the FCV command and operates the cooling fan at the speed requested (by varying the power applied to the fan motor). 

The fan controller has the capability to detect certain failure modes within the fan motors. Under certain failure modes, such as a motor that is drawing excessive current, the fan controller will shut off the fans. Fan motor concerns will not set a specific DTC. With the fan motor disconnected from the fan controller, voltage may not be present at the fan controller. 

Fan Speed Sensor (FSS)

The FSS is a Hall-effect sensor that measures the cooling fan clutch speed by generating a waveform with a frequency proportional to the fan speed. If the cooling fan clutch is moving at a relatively low speed, the sensor produces a signal with a low frequency. As the cooling fan clutch speed increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the FSS as a feedback for closed loop control of the cooling fan clutch. 


Cooling Fan Clutch 
The cooling fan clutch is an electrically actuated viscous clutch that consists of three main elements: 

      a working chamber 
      a reservoir chamber 
      a cooling fan clutch actuator valve and a fan speed sensor (FSS) 


The cooling fan clutch actuator valve controls the fluid flow from the reservoir into the working chamber. Once viscous fluid is in the working chamber, shearing of the fluid results in fan rotation. The cooling fan clutch actuator valve is activated with a pulse width modulated (PWM) output signal from the PCM. By opening and closing the fluid port valve, the PCM can control the cooling fan clutch speed. The cooling fan clutch speed is measured by a Hall-effect sensor and is monitored by the PCM during closed loop operation. 


The PCM optimizes fan speed based on engine coolant temperature (ECT), engine oil temperature (EOT), transmission fluid temperature (TFT), intake air temperature (IAT), or air conditioning requirements. When an increased demand for fan speed is requested for vehicle cooling, the PCM monitors the fan speed through the Hall-effect sensor. If a fan speed increase is required, the PCM outputs the PWM signal to the fluid port, providing the required fan speed increase. 


Based on this, you may have to test for an ac signal, not a dc voltage. All of the troubleshooting procedures involve starting with reading the DTC's.

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Thank you both for the details.


And I guess you are right, I don't know 100% that the ECT reading from my OBD scanner is accurate, it just seems about right as I watch it climb from start to around 200 F. No fluctuating, smooth climbing in temp and about 10 mins to around 200 F seemed about right to me.


Here are a few points I should have maybe mentioned previously.


1 - when my MKX overheated, and coolant was spewing everywhere, I immediately noticed my fans weren't blowing.(neither of them) and the AC was running when I overheated.


The next day I tested both the fans direct to the battery and they both turned on.


2 - I did some research online and concluded it must be the fan control module. So I ordered a fan assembly which includes the FCM.

But as I live on a small island (Aruba), ordering car parts typically takes about 2 - 3 weeks to get here.  So in the meantime, I was trying to hook up my fans to a toggle switch (using a couple of relays) to control the fans manually so that I could still drive it until the new parts get here.  If anyone wants to see, this is basically how I hooked it up http://www.gtsparkplugs.com/images/two-speed-manual-fan-relay-wiring-840x.jpg


Here are a few interesting things I found while trying to do this...


1- with both fan wires disconnected from the fan control module, (i have the tow package so there are 2 sets of wires - one for each fan) I used my multimeter to test and noticed there is continuity between the red wire and the black wire.  I tested both sets of fan wires and both seems to have continuity between red and black. What the...


Is that possible? Am I missing something? Or doing something wrong? 


Are the red wire and black wire shorted together? Or are these magic fans designed to handle this? I am puzzled.


To further test this, I added power from the battery to the red wire of the fan (still disconnected from the FCM) and with a test light found power in the black wire also. So then I reversed it and grounded the black wire  and tested for ground on the red wire -  sure enough there was ground now on the red wire.


This really doesn't make sense to me, but I proceeded to conect the  2 automotive  relay 's to the fans and with a toggle switch, I was able to create a circuit that turned my fans on and off.  I ran it for 10 minutes and no wires got hot or no fuses blew so I guess all is good.   


But I am really puzzled about the fan wires having continuity..


Either way, I continued on and decided I would try to eleminate the toggle switch from my new temperary solution - and thought I could possibly use the little white wire (going into the FCM) as my trigger wire to turn on my newly created cooling fan circuit.  Unfortunately  I didnt' see any power in that wire. But as I mentioned, I tested it with a test light only until about 206 F.. then got scared it was too hot and manually turned on my fans to cool the engine.  So now I am wondering if I was only 4 degrees, and literally a few seconds away from getting a positive signal from that white  wire. Maybe I will test again until 220 F this time.


If that little white wire gives me power at 210 F, that would be perfect, as I could use that as my trigger wire to turn my cooling fans on and off automatically 

Which would mean I wouldn't have to drive with my OBD scanner connected (to monitor my coolant temp) and know when to turn my fans on and off manually.


Any input or insight on the above would be greatly appreciated


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If you turn you ac on max both fans should come on full.


The reason you get continuity is the fan motor is a continuous wire (its just a coil). You should see a resistsnce of a few ohms (maybe 20-30 ohms).


I hope you ordered a Motorcraft fan assembly. The aftermarket units have reported problens.

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8 hours ago, enigma-2 said:

If you turn you ac on max both fans should come on full.



Just a comment from my experience with the 2011 MKX, although the fans come on with A/C, they run at a faster speed when ambient temperature was 30-32 degrees Celsius. 

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The fans do not come on when the AC is turned on.

In fact, that was the first thing I noticed the day it overheated - fans were not running, and AC was on full blast .



So I've now got the fans wired to a toggle switch (with 2 relays) so I can manually turn them on and off...until my new fan assembly gets here (about 3 weeks) but I am trying to find a wire that i can connect to that would trigger them to come on automatically at the right temp. So like maybe aN engine coolant temp sensor wire or something. 

Anyone have any ideas for this?

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