The fan in the car is really somewhat simple, but not as simple as the old days when the fan was belt driven. Back then, if the fan wouldn't turn, either the ENGINE wasn't turning, or the belt was GONE!

In the XF, the Fan motor is operated by the ECM (Engine Control Module - it's over in the box by the battery and runs a LOT more than just the fan) and the FCM (Fan Control Module- it's the little Grey ribbed box on the driver's side of the engine compartment, up almost behind the headlights).

There is a "handshake" taking place all of the time between these two modules. The ECM "pings" the FCM and the FCM answers back. Here we have the FIRST thing that will light the "low coolant light".

If the FCM, for ANY reason, does not respond to the ECM's "pings", the ECM will wait a few seconds - very few, and will light the Low Coolant light to tell you that SOMETHING is wrong with the cooling system. Maybe the FCM is bad, maybe the wiring between FCM and ECM is bad - or maybe the fuse to the FCM is blown. If the fuse is NOT, in fact, blown then the FCMis powered up and knows it has lost contact with the ECM, it "knows" that it no longer knows the engine temperature - so it turns the fan on full speed to provide airflow in case it is needed.

Now....

Until the engine warms up, the fan does not turn EXCEPT that IF you turn the A/C on, the ECM tells the FCM to run the fan as slow speed. Here we have the NEXT reason the low coolant light will come on.

The FCM measures the current drawn by the fan motor, if it is not within the specs of what the FCM wants to see, the FCM tells the ECM to turn the light on. But the light will only be lit if the motor is supposed to be running either cause the engine is hot OR the A/C is on! If your engine is cool and the A/C is on, and the LIGHT is on - turning the A/C off in this case will cause the FCM to shut the fan off. Within a few seconds, the light will go out IF the problem is the fan motor's current draw is not within specs cause it is disconnected or the motor is defective.

So, let's say you turn the A/C on, and you get the light. You notice the fan isn't turning. You then check the fuse and it's not blown. What do you do? Well, like I said, replacing the fan motor is probably all you need do. BUT, if you want to PROVE the motor is bad, keep reading.

Remember, you have the A/C on. The FCM should have the fan running at slow speed. To do this, the FCM applies 12 volts to the Green/Black wire going to the fan motor. (The brown wire is the "return" or ground wire from the motor). Now, since the FCM wants the fan to run SLOWLY, it applies PULSES of 12 volts. Your digital meter will read about 4 volts when it sees these pulses. An analog meter will, depending on it's design, show something around 3 to 6 volts and the meter needle will probably be bobbing around a bit - this is normal.

If you measure the voltage on the BROWN wire, you should NEVER see ANY voltage on it. If you do, THERE is your problem. Follow the brown wire back to the FCM and on to the ground lug - it's open somewhere!

Before you proceed - TURN THE ENGINE OFF IF IT'S RUNNING!

So, how do you measure these voltages? If you are careful, it's easy. Take some stranded wire and strip it back 2-3 inches. Now, cut out TWO strands, yes, individual strands of wire. Unplug the fan motor harness from the FCM harness (use a little screwdriver or key to lift the locking tab as you pull the connectors apart). Now, insert one strand of wire into each side of the FCM's plug. Push the connectors together - making sure the two strands dont' touch each other. If you look closely, you'll see that I have done this in the pic below.

You now have easy to reach leads to measure with. I tied the ground of my meter right back to the negative battery terminal. I then measured the Green/Black wire's voltage when researching this write-up.

SUMMARY:
Turn the A/C on, this assures that the fan motor is commanded to run at slow speed. (If the engine is real hot, the fan will be commaned to run at high speed anyway, but turn the A/C on to make sure you aren't wasting your time looking for voltage when it's not really there anyway).

If you see voltage on the Green/Black wire and NO voltage on the Brown wire - your fan motor BETTER be turning. If not, the motor is bad.

If you see no voltage on the Green/Black wire, verify:
1) That your meter is good (measure right across the battery!)
2) Verify your meter ground is tied to a good ground - the battery negative terminal is a good place to go.
3) Verify that you have the A/C on.
4) Verify the fuse to the FCM is good. (Lotsa luck with that, can't determine from MY manual which fuse it is.)
Without voltage on the Green/Black wire and NO voltage on the Brown wire - the motor won't be turning. WITH voltage on the Green/Black wire and no voltage on the brown wire - the motor HAS to be turning if it is not defective. PERIOD.



If the low coolant light is on due to a fan problem, either we have a breakdown in communications between the ECM and FCM (we have bad wiring, a bad FCM, a bad ECM, a bad fan motor fuse or bad power to the FCM.) (Power to the FCM comes from the ignition switch when the car is started. I am unsure if there is a fuse in this lead, still looking for that in the manual)

OR

the FCM does not like the amount of current it sees the fan drawing. (The current is too high or it's too low due to a bad motor or the motor not
being plugged in.)