COOLANT FAN OPERATION
I have split the circuit diagram into 2 parts to clearly show how the fans are controlled.
Slow speed operation
On the above diagram you can see the wiring path that comes into play when the fans run in slow speed.
On the right hand side of the circuit you can see there is a permanent supply through fuse 39 into the first fan. The relay is open so both fans are off.
When the PCM wants the fans to run slow it energises the relay by grounding pin 68 to earth.
The relay has a switched live feed to its coil from the ignition switch. This prevents the fans coming on if the ignition is off.
When the relay is energised the contacts close and the two fans are connected in series.
This means that the 12v supply is split evenly between the 2 fans and so they get 6v each.
This is what makes the fans turn slowly.
This diagram is for all cars except 2.9 12v and diesel. They have a 3 way temp switch, Normal, Temp1, Temp2. The circuit is the same but uses this switch instead of the PCM.
Fast speed operation
The above diagram is for FAST SPEED. It is basically the same as before but note each fan has its own fuse and the relay is switching fan 1 direct to earth and is switching 12v to fan 2.
Each fan now has 12v across it’s terminals and runs at maximum speed.
The PCM grounds pin 17 to activate the fast speed.
This diagram is for all cars except 2.9 12v and diesel. They have a 3 way temp switch, Normal, Temp1, Temp2. The circuit is the same but uses this switch instead of the PCM.
Please note in order for the slow speed to work correctly both fans need to be connected and working.
If 1 fan is disconnected then both fans will not run.
However the pcm will turn on the high speed signal on when temp reaches a higher point and the 1 remaining fan will now start spinning.
You can use the OBDII cable and software to monitor the PCM outputs to the fan relay whilst you are driving and you can see if they are being commanded to run slow or fast.