CDT Cooling strategy
The ECM controls cooling fan operation for the engine, automatic gearbox and A/C condenser. With the ECM controlling cooling fan operation, it can adjust injection duration and timing to compensate for additional engine load imposed by the alternator during cooling fan operation. The ECM can request one of three fan speeds depending on coolant temperature, EAT ECU and A/C requests. These fan speeds are:
- Low: 250 rev/min.
- Medium: 800 rev/min.
- High: 1750 rev/min.
Priority will be given to the highest fan speed request. When A/C is requested, fan speed is set to low, unless the EAT ECU or ECT requires a higher fan speed. If the cooling request circuit of the trinary switch is open the fan speed is then set to medium. High speed fan operation will be selected by one of the following conditions:
- Engine temperature is greater than 119 °C (246 °F).
- EAT ECU request for increased cooling.
The ECM achieves fan speeds by sending a 140 Hz PWM signal to a PWM converter located within the fan relay module. The PWM converter is connected to three relays, also located within the fan relay module, and determines which relays to energise by the duty cycle of the pulse:
- 13%: Relay one energised to give low fan speed.
- 40%: Relays one and two energised to give medium fan speed.
- 86%: Relays one, two and three energised to give high fan speed.
Note for the V6 and CDT three speed fans the high speed relay does not switch a third +12V supply to the motor as shown in the wiring diagrams instead it switches in a second ground brush in parallel with the normal ground brush.
This explains why when most motors fail only high speed works as the normally used ground brush on the Black wire wears out prematurely leading to no low or medium speeds.
Later Vehicles (V6 and CDT)
Later vehicles were all fitted with a two speed fan, these benefited from two ground brushes being wired in parallel to extend their service life. The two speeds were then controlled via a large (visible through the grill) external resistor mounted on the fan cowl. It would appear that the resistor has been known to be a weak point again if it fails leading to operation at only one speed. The resistor is however easily replaceable and a 100W .47 ohm resistor from somewhere like Maplins is believed to be suitable but may need mounting on a heat sink.
Note For the
CDT the fan
will run when AC is selected with or without engine running but for the petrol version the engine must also be running.
Aircon behaviour and Ambient Temperatures,
It was originally believed that when Aircon was selected the fan always came on at the lowest speed this is not correct and operation is determined by the temperature difference between the chosen ATC setting and the outside temperature if that difference with an outside temp of 12.5 degrees is more than approx 10 degrees C than fan will not run. 10 degrees here is used as an example as this value may increase dependant on outside temp i.e at zero degrees the difference may be even higher.
E.g
It is +12.5 in my car at the moment
I stepped my AC up from my normal 20 degrees (car is a CDT so no need for engine running) when it got to 23 degrees fan stopped step back down again and fan restarts at 21 degrees.