Cooling fan


  • Preface
  • Fan with viscous coupling
  • Electric fan control by means of a thermal switch
  • Electric fan control by means of a control unit
  • Electrical fan control by means of a control unit (relay control)
  • Electrical fan control by means of a control unit (PWM control)
  • Possible Malfunctions That Keep the Cooling Fan Running

In a car we find many types of cooling fans: in the engine compartment, in a multifunctional radio, used in battery packs of hybrid and electric vehicles, see: alternative drive. This page focuses on the engine cooling fan.

The cooling fan of a car with an internal combustion engine protects the cooling system from overheating. The cooling fan comes in various versions (see the different paragraphs on this page) but all have one common feature: the plastic fan blades are located in the front, near the radiator (sometimes in the front, usually in the back). The fan will run when the coolant has warmed up, or when the air conditioner is turned on.

In the above image we see a BMW electric cooling fan in a plastic jacket. The cooling fan is disassembled from the engine compartment by a mechanic by sliding it up from its guides.

The following sections deal with the different cooling fan control methods.

Fan with viscous coupling:
In addition to the electronically controlled fan, there is also a self-thinking / regulating fan, namely the version with viscous coupling. No more electronics involved here. A bimetallic strip and liquid silicone fluid ensure that the fan is switched on and off when temperatures change by connecting two storage chambers (the storage chamber and the working chamber) together.

The viscous coupling is fitted with the flange on the coolant pump confirmed. In the picture we see part of the flange. The viscous coupling concerned is screwed to the coolant pump with four bolts. There are also versions with one central fastening nut.

The viscous coupling is behind the radiator. The air flowing through the radiator heats up the viscous coupling. A bimetallic strip also heats up and thus warps. With the warping, the bimetallic strip opens a leaf spring valve and the silicone fluid can flow from the storage chamber to the working chamber. The fluid allows the rotational movement of the drive disc (motor side) to be transferred to the fan housing (fan side). The silicone fluid can flow back to the storage chamber via the return channel.

  • The fan is switched off when the engine is cold. The flange on the coolant pump rotates, but the fan housing is stationary. In this situation, no chambers are connected to each other in the viscous coupling;
  • The fan switches on when the engine is warm. The silicone fluid in the working chamber ensures that the fan housing is taken along and starts to rotate.

The degree to which the bimetallic strip is warped (which in turn depends on the air temperature) determines how much liquid can flow into the working chamber. More liquid in the working chamber results in less slip, and therefore a higher fan speed. There is always minimal slip in the viscous coupling.

Driving wind cools the viscous coupling. Therefore, the cooling fan will mainly run when standing still or driving slowly.

We can recognize by the sound whether a car has a cooling fan with an electric motor or is driven with a viscous coupling. The viscous coupling is driven – via the multi-ribbed belt – by the crankshaft. A higher crankshaft speed results in a higher fan speed. If the fan blows harder when the engine speed increases, and switches off after a few seconds due to cooling, the car is equipped with a viscous coupling. An electric fan will not run faster or softer when the engine is idling than when it is accelerated.

The following figure shows the disassembly operation of the viscous coupling with a central bolt connection. The bolt connection – and thus the viscous coupling including the fan – can be removed with two large open-end wrenches. The coupling of the coolant pump can be dismantled by moving the wrenches apart in opposite movements. The disassembly option depends on the type of car. It is not always possible to unscrew the fan with two open-end wrenches:

  • there is only one nut on the viscous coupling and there is no blocking possibility. By putting a wrench on the nut and giving it a blow with a hammer, the nut comes loose from the coolant pump the first part. Attention: this can damage the bearings and seal of the coolant pump!
  • the fan can be blocked with a number of recesses with special tools.

Electric fan control by means of a thermal switch:
The electric cooling fan in this system is switched on and off with a temperature-dependent switch, or the thermal switch. This component is located in the radiator.

The thermal switch is located above the hose that serves as the return hose; the coolant cooled in the radiator returns to the engine via this hose. During driving mainly the wind provides sufficient cooling. When the coolant on the outlet side of the radiator gets too hot, the contacts in the thermal switch close. This creates an electrical connection in the control side of the relay circuit and turns on the cooling fan relay. The fan is controlled and starts to run.

While the fan is running, the coolant in the radiator cools down again. When the temperature is low enough, the thermal switch breaks the electrical connection. The relay, and with it the cooling fan, switches off.

The following electrical diagram shows the control method of the cooling fan. In the diagram we see:

  • that it concerns a waterfall scheme, with terminal 30 at the top (battery plus), terminal 15 below (ignition lock output) and terminal 31 at the bottom (battery ground);
  • the relay with on the left the connections 86 and 85 (input and output control current) with on the right side 30 and 87 (input and output main current).
  • the thermal switch between terminal 85 and the ground of the battery
  • the cooling fan between 87 and the ground of the battery.

The thermal switch operates the control current side of the fan relay. When the temperature in the radiator threatens to rise too high, the switch closes. The circuit in the control current side of the relay is closed; current flows through the coil between terminals 86 and 85. The coil becomes magnetic and closes the switch between terminals 30 and 87. As a result, a main current flows from the plus of the battery through the electric motor to the ground. The fan will run until contact with the relay is broken.

Electric fan control by means of a control unit:
Nowadays we see more and more cooling fans that are controlled by a control device. With this version, a thermal switch is no longer required: the control unit reads the values ​​of one or more coolant temperature sensors and determines the control of the cooling fan on the basis of this. The advantages of the ECU control are:

  • Control (switch-on and switch-off moments) can be regulated much more accurately than with the version with a thermal switch;
  • One cooling fan can take over the function of previously two separate (often one large and one small) fans.

The control unit determines when the fan turns on or off and at what speed it runs. The current to the fan does not pass through the control unit: the current is so high that too much heat would develop in the control unit. The ECU controlled fan systems can be constructed in two ways:

  1. Relay control;
  2. PWM control.

These two systems are described in the following sections.

Electronic fan control by means of a control unit (relay control):
As described in the previous section, the ECU control replaces the control system with the thermal switch. The following schedule shows the circuit of a cooling fan circuit from a Fiat Grande Punto 199. In this scheme we see the following main components:

  • R02: series resistor fan;
  • M05: radiator fan;
  • K07: high speed relay;
  • K07L: low speed relay;

The engine control unit determines, based on the coolant temperature and the value of the high pressure sensor in the air conditioning system, whether and at what speed the cooling fan should run. When the air conditioning is switched on, speed 1 is switched on as standard and when the engine is (too) warm, speed 2 is switched on. The fan (M05) can be controlled at two speeds:

  1. for the low speed, the engine-ECU connects the coil of relay K07L to ground. The relay switches on the main current, which reaches the electric motor of the fan via the series-connected series resistor R02.
  2. for high speed, the ECU switches relay K07L off and K07 on: the electric motor is now supplied with voltage and current without a series resistor. The fan will run at maximum speed. This happens, for example, if the engine is very hot when you are in a traffic jam, or during a malfunction in the temperature circuit: for safety, the ECU controls the cooling fan at the highest possible speed.

The two pictures below show the series resistor R02 (left) and the location of the series resistor in the cooling fan shroud (right). The white and green plastic part of the series resistor is hollow inside: the cooling fan blows air through it. The metal strips transfer the heat from the resistance to the flowing air. This element prevents overheating of the series resistor.

Source: HGS data
Series resistor R02
Location of the series resistor

The benefit of the relay circuit and series resistor is that it is a relatively simple system. In the event of a malfunction, the voltages to and from the relay can easily be measured. For the troubleshooting method, see the page about the relay.

The disadvantage is the use of the series resistor in position 1. A resistor absorbs energy, which ultimately leads to energy loss. In addition, the resistor is sensitive to defects. If the resistance burns out, the fan will no longer work at speed 1. If it is suspected that the series resistor is defective, the resistance can be measured. Remove the plug and measure the resistance on the pins of the component. If the result is “OL” or “1.” there is a so-called infinitely high resistance and indicates that it is defective. A resistance of several ohms is OK.

If a car is equipped with one fan relay and the fan starts to rotate at a high speed when it is switched on, this is at the expense of comfort. The sound of the fan switching on and off can be experienced as disturbing. In addition, there will be a peak in energy demand when switching on: consumers such as the lighting will dim for a short time after switching on the relay and starting the fan.

Electronic fan control by means of a control unit (PWM control):
With the PWM-controlled cooling fan, the rotational speed of the fan can be continuously increased or decreased. Where with a thermal switch the fan starts to run at a maximum speed after switching on, or can run at a low or high speed with a series resistor, with a PWM control the cooling fan can run at any desired speed. Advantages over the fixed speed system are:

  • More comfort: the fan is a lot quieter at the lowest possible speed than when it runs at a (too) high speed with an on-off control. The constant or low speed will also have no influence on the lighting that dims briefly in the system discussed earlier;
  • Energy-saving: if little cooling is required, the fan does not need to cool much. A slowly rotating fan costs less energy (including fuel);

The following schedule is from the cooling system of a Mercedes C-180. In this diagram we see the following components:

  • P05: main fuse box;
  • K04: main relay;
  • A10: engine compartment electronics module;
  • A11: engine ECU;
  • M05: radiator fan;
  • B13: coolant temperature sensor.

In this diagram we see that the cooling fan gets a constant plus on pin 2 via the fuse box, on pin 3 a switched plus when relay K04 has been switched on by the ECU, and on pin 4 gets a control signal from the engine-ECU.

The engine-ECU controls the cooling fan with a PWM signal. Control depends, among other things, on the motor temperature.

In the event of a malfunction of the cooling fan, we can check whether the motor receives a constant and switched positive (pin 2 and 3) with respect to ground (pin 1). If these voltages are correct (at least 12 volts with the engine running), we measure whether the control signal (PWM) from pin 16 on the ECU arrives at pin 4 of the fan. 

In the housing of the cooling fan M05 we also see an ECU: this is the control unit of the cooling fan. The engine-ECU sends a drive signal to the cooling fan-ECU at all times; even if it's not supposed to run. In this way, the cooling fan-ECU will recognize that the communication is good and that the fan should be turned off. When this signal is missing or incorrect, the ECU can no longer recognize whether the fan should remain switched off, or at what speed it should run. For safety reasons, the ECU drives the cooling fan motor at full speed. The driver of the car will notice that when he/she switches on the ignition, the fan will blow very loudly.

It is possible that the fan continues to run hard with the ignition on or off (depending on the type of car). If the engine-ECU control signal is correct, the cooling fan-ECU may be defective.

Another malfunction can of course be that it is suspected that the fan is not running at all. In order to keep the fan running during the diagnosis, we can control it by means of diagnostic equipment via the actuator test and at the same time measure the supply and control voltages.

The next screen shows the Coolant Fan Control Circuit 1 actuator test in the VCDS program. 

After clicking “Start”, the VCDS program commands the engine-ECU to drive the cooling fan. Control then takes place: every five seconds the fan runs at maximum speed and switches off again.

The scope images below show the PWM drive signals with the fan off (left) and at full speed (right).

The fan can rotate at any desired speed by making the active part in the signal longer or shorter.

Signal when fan is switched off
100% control.

Possible faults that keep the cooling fan running:
A high-speed cooling fan may continue to run even when the engine is off. Here is a summary of the most common faults that cause the cooling fan to go into a so-called “emergency run procedure”.

  • One or more error codes: read out the error codes from the engine management system or the air conditioning. There may be an error code related to the coolant temperature sensor, the high pressure sensor or its wiring;
  • The coolant temperature sensor indicates an illogical value. Check the current temperature with the help of the live data;
  • The radiator is clogged. This can be either a coolant duct that prevents the coolant from circulating properly, or a blockage to the air flow. The latter is easy to check: check the radiator for visible damage.
  • The relay sticks: in principle this only applies to the version with a series resistor;
  • There is no proper communication between the engine ECU and the cooling fan ECU: this is true for the PWM controlled fan ECU. The signals on both ECUs can be measured with an oscilloscope. There should be no difference here. Do you measure a voltage difference? Then you may be dealing with an interrupted wire, a contact resistance or short circuit.
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