Variable-speed engine cooling fan

The purpose of this test is to evaluate the correct operation of the cooling fan circuit based on the cooling fan current draw in relation to the control signal voltage and duty cycle.

How to perform the test

Channel A

Plug the BNC moulded plug of a test lead into Channel A of the scope. Connect a Back-pinning Probe to the positive connection of the test lead (coloured) and place a black clip on the negative connection (black) and clip to a suitable earth connection within the engine bay. Probe the signal wire (thinnest of the three wires) to the fan multi-plug connector.

Channel B

Plug the BNC moulded plug of the low-amp current clamp into Channel B of the scope. Set the clamp to the 60 amp setting and auto-zero it. Place the clamp around the positive wire of the two heavy-duty wire connections of the fan multi-plug. (It may be necessary to identify this by turning the ignition on and using the scope set on a 0 to 20 volt range, using a multimeter, or checking technical data). With some vehicles it may be necessary to use a higher rated current clamp such as the Pico 2000 A clamp on its 200 A setting

Figure 1 shows both connections to the fan multi-plug. Figure 2 shows a suitable earth connection. With the example waveform displayed on the screen you can now press the space bar to start looking at live readings.

Waveform notes

The example connection was made on a BMW vehicle. This system varies the fan speed by a change in duty cycle of the signal. The fan is at rest at 10% and at maximum speed at 90%. The fan or fans have a constant-voltage supply with the earth return path being switched though the vehicle's electronic control unit (ECM).

Technical information

Many motor vehicle manufacturers now use variable-speed engine cooling fans within their model ranges. The advantage of a variable-speed engine cooling fan is that the engine is better able to control its operating temperature under varying conditions. The cooling fan's speed reduces as the vehicles road speed increases, because a larger volume of air naturally passes through the radiator. There may also be an input into the vehicle's electronic control module (ECM) from the vehicle's climate control unit.

The signal (blue trace) shows the cooling fan's earth return as a 0 to 12 volt square wave that is modulated at a frequency of 110 Hz. The ECM adjusts the fan's speed by altering the square wave's pulse width.

A signal that has a lower duty cycle (on-time) results in a slower fan speed and as the duty cycle increases, the fan speed also increases.

The current draw on the fan is measured using the amps clamp. The current draw is approximately 50 amps when the fan is running at full speed. The current waveform is shown in the red trace in Figure 3.

Depending on the manufacturer, the fan or fans may continue to run after the ignition has been switched off until the engines reaches a predefined temperature.

Faulty waveforms - troubleshooting

Ensure that the fan operates at varying speeds by monitoring the duty cycle on the cooling fan's earth return. This can be monitored using either an oscilloscope or a multimeter set to dwell. If the fan's duty cycle fails to increase with increasing engine temperature, the vehicle's ECM needs to be tested by a specialist to ensure that this function is working.

Operate the climate control and ensure that the duty cycle increases. The increase in duty cycle is required to pull more cool air through the air con's condenser, located in front of the radiator.

Note: Before condemning any of the vehicle's components, ensure that the particular model has the function being tested as there is a wide variation between both manufacturers and individual models.

Pin data

The connections to test this fan on a BMW vehicle were made at the multi-plug by the top of the radiator cowling. The three wires were: power supply (red/blue), an earth (brown) and the signal from the ECM (yellow/red).



Suitable accessories





  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


  • Premium Test Lead: BNC to 4 mm, 3 m




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Guided test: Cooling fan