Products suited to this guided test*
  • 20 A / 60 A DC (low amps) current clamp

  • 200 A / 2000 A (high amps) DC current clamp

  • Back-pinning Probe Set

  • Flexible Back-pinning Probe

  • Large Dolphin/Gator Clips

  • *At Pico we are always looking to improve our products. The tools used in this guided test may have been superseded and the products above are our latest versions used to diagnose the fault documented in this case study.

Cooling fan - variable speed

The purpose of this test is to investigate the engine coolant cooling fan operation by monitoring the control unit signal and motor current draw through varying fan speeds.

How to perform the test

View connection guidance notes.


  • The cooling fan can switch on without ignition on or engine running.
  1. Use manufacturer data to identify the cooling fan circuit functions.
  2. Connect PicoScope Channel A. to the fan switched earth circuit.
  3. Connect the high amp clamp to PicoScope Channel B, zero the clamp before inserting into the fan power supply circuit.
  4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  5. Start the scope to see live data.
  6. Run the engine to operate the cooling fan, observe different fan speeds.
    (Note: activation of AC would normally increase fan speed).
  7. With your waveforms on screen stop the scope.
  8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.


The orientation of the current clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.

Example waveform

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).

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, cooling fan current or cooling fan voltage.

Further guidance

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.

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.

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).


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

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Guided test: Cooling fan - variable speed