Cooling fan - variable speed

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.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

Warning

  • The cooling fan can switch on without ignition on or engine running.
     
  1. Use manufacturer data to identify the fan switched earth circuit.
  2. Connect PicoScope channel A to the fan switched earth circuit.
  3. Connect the low amp current clamp to PicoScope channel B, select the 60 A scale and zero the clamp before inserting into vehicle circuit.
  4. Insert the low amp current clamp into the fan switched earth circuit.
  5. Operate the cooling fan.
  6. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  7. Select Go or press the space bar to see live data.
  8. With your live waveform on screen press Stop or the space bar to capture your waveform.
  9. Use the Waveform Buffer and Zoom tools to examine your 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).

GT071-2

Disclaimer
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.

Suitable accessories

  • 20 A / 60 A DC (low amps) current clamp

    £111.00

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

    £259.00

  • Premium 6-way breakout lead set

    £269.00

  • Back-pinning Probe Set

    £40.00

  • Flexible Back-pinning Probe

    £3.00

  • Large Dolphin/Gator Clips

    £10.00

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Guided test: Cooling fan - variable speed