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Golf/Bora MK4 Fans Repair

Prepared By:-

Tamer Emin

August 2007




The VW Golf MK4 uses electric looking fans with two speeds, slow and full. The slow speed is required by the air conditioning system. It runs continuously while the air conditioning is on.

The cooling fan motors include a series power resistor within their casing to provide the slow speed. It is a common failure that this resistor breaks, leaving only high speed operation. Without any air circulation, the air conditioning condenser heats up enough to force the fan controller to turn on full speed fan operation.

The owner of the car only notices this problem when full speed fan operation starts and stops intermittently while the air conditioning is on.



The Fan Control Module

The fans are switched from off, low and high by a small device under the battery tray, called the fan control module.

It receives signals from various places, including radiator temperature sensor and air conditioning, and controls two relays for low and high speed.

The relays are embedded into the controller and are not replaceable. There are two multi-plugs connecting to the controller. The 14 pin plug contains the signal wiring. The 4 pin plug connects straight to the relays inside of the controller.

 Within the 4 pin plug there are two large connectors, they switch the full speed wire of the fans. The two remaining smaller connectors switch the low speed. Refer to the diagram below.








This solution works by driving the fans high speed wire with a pulse width modulated supply from a PWM speed controller.

The speed controller is powered from the fan control module's low speed output. So that, when the fan controller switches the low-speed output, the fans run at low speed. When the fan controller switches to high speed, the speed controller is bypassed and the fans run at full speed. Power electronics are used to efficiently reduce the speed of the fan motors without dissipating excess energy as heat.




Simple circuit diagram

The diagram below shows the standard two fan configuration used in the Golf 4, everything apart from the PWM Controller is standard. As you can see, it only needs three connections to take over the function of the blown motor resistors.


PWM Controller Design

The speed controller circuit diagram is shown below. It uses a 555 timer IC to generate the waveform. The output frequency is about 20 khz, and has adjustable duty cycle from 8% to 60%. In testing, it was able run the fans fast enough to keep the air conditioning happy, whilst drawing only 6A of current for both motors!

Interest in ready made controllers has been quite healthy, and as a result, I have produced a few units ready for sale. I can supply a unit for 85.00 inclusive of VAT and delivery costs.

You can securely order one right now, by visiting the Nime Shop here

Alternatively, email us at Enquiries for questions concerning the product, installation or payment.


You can view the installation guide for the speed controller right now. Click here to view.