Viscous Fans

Understanding the Role of Viscous Fan Couplings in Engine Cooling

BG Automotive give an overview of Viscous Fan Couplings and explain how their range offers a great sales opportunities for the independent aftermarket.

Viscous fan couplings (also known as viscous fans or viscous fan clutches in the aftermarket) are one of the lesser known products of the engine cooling system despite being part of more than 960,000 vehicles in the UK. The from BG Automotive provides a great opportunity to tap into this under-capitalised segment.

Viscous Fans Explained

Viscous fan couplings are used on vehicles (cars and trucks) with inline engine positions, mainly rear wheel drives. These fans are needed at slow speeds and while idling. A faulty fan can cause an engine to overheat while idling or in heavy traffic, or poor mileage and loss of horse power.


Viscous fan couplings are located between the water pump pulley and the radiator.


• Controls fan speeds to aid engine cooling
• Aids in the efficiency of the engine by engaging the fan, when necessary
• Reduces load on the engine


Either assembled directly to a flanged shaft mounted onto the pulley of the water pump, or alternatively it can be threaded directly to the water pump hub.

The Mechanism

This product is based on the bimetal sensor located at the front of the viscous fan. This sensor expands or contracts, depending on the temperature imparted through the radiator. This intelligent component improves engine efficiency by regulating the RPM of the engine fan and cool air supply.

At Cold Temperatures

The bimetal sensor contracts closing the valve, so the silicone fluid remains in the reservoir chamber. At this stage, the viscous fan coupling is disengaged and turns at around 20% of the rotation speed of the engine.

At Operating Temperatures

The bimetal sensor expands, rotating the valve and allowing the silicone fluid to move through the entire chamber to the outer edges. This generates enough torque to drive the cooling fan blades at engine operating speeds and remain at a consistent temperature. At this stage, the viscous fan coupling is engaged and turns at around 80% of the rotation speed of the engine.


It is good practice to always check the condition of the viscous fan coupling when the water pump is replaced. A worn coupling will directly affect the water pump’s lifespan. A failed viscous fan coupling can stay stuck in the engagement position meaning it will always run at 80% of the engine rotation speed. This can cause disruption with high noise and vibration, making a loud whirling sound as the engine is revved and will increase fuel consumption.

On the other hand, if the viscous fan coupling fails in the disengaged position, it will not draw air through the radiator. This in turn will lead to the engine overheating as the cooling process is disrupted.

Common Causes of Failure

• Leaking silicone fluid, disengaging the fan coupling.
• Bimetal sensor losing its properties due to surface oxidation, causing the coupling to be stuck disengaged.
• Bearing failure, although uncommon can occur if the viscous fan coupling has not been replaced after high mileage. This results in the condition of the blades deteriorating.

Understanding the Sensor

The bimetal sensor governs the functioning of the viscous fan coupling. Primarily, there are two kinds of bimetal sensor systems: plate and coil. Both of them work under the same principle, as explained previously.

The only difference is that whilst the coil expands and contracts to turn the rotation plate, the bimetal contracts and bends. This moves the slide plate and allows the silicone fluid to move from the reservoir chamber to the cavity.

Quality & Testing

BGA viscous fan couplings are fully tested before release to the aftermarket to ensure high quality of the replacement products. The process involves testing the engagement and disengagement temperatures of the product; whilst comparing the data against an OE part to warrant correct performance.

Market Potential

Being a high-value ticket item at OE level, dealers service only a very limited business while the aftermarket for this product is shared between 8-10 suppliers. Although these suppliers have gained a small foothold within the market, none of them offers a full range. Factors, therefore, need to multi-source parts to support the varied UK car parc, which is neither practical nor cost effective.

Aftermarket specialist BGA have the most comprehensive range of viscous fan couplings, consisting of 60 references. BGA has further planned NTR introductions to expand the range to around 80 references throughout 2016, to keep the range relevant to the UK market. BGA cover 94% of the UK vehicle parc fitted with viscous fan couplings and across 700 lines of application, including European and Asian makes. They also cover LCVs and the range also caters for the likes of Ford Transit Mk 4, 5, 6, Mitsubishi L200 and VW Passat.

Sales Opportunity

Viscous fan couplings have been a logical addition to the BGA cooling portfolio. Factors too should treat it as an associated sale for the relevant applications. When replacing the water pump, it is best practice to check that the viscous fan coupling is operating correctly as well.

In the current market this part could be overlooked during the replacement of the water pump. This could lead to factors missing out on a sales opportunity, whilst the vehicle owner potentially would have to pay a second labour charge to replace the coupling at a later stage.

Supported by excellent stocking and logistics from ourselves, the comprehensive viscous fan couplings range from BGA is clearly a reliable choice for the motor factor.


• Viscous fan couplings are a popular yet under-capitalised segment in the aftermarket.
• BGA has launched 60 viscous fan coupling references with plans to introduce 20 more throughout 2016.
• The range covers 94% of the UK car parc fitted with couplings across 700 lines of application.
• The range includes European and Asian makes and LCVs.
• BGA provide the most comprehensive portfolio among UK suppliers.
• Competitively priced with thorough testing against OE parts; offering excellent quality & performance.
• All parts available from FPS on a same/next-day basis.


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