Effects of radiator fan control bypass

Radiator fan speed is normally controlled by the ECU or fan control module. If the system is bypassed, it has negative side effects like high fuel consumption, shortened fan lifespan.
Last modified
11 Feb, 2024

Sometimes, in an attempt to solve problems, people create bigger problems. This is exactly what happens when people attempt to prevent or mask overheating problem by bypassing the electrical fan control system.

The fan control system is what regulates the speed and operation of the radiator cooling fan.

Cooling fan bypass

The radiator fan is designed not to run at a constant speed. The speed is varied by the ECU or fan control module, depending on the coolant temperature.

The temperature of a car's engine is controlled to make the engine run within specified operating temperature range. The primary means of temperature control is by using a radiator fan to cool the coolant that circulates in the cooling system.


Different ways of controlling electric radiator cooling fan speed

1. Relays can be combined to vary the fan speed by having no speed, low speed, medium speed and high speed.

2. The engine control unit (abbreviated as ECU or ECM) with a fan control module is used to control the fan speed.

In case 2, pulse width modulation technique is applied to vary fan speed. This is one of the applications of PWM in automobiles.

There are reasons why people bypass radiator fan speed control by doing direct fan connection even though it is not good for the health of the car. There is a simple way of doing the bypass. This practice has negative effects on the engine and we shall look at the effects.


What happens when radiator fan speed control is disabled by hot wiring the fan?

1. Fan lifespan is reduced

The fan works at full speed which overworks and wears it fast.

2. Alternator is overworked and lifespan is reduced

The power consumption of a fan that works at full speed is more. It places high current demand on the alternator and this can reduce the alternator's lifespan.

3. Reduced battery lifespan

As the load on the alternator increases, there is more current demand and more voltage drop on the system. This can negatively affect battery charging and battery life.

4. Increased fuel consumption

A cooling system with radiator fan at maximum speed will cool the engine below the normal operating temperature and this reduces fuel efficiency. Also, the engine oil won't thin out to it's viscosity at 100 degrees Celsius and this reduces fuel efficiency.

5. The engine wears faster

The enternal components of the engine are more free and better lubricated when the engine works at it's designed operating temperature. Wear is accellerated when the engine is overcooled.

6. There is sludge formation in the engine

An engine that doesn't run hot enough is not able to burn off moisture in engine oil effectively. This mixture of oil and moisture causes sludge formation in the engine.

7. The car becomes noisy

This is because fans make noise when they rotate at high speeds.

8. Engine bay becomes dirty

A fast rotating fan picks up dust from the ground easily.

Fan speed control should only be deactivated if it is causing the radiator fan not to work and the deactivation should be temporary.