Radiator fan speed varies depending on coolant temperature. When the speed control circuit is faulty, the system can be temporarily bypassed.
Straight wiring a radiator fan is not something that we do or recommend. It should be done only temporarily when the radiator fan speed control system is faulty and causing the fan to not function. Direct connection of radiator fan to power has negative effects.
There are more than one way of doing a direct fan connection. Apply the one that is easier or the one that suites your need.
Methods of direct radiator fan connection
1. Pulse width modulation signal cable disconnection
This method is very simple. All that is needed to be done is to disconnect the PWM signal cable that the ECU uses to control radiator fan speed. When that is done, the fan spins at the maximum speed all the time and it uses ignition power.
2. Adding a relay and connecting to ignition circuit
The second method is by using an external relay. The relay power is tapped from the ignition circuit from the engine bay fuse box. The switch terminals of the relay are connected to positive alternator terminal and the positive terminal of the fan. In essence, the ignition current powers the relay which closes the relay's switch and cause current to flow from the alternator to the fan.
3. Adding a relay and a power resistor
In this method, a relay is connected as in the above but if the speed of the fan is too high, a power resistor which is able to dissipate heat is connected in-between the positive terminal of the fan and the output of the relay. The resistor helps to reduce the speed of the fan.
4. Adding a relay and a temperature switch
In this case, a temperature switch which senses coolant or engine block temperature is connected to the supply of a relay. It acts as a switch for the relay. The advantage of adding a temperature switch is that; fan won't turn on immediately there is ignition power. It will turn on only when the temperature attains that of the temperature switch rating.
5. Connecting a relay, temperature switch and a power resistor
In this set-up, soak resistor is added to the setup above to limit the fan speed just as in case 3. The value of the resistor will depend on the desired voltage supplied to the fan hence it also depends on the desired fan speed.
Caution: The resistor dissipates a lot of heat hence a heat sink should be attached to heat. Resistor without heat sink should not be used.
There are other more complex circuits including ready-made kits to achieve this and fan speed control but this tip is limited to simple do it yourself merhods.
