Circuit/System Description
The Cooling Fan Relay 1 or the Fan Low Relay, and the Cooling Fan Relay 3 or the Fan Hi Relay are controlled by the engine control module (ECM). Battery voltage is supplied to the cooling fan relay coil. The ECM monitors the voltage on the control circuit of the relay. When the relay is OFF, the voltage should be near battery voltage on the control circuit of the relay. When the relay is commanded ON, the voltage will be low on the control circuit of the relay. The ECM provides ground on the control circuit of the relay using a low side driver.
The engine control module (ECM) commands the fans ON in either high speed or low speed, depending on cooling requirements. In low speed, both fans are turned ON at a reduced speed. High speed has both fans turned ON at full speed.
In low speed operation, the ECM applies ground to the coil side of the cooling fan 1 relay. This energizes the coil and applies voltage directly to the left cooling fan through the switch side of the low relay. The left fan is connected in series to the right cooling fan through the normally closed contacts of the fan 2 relay. The series circuit operates both fans at low speed.
In high speed operation, the ECM applies a ground to the coil side of the cooling fan 3 relay, the fan 2 relay, and the fan 3 relay. When energized, the fan 3 relay applies voltage directly to the right cooling fan through the switch side of the relay. Simultaneously, the fan 1 relay and the fan 2 relay provide ignition voltage and a direct path to ground for the left cooling fan. With both fans connected in a parallel circuit, each fan receives full voltage and operates at high speed.
The engine control module (ECM) commands the fans ON in either high speed or low speed, depending on cooling requirements. In low speed, both fans are turned ON at a reduced speed. High speed has both fans turned ON at full speed.
In low speed operation, the ECM applies ground to the coil side of the cooling fan 1 relay. This energizes the coil and applies voltage directly to the left cooling fan through the switch side of the low relay. The left fan is connected in series to the right cooling fan through the normally closed contacts of the fan 2 relay. The series circuit operates both fans at low speed.
In high speed operation, the ECM applies a ground to the coil side of the cooling fan 3 relay, the fan 2 relay, and the fan 3 relay. When energized, the fan 3 relay applies voltage directly to the right cooling fan through the switch side of the relay. Simultaneously, the fan 1 relay and the fan 2 relay provide ignition voltage and a direct path to ground for the left cooling fan. With both fans connected in a parallel circuit, each fan receives full voltage and operates at high speed.