Cooling Fan Description and Operation (Single Fan, 3 Speed)

Cooling Fan Description and Operation (Single Fan, 3 Speed)

The engine cooling fan system is composed of one cooling fan, a series of 3 relays, the engine control module (ECM), and the associated wiring. The cooling fan motor assembly includes two resistors. This combination of components enables the ECM to operate the cooling fan at 3 speeds using two fan control circuits. The underhood fuse block supplies the cooling fan relays with B+ at the switch side, and ignition voltage at the coil side of the relays through separate fuses. When the cooling fan is commanded ON, the ECM grounds the appropriate relay control circuit, which energizes the relay coil. The magnetic field, created within the relay by the energized relay coil, overcomes the spring tension of the bi-metal switch armature causing the relay switch to close. When the relay switch closes, B+ is supplied to the cooling fan motor through the relay controlled output circuit, causing the cooling fan motor to operate. A dedicated ground is supplied to the cooling motor to complete the circuit.

Low Speed Operation

The ECM applies ground at the FAN 1 control circuit for the coil side of the KR20C cooling fan low speed relay. The energized KR20C relay switch closes to supply B+ through a fuse, to the internal low speed resistor of the engine cooling fan motor. The result is cooling fan operation at a reduced speed. The energized KR20C relay also supplies B+ to the switch side of the KR20D cooling fan high speed relay. The KR20D relay remains inactive because the ECM is not commanding the FAN 2 control circuit ON.

Medium Speed Operation

The ECM applies ground at the FAN 2 control circuit for the coil side of the cooling fan medium speed and high speed relays. The KR20D high speed relay is energized, but remains inactive because the KR20C relay is commanded OFF and not supplying B+ to the switch side of the KR20D relay. The energized KR20P medium speed relay switch closes to supply B+ through a fuse, to the internal medium speed resistor of the engine cooling fan motor. The result is cooling fan operation at a medium speed.

High Speed Operation

The ECM applies ground at the FAN 1 control circuit for the coil side of the KR20C cooling fan low speed relay. The energized KR20C relay switch closes to supply B+ to the switch side of the KR20D high speed relay. Simultaneously, the ECM applies ground at the FAN 2 control circuit for the coil side of the cooling fan high speed and medium speed relays. The KR20D high speed relay switch closes to supply B+ from a fuse and through the KR20C relay switch, and directly to the engine cooling fan motor. The result is cooling fan operation at full speed. In this state, all of the cooling fan relays are commanded ON and voltage is available at all 3 motor supply voltage terminals. The fan operates at high speed due to the principals of ohms law and the path of least resistance. This ensures that the cooling fan will continue to operate in the case of a relay or circuit fault related to an individual speed.