Radiator Fan Relay Control Circuit

NAME OF CODE
Rad Fan Control Relay Circuit

WHEN MONITORED
With the ignition key on, battery voltage greater than 10 volts, and the PCM requests the fans to be turned on.

SET CONDITION
An open or shorted condition to detected in the radiator fan rate I circa it

THEORY OF OPERATION
The solid state radiator fan relay controls the operation of the radiator fans. One connector of the relay is supplied with fused B(+) to power the fans. This same B(+) connection powers the internal electronics in the relay The chassis ground connection (physical attachment to the chassis) on the relay provides a heat sink for the relay and a ground for the internal electronics, The ground connection from the relay connector is a redundant ground for the internal relay electronics. If the relay attachment to the chassis becomes loose or corroded, the relay could fail due to its inability to dissipate heat through the chassis attachment. When the relay control circuit is grounded by the PCM, current flows through the relay to the fans As the engine temperature rises to a preset level, the PCM will start sending a "Pulse Width Modulated" (PWM) ground signal to the relay (see below). The dwell time of this signal will increase or decrease from 30% to 100% based on the engine coolant temperature and A/C pressures. This method of controlling the fans provides an infinitely variable fan speed.

NOTE: This shows the signal from the relay at the fan, the signal from the PCM is inverted.
NOTE: PL is always 100% d.c. when ON.

POSSIBLE CAUSES
- Relay Failure
- Fused B(+)
- Radiator fan relay control circuit open or shorted
- PCM failure
- Connector terminals
- Connector wires