Low Speed Fan Control Relay Circuit

LOW SPEED FAN CONTROL RELAY CIRCUIT

When Monitored
With the ignition key on and battery voltage greater than 10 volts.

Set Condition
An open or shorted condition is detected in the low speed radiator fan relay control circuit.

Theory of Operation
The low speed radiator fan relay controls the low speed operation of the radiator fan. When the vehicle is equipped with a Power Distribution Center (PDC), the low speed rad fan relay is inside the PDC. One side of the relay control coil is supplied with 12 volts when the ignition switch is turned to the on position. The circuit is completed when the other side of the relay coil is grounded by the Powertrain Control Module (PCM). The PCM grounds the relay control circuit depending on coolant temperature . LH vehicles have the additional capability of grounding the control circuit according to A/C head pressure. The relay will be grounded when the temperature is above a predetermined value (210°F is typical). The ground will be removed when the temperature drops to a predetermined value (199°F is typical). LH vehicles will also ground the relay when A/C head pressure is above 209 psi and, provided the temperature is not too high, they will remove the ground when the pressure drops below 125 psi.

Possible Causes
- Relay coil open or shorted.
- Fused ignition switch output circuit open.
- Low speed radiator fan relay control circuit open or shorted.
- Inoperative circuit driver in Powertrain Control Module.