Radiator fan relay

by Bob O’Neill and Bob Lincoln

Because the radiator fan requires a high level of current, a relay, managed by the logic module, turns the fan on and off. The logic module uses the temperature switch on the radiator for 1984 models; the radiator is turned on when the coolant temperature reaches 210 degrees and turned off when the coolant temperature cools to 180 degrees.  When the A/C compressor is engaged the logic module also turns on the radiator fan, regardless of temperature.

In later models, the relay has two separate inputs. One of these inputs is the Coolant Temperature Sensor, which is connected to the logic module, which will turn the fan on and off based on vehicle speed and coolant temperature. The fan will be turned on at 220° and off at 210° if the vehicle speed is below 35 miles per hour. The fan will be turned on at 240° and off at 230° if the vehicle speed is above 45 miles per hour.

The A/C cutout relay is engaged at wide open throttle (WOT). This relay is connected to the second input of the fan relay and will turn the fan on when the A/C compressor is engaged.  Later models do not have an A/C cutout relay, but instead the logic module opens the A/C relay when the Throttle Position Sensor tells the ECM that the engine is at WOT.

If the coolant temperature is below 150° in 1984 models, the logic module will monitor the relay input. Code 35 will be triggered if the relay is off when the clutch is on. Code 35 will also be triggered if the relay is on and the clutch is off.

On 1985 and later models, the logic module monitors the output of the relay driver circuit when the logic module turns on or off the relay. If it the logic module doesn’t sense zero volts when the driver is on or 12v when the driver is off it triggers code 35.

LINKS: Heat issues | Overheating | Code 35

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