How to Troubleshoot a Cavalier Radiator Fan

by Don Bowman

A radiator cooling fan on a Chevy Cavalier is controlled by the computer. Its circuit goes from the battery positive to the fuse. From the fuse it travels to two relays: the low-speed and the high-speed relay. Both of these relays have a separate hot wire to the fan. They are actuated by the computer, which uses the engine coolant temperature (ECT) sensor to determine when to turn the fan on and which relay to actuate dependent on the temperature. The ECT sensor is a 2-wire thermistor. As engine coolant temperature increases, the thermistor resistance and signal voltage decreases. On a cold engine, the voltage is sent through a high resistance circuit in the computer. On a warm engine the voltage is sent through a low-resistance circuit in the computer. This dual circuit in the computer improves the accuracy of the ECT sensor over all temperature ranges.

Step 1

Inspect the fuse in the fuse relay box. Start the engine and pull the connector off the ECT. This is a two-wire connector with a yellow and a black wire. It is located near the coolant outlet on the upper rear of the hose. When the connector is pulled off of the ECT, within a second or two the fan should come on. If it does come on, the ECT is suspect.

Step 2

Check the voltage with the meter by backprobing the sensor when the engine warms up. Probe the yellow wire on the sensor with the red wire or input wire on the voltmeter and probe the ground with the black wire on the voltmeter. The voltage should be between 1.5 to 2.0 volts. If not, shut the engine off.

Step 3

Disconnect the connector on the ECT and check the ohms across the terminals of the sensor. At 194 degrees there should be 250 ohms. At 158 degrees there should be 450. Just interpolate in between and if it is close it is okay. If the ohms are not in the ballpark, then replace the ECT. The voltage goes from the computer through the sensor and back to the computer. If the sensor does not have the right resistance to change the voltage returning to the computer the wrong signal will be sent.

Step 4

Inspect the radiator fan by spinning it with the fingers and see if it is free moving. Do this only with the engine off and cool. The fan can come on at any time even with the key off when the engine is warm. Install a jumper wire to the red wire at the harness connector and the black wire to ground. Touch the red jumper to battery positive side and the fan should operate if it is not burnt out. If it does not come on, check the connection for a good power and ground on the jumpers. If the connections are good, replace the fan.

Step 5

Check the relays if the sensor was good and the fan worked. Pull the relays out and probe the terminals with the key off for power. There should be power to one terminal in both relays. If not, then there is an unlikely problem with the wiring from the fuse to the relays under the backside of the fuse box. Maybe a pin is pushed out or there is a burnt wire. If there is power, swap the relays around and try again. One of the relays is probably burnt out. If the fan still does not work replace both relays. If it does work replace the burnt relay.

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