Cooling fan problems

User BarnieB in our Ask a Mechanic forums had a problem with his 2001 Pontiac Montana's cooling fans. They were not coming on when the engine became hot. He tried changing the breakers (fuses), but with no luck.

With problems like this, going to the fuse box was a good start since that's a simple check and an easy fix if it's the problem.

The next thing to check are the fans themselves. On most vehicles, the cooling fan or fans for the radiator are either electric (in which case they might not work, as here) or engine-mounted (in which case they are "always on"). Unplugging the electric fans allows you to add wire length to the fan-side wiring and connect the fan directly to the vehicle's battery. If the fan turns on (direction will likely be determined by which wire is on which battery post), then it's working OK. If not, then you know the fan is bad and either it or its motor needs replacement, depending on the model.

If the fan works, plug it back in and (if present), test the next fan.

Now things get a little more technical. Circuits from the fan to the battery need to be checked. In our Pontiac example, this means going from the fans to the relay in the engine-mounted circuit box. In between those is a temperature sensor, mounted on or near the thermostat housing. If that sensor is bad, it would likely set off an engine error code that will appear on the dashboard. Your vehicle's manual will also likely have the test procedure to check that sensor yourself. If it is OK, the thermostat could also be malfunctioning, which will lead to vehicle overheating even if the fans are working.

Other vehicles may also have temperature sensors on the engine itself to determine when the fan(s) should be activated. Refer to your vehicle's electrical schematics to find them.