Electric fans generally are controlled by a relay, which in turn is controlled by the Engine Control Module (ECM). The inputs the ECM uses to turn the fans on are different for an A/C request and would make this test invalid.
The latest evolution of electric radiator fan control has eliminated all relays and has the ECU control the fan(s) directly via a fan-control module that's either integral to the fan or separately mounted.
It can depend on the vehicle, but usually it's the coolant temperature sensor. Or it could be the engine temperature sensor, or radiator temperature sensor, or if the AC is on then it will most likely run both fans all the time.
Coolant Temperature Sensor
Both systems use a temperature sensor to determine when to start the fan. If the temperature sensor is faulty, the control unit doesn't interact with the radiator correctly. Some cars have a separate engine coolant temperature sensor for the radiator fan and the engine control unit.
They are controlled by relays and the ground on the control side of the relays is connected to two pins on the PCM. One is labeled as “High Speed fan” and the second as “Low Speed fan”.
The fan receives power through a complex wiring system consisting of relay(s), fuse, and cables. For automatic on and off, electric fans use a temperature switch and coolant temperature sensor.
The motor activates automatically when the engine temperature reaches a certain level and will continue to operate until the temperature drops below that threshold. The fan may also have different speed settings depending on the temperature of the engine.
The engine temperature should trigger the fan to turn on. It will also turn the fan off once the engine has cooled to under 200 degrees. You may have heard of a part called the radiator fan switch or temperature fan switch. This is linked to a thermostat and works much like your home's HVAC system.
It could be a problem with the fan motor, temperature sensor, fan control relay, or wiring. The Check Engine Light might come on. With modern cars, a DTC will be triggered that illuminates the light on the dash when it exceeds normal operating temperatures. It might be accompanied by a message on the cluster too.
The fan stops blowing air: Anyone who has driven a car will realize that there is a problem when the fan isn't running when the air conditioning is on. If the cooling fans are not running at all, then this may be a sign that the fan control module is damaged.
The Cooling Fan Motor Relay is an electrically operated switch that turns the coolant fan motor on and off, as commanded by the Engine Control Module or Powertrain Control Module (ECM/PCM) or, in older vehicles, the coolant temperature switch.
A lot of older cars will have a mechanical fan clutch, which is driven by a belt and pulley system. This means it is powered directly by the engine. Modern cars tend to have electrical fans which are powered by the vehicle's electrical system.
Unplugging the coolant temperature sensor results in an immediate signal to the control module that is outside of “rational limits”. Since this vital input is no longer useable, the default strategy is to turn on the radiator fan relay.
The thermostat acts as the control center for your HVAC system, including the fan. If the thermostat is faulty, it may not send the proper signals to the fan, preventing it from turning on or functioning at the desired speed.
To accomplish that task, thermostat functions are controlled by the engine's electronic control unit (ECU), ensuring precise regulation of temperature based on the engine's loads. The combustion process in a passenger car engine runs optimally at an operating temperature of approximately 230°F.
Electric Fans: These fans are controlled by the ECM (Engine Control Module) based on the information from temperature sensors.
There are a few ways to test the radiator fan. You can connect the radiator fan to the car battery with jumper wires to see if the fan runs normally. You can connect a low impedance lamp in series with the motor, turn it through, and see if the lamp ever goes dark, which is the most sensible way.
In short, common issues that can cause an air conditioning unit fan to stop spinning are electrical issues, problems with the capacitor, contactor issues, a burnt out air conditioner motor, a broken fan belt, or clogged air filter.
If there's insufficient coolant, this heat transfer is disrupted, expelling cold air instead of warm. Coolant levels can drop for various reasons, but leaks are the most typical culprit. These leaks may occur in the radiator, hoses, or the water pump. Even a failing head gasket can cause coolant to escape.
If the cooling fan relay has failed, you may see warnings, such as the check engine light and the temperature warning light, pop up on the dash, but first, you may notice that the A/C stops cooling. More about that in the next paragraph. The check engine light illuminating may indicate a faulty cooling fan relay.
First, run the AC and see if the radiator fan runs: it's supposed to run with the AC by default. If it doesn't run, you likely have a low level of refrigerant in the system, a faulty switch or a sensor, or a bad fan motor, fuse, relay, or a wiring problem.
Q: Where Is the Cooling Fan Switch Typically Located? A: The cooling fan switch is usually mounted on the radiator assembly. The cooling fan switch is usually mounted on the radiator assembly.
Loose or corroded connections can disrupt the electrical flow and prevent the fan from receiving power. It's also worth checking the temperature sensor or switch that signals the fan to turn on when the engine reaches a certain temperature. If the sensor is faulty, it may not trigger the fan to engage.