Q: My coolant fans is not running when ac is turned on.

With A/C systems, it is important to know in what order things turn on and off. From your description, it sounds as though the compressor clutch is only engaging for a short period of time. This is usually due to a low refrigerant level. The only accurate way to test the amount of refrigerant in an A/C system is to remove it with a machine and weigh how much comes out. Many people confuse pressure in the system for the system being full. Volume and pressure are two different properties of a gas, which the refrigerant in your A/C system is a gas when it is allowed to escape to the atmosphere.

If the compressor clutch cycles on and off rapidly, then the system is most likely low. The best remedy for this is getting your A/C serviced.

From you description, it sounds as though the system isn’t building enough pressure, and by relation enough heat to turn the cooling fans on. In other words, the system is low on refrigerant. I would confirm this by connecting A/C gauges to the system to see what the pressures are on the high and low side of the system. If the low side sucks into a vacuum quickly, I would suspect a low system. If the high side spikes and the fans don’t turn on, I would suspect a problem with what is called the A/C pressure transducer (high side switch), one or both fan relays, the fan motors themselves, an ambient temperature switch or the sun sensor.

If the compressor is running long enough to build heat in the high side of the system, and the cooling fans don’t turn on, I would begin finding the fan relays and add power to the appropriate circuit to see if the fans turn on at all. This will require a wiring diagram. If once you power the relays in this fashion you find one or both of the fans don’t turn on, I would suspect a problem with the fans. If both fans operate then it is time to take a look at the pressure transducer. Don’t let this strange name fool you, it is simply a sensor that relays the amount of pressure in the system to the PCM and the PCM decides when to turn on the fans and or compressor clutch. Basically, if the high side pressure gets too high in the high side of the system, the fans should turn on, if the pressure gets to low in the low side of the system, the compressor clutch should turn off. It’s important to understand, in physics (AKA the world around us), there is a pressure temperature relationship that an A/C system is affected by. The higher the pressure the higher the temperature, the lower the pressure the lower the temperature. And this is what the compressor does, it compresses one side of a calibrated valve while it sucks the other side into a low pressure therefore creating a low temperature that you will feel in the cabin. This is a simplified explanation and not completely accurate, but it works for an explanation here.

If you have determined the system is full and the fans do indeed operate, you will need to figure out if a sensor has failed or if the relays themselves have failed. There are two relays on your car. A high speed and a low speed. I would swap these relays and see if anything changes. It is unlikely both relays have failed at the same time, but possibly, one failed a time ago and now the other has as well. If you feel this is a possibility, buy two new ones and replace them. Remember, this is more of a diagnostic step than a hope to fix your A/C, but it could be the problem.

There are three potential sensors that could be a problem. The ambient temperature sensor and the sun sensor are not common failures. The transducer is another story. This part can get really complicated, but the first thing to do is run the A/C with gauges connected to observe the system pressures and monitor the circuit from the PCM to the relays. You are looking to see if the PCM sends power to one or both relays to turn on the coolant fans. This should happen as the high side of the system approaches 214 psi. If it exceeds this amount of pressure and the system doesn’t turn on, one of the sensors in the system that the PCM uses to know when to turn the system on and off has failed.

If you have made it this far, I applaud you. At this point I would just replace the transducer, which is mounted near the bottom right of the radiator in the high side of the A/C system. They are not to expensive and a common failure. Testing of the high side switch, which is what they are better known as, is problematic. The ambient temperature switch and the sun temperature switch are as well. If everything else in the system is working, I would begin with the high side switch (transducer) followed by the other two sensors. They are much less common failures therefore are harder to find.

Good luck. I hope you have a successful diagnosis of your A/C system. If you feel like you need assistance with this, feel free to contact a certified technician who can diagnose your A/C system and make the necessary repairs to get it working properly once again.