Is the air from your car intermittently transitioning from cold to lukewarm and humid when you are on a drive? There is a chance that the AC clutch engages intermittently in your car. In that situation, AC blows cold air for like 5 to 20 minutes, and then it goes off again. Some users also manage to get cold air again from AC at high RPMs i.e. above 4000. All these signs are attributed to the short cycling of AC. So, let’s dive in and discuss all possible causes of engaging and disengaging the AC clutch.
Does your car have an air recirculation button? You can check my guide on why the air recirculation button of the car AC turns off automatically.
Table of Contents
How Does the Clutch System of Car AC Work?
There is a belt drive supplying energy continuously from the engine crankshaft pulley to the AC compressor. The serpentine belt runs over the crankshaft pulley. The serpentine belt powers the AC compressor, water pump, power steering pump and other accessories.
The air conditioning compressor has an electromagnetic clutch that engages with or disengages the compressor pulley. The AC clutch has a coil that generates a magnetic field when power is distributed to it. As it generates a magnetic field, it will attract the A/C clutch drive plate inward and engage with the pulley. This locks the pulley and the drive plate together. The drive plate runs the compressor.
If you look at the compressor pulley, you will see slots in it. These slots allow magnetic field lines from the coil to pass through and pull the friction plate in contact to complete the clutch engagement.
The drive plate of the AC compressor consists of the following parts:
- Friction plate
First, the hub is connected to the compressor. So, when the friction plate engages with the pulley, the rotation of the pulley is transferred to the compressor via that hub.
Second, the springs connect the hub to the friction plate, these “springs” can be steel or rubber. Lastly. the friction plate mates directly to the pulley friction surface.
The compressor pulley always rotates when the engine is turned on as the crankshaft is also rotating. The AC compressor will only run when the electromagnetic clutch plate will engage with the pulley.
AC compressor is responsible for cooling down the air as it compresses the refrigerant fluid, which is usually freon.
When freon passes over the coils of the evaporator of AC, it absorbs heat from the air and lowers its temperature. If the AC compressor is not running, it will not compress the refrigerant. As a result, your car AC will blow hot air.
How A/C Compressor Clutch Is Controlled?
Low and high-pressure switches, relays and transducers control the engaging and disengaging of the A/C compressor clutch.
In older vehicles, low and high-pressure switches are used. While in modern vehicles with PCM (Powertrain Control Module), transducers are used.
In the refrigerant, the lubricating oil for the compressor is mixed for the proper operation of the compressor. the oil lubricates O-rings in the compressor. Refrigerant oil specifications vary between A/C compressor designs and manufacturers.
If the pressure of the refrigerant is lower than a certain level, the system won’t carry enough refrigerant oil to the compressor, causing it to fail. In that condition, the A/C clutch will disengage.
The high-pressure switch is present after the A/C compressor. Its purpose is to detect any blockages in the system that would lead to more pressure. If pressure buildup is too high, it could explode the system. In that case, the clutch will disengage from the compressor.
Apart from that, these pressure switches are also important to maintain a suitable pressure of refrigerant inside the evaporator. Too low pressure of refrigerant inside the system causes evaporator coils to freeze up, resulting in obstruction of airflow.
Similarly, too high pressure of the refrigerant at the inlet of the evaporator gives poor heat transfer at the evaporator.
In modern engines, pressure transducers are used instead of pressure switches. The pressure transducers convert refrigerant pressure to electric signals. PCM receives these electric signals and commands the system to engage or disengage the A/C clutch.
In some vehicles, the evaporator temperature sensor turns on and off the clutch relay. The evaporator temp sensor cycles out the compressor at around 35 degrees Fahrenheit to prevent freeze-up and come back on at around 40 degrees.
How Often Should Car AC Clutch Engage?
There is no standard timing for when your car’s air conditioning clutch engages. The frequency of A/C clutch engagement depends on the type of car and the climate where it is driven. AC compressors work by cycling on and off to maintain the desired temperature inside your car cabin.
Keep in mind that the air conditioning compressor does not always engage. The cycling of the AC compressor depends on cabin temperature, ambient temperature, compressor design, or airflow across the evaporator coil. In normal conditions, the A/C compressor cycles after every 15 to 20 minutes or three times per hour.
The rapid cycling of the air conditioning clutch after every 5 to 15 seconds can be due to the low pressure of the refrigerant as the lower pressure is not operating the low-pressure switch to allow the current to pass through it.
Types Of A/C Systems in a Car
In a car, there are two types of A/C systems:
- Expansion Valve with Receiver/Drier
- Fixed Orifice Tube with Accumulator
Both A/C systems have a compressor, condenser and evaporator. Both the orifice tube and expansion valve are placed between the condenser and evaporator.
Both orifice and expansion valve cause pressure and a temperature drop of the refrigerant, but the expansion valve also controls the refrigerant flow as it has a spring that controls the opening of the valve.
On the other hand, the orifice tube has a fixed opening. It does not vary the amount of refrigerant flowing into the evaporator the way an expansion valve can. So, in A/C systems with orifice tube, the compressor is on and off (AC clutch engages and disengages) at appropriate times to control the refrigerant flow.
In A/C systems with an expansion valve, a drier is used between the condenser and the expansion valve to capture dirt, rust, or any foreign particles. The drier has also desiccant to protect the system by removing harmful moisture from the refrigerant.
In the orifice tube system, there is no drier. Instead, the orifice tube is responsible for filtering out the impurities and debris from refrigerant when it comes through its screen. In orifice tube systems, you will find an accumulator on the low-pressure line between the evaporator and compressor.
The function of an A/C accumulator is similar to the receiver/drier. But it is designed a bit differently and is typically much larger. The accumulator separates the liquid refrigerant from its vapors so that it does not harm the compressor. If any liquid enters the compressor, it will damage it.
Causes Of A/C Clutch Engaging and Disengaging Intermittently
Slipping of A/C compressor clutch
- Slipping Clutch of Compressor
- Low or high freon(refrigerant) pressure in A/C system
- Bad magnetic coil of the compressor
- Loose electrical connection in compressor clutch coil
- Icing up of evaporator coil
- Defective evaporator thermostat
- Blockage in condenser, expansion valve or orifice tube
- Slipping serpentine belt
1. Slipping Clutch of A/C Compressor
The most common cause of AC clutch engaging and disengaging intermittently can be a slipping clutch plate. A slipping clutch is a sign of severe wear or an increased gap between the friction plate and compressor pulley.
Clutch plates and pulleys are designed to fit snugly together. If they wear and become loose, the clutch will slip and fail to engage. Clutch plate damage can also happen if the air gap between the pulley and clutch plate widens beyond a certain point. This is usually due to an overheating compressor or damaged bearings.
The air gap between the friction plate of the clutch and pulley is determined by the automobile company and is related to the strength of the AC clutch coil.
If the air gap between the friction plate of the clutch and the pulley is increased, the magnetic field will not be able strong enough to pull and tightly hold the plate against the pulley. As a result, the AC clutch will disengage intermittently and cold air will not be blown from AC vents.
To check the clearance between the compressor pulley and the friction plate of the clutch, you can use a feeler gauge.
Make sure to check the gap when the engine is not warm. The nominal air gap settings between pulley and friction plate nominal can vary between 0.3 mm to 0.8 mm. If the air gap is greater than 1.0mm, you need to change the clutch assembly. When you remove a clutch plate, you will see a shim (a small metal disc). The shim sets the desired gap between the pulley and friction plate. You can adjust the air gap with a shim of different thicknesses.
Also, check if the color of the friction plate of the compressor clutch is faded or if there are burnt signs on it. If it is, you have to replace the clutch assembly.
2. Bad Electric Connection In Compressor Clutch Coil
If there is less than 12v at the AC compressor coil, it would also cause the AC clutch to disengage intermittently. As a result, the clutch would slip intermittently and this would cause the compressor clutch to disengage intermittently.
This problem could happen because the electrical wire at the compressor coil is damaged or disconnected. It may happen due to a short circuit. Or, it could be caused by a blown fuse. You should check the fuses on the air conditioner compressor coil and replace them if necessary.
To test for a short to-ground of compressor coil circuit, set a digital multimeter to the ohms scale and touch one lead to a clutch coil terminal and the other to a ground point on the compressor. If there is a short circuit, you will not see any conductance. Similarly, perform the test for the other terminal of the coil.
After that, check the resistance across the terminals of the compressor clutch coil. If the resistance is less than 3 ohms or more than 5 ohms, it means the coil is bad.
Now, check the clutch relay. If it is faulty, it will not supply a suitable voltage to the coil. With time, electrical contacts in the clutch relay develop pitting that prevents current flow. So, unplug the connector for the compressor and see if it is getting 12V with a multimeter. If not, it means the clutch relay has gone bad.
3. Low or High Refrigerant Pressure
AC compressor clutch also engages and disengages intermittently if the pressure of freon on the inlet of the evaporator is less than 30psi or more than 250psi after the compressor outlet. That pressure limitation is different for each refrigerant.
The AC clutch only engages if there is sufficient pressure in the system (30psi) to close the low-pressure switch and not so much high pressure to open the high-pressure switch and stop the current flow.
This means that the AC clutch will engage and disengage if the refrigerant pressure in the system is low and re-engages as the pressure between the high and low-pressure sides equalizes.
The low pressure on the low-pressure gauge i.e. between evaporator and compressor might indicate that the AC system is low on freon. However, before proceeding further and adding more refrigerant inside the system, make sure that there is no leakage in the system. For leakage, you should check the o-rings of the AC compressor at its inlet and exhaust ports.
You can recharge the A/C system with a refrigerant that has a dye in it so you can trace all the hoses and connections when the system leaks down again.
Similarly, high pressure on the high-pressure gauge i.e. between the compressor and evaporator might indicate that the system is overcharged with the refrigerant. However, the high pressure in the A/C system can be due to blockage in the expansion valve or orifice tube.
The condenser can also be plugged up with the debris and cause high pressure. If the condenser has a blockage or restriction of airflow, the high-pressure side reading would be high while the low-pressure side readings would drop over time. Also, check for bent fins of the condenser.
Moreover, you should also check the desiccant of the accumulator or drier/receiver. If their capacity of holding the moisture is full, it will also affect the pressure on the low and high-pressure lines of the AC system.
4. Slipping Serpentine Belt
Since a serpentine belt transfer power from the crankshaft pulley to the compressor pulley, chances are that it is also causing engaging and disengaging of the AC clutch plate. The serpentine belt has both a tensioner and an idler pulley. There is a chance that the pulley of the serpentine belt does not turn smoothly and there is a play from side to side that can prevent the serpentine belt from stretching. I have explained that concept in this guide.
So, if the AC clutch engages intermittently and your car’s compressor turns on and off rapidly, the most common cause is that the AC system is low on freon or it is having low pressure due to leaks or faults in the components of AC system. Bad electrical connections of AC compressor coil also cause clutch to disengage intermittently, resulting in short cycling of AC compressor.