An AC clutch not engaging when hot is almost a given for riders in warmer locations like Arizona. However, it would be unwise to assume that the issue is limited to car owners in these regions. Mechanical problems, inadequate refrigerant and fluids, open circuits, and ECM failure are more likely to cause your AC clutch compressor to stop working.
Here are 14 reasons behind an AC clutch not engaging:
- AC Switch Failure
- Insufficient Refrigerant Volume
- Low-Quality Oil
- Worn Belt
- Obstruction or Blockage
- ECM Judgement Calls
- Poor Grounding
- Faulty Pressure Sensors
- Busted AC Clutch Compressor Relay or Fuse
- Damaged Clutch Coil and Relay Diode
- Clutch-Hub-to-Pulley Air Gap
- Current Overdraw
- Premature Part Failure
- Flawed Control Module
Before attempting to resolve this problem, the first set of questions to probably ask is, “What is an AC compressor clutch?”, “How does one work?” and “What are the causes of an AC clutch not engaging?” And you are just in luck, as this guide intends to answer these queries (and more).
What Is an AC Clutch Compressor?
The AC clutch compressor or AC clutch is a mechanical element of an engine responsible for pushing cool air through and pressurizing a vehicle’s air conditioning system. It operates via a self-powered electromagnetic switch and connects to a complex clutch system (hence, the name).
Charge ports, suction/discharge connections, a wobble plate, connecting rods, and a clutch assembly (to name a few) comprise this supplementary component. The clutch assembly is then horseshoed into three sections: the hub, pulley, and clutch coil.
In an ideal setup, the AC clutch “connects the rotor pulley to the compressor input shaft” once the field coil is energized. Meanwhile, its compressor turns refrigerant vapor into high-pressure vapor and discharges the same into the condenser.
Simply put, the entire assembly converts gas into liquid (producing a cooling effect when required), and your AC unit will not function without it.
Why Is Your AC Clutch Not Engaging?
On certain occasions, the ECM would intuitively turn the AC clutch compressor off to allow other vehicle functions to operate smoothly. Some of these instances will be explained in detail in this section, alongside other possible triggers for an AC clutch not engaging:
1. AC Switch Failure
If your AC clutch fails to engage, the first thing you should probably look into is your car’s AC switch module. This switch is prone to wear or damage since you use it regularly to turn on the AC unit.
Moreover, a faulty AC switch disrupts the flow of electric current to your compressor clutch — making it easy to confuse the issue with an AC clutch compressor not engaging.
Engine overheating and worn electrical contacts are the top two reasons for an AC switch failure. At the least, replacing the faulty switch is required if you want your AC clutch compressor to work again.
Worst-case scenario, you may also need to deal with the culprits behind your overheating and electrical issues.
2. Insufficient Refrigerant Volume
Your vehicle’s A/C system is an exacting system. It requires precise volumes and ratios of relevant fluids to function properly. Expectedly, the same goes for the nature of your AC clutch compressor, which will not work on low refrigerant volume.
The volume of refrigerant in your car’s cooling system impacts the pressure switches of the AC clutch (in this case, it is the low-pressure switch). Hence, not meeting the specific volume requirement would aptly explain your AC clutch not engaging.
Several things could cause low refrigerant volume, including loose fittings, a faulty valve, and a compromised evaporator coil. It is best to check for leaks in these areas and replace damaged components (if any) before increasing your refrigerant volume.
3. Low-Quality Oil
Another thing to prioritize when inspecting cooling system components is fluids. You should ensure adequate oil for your AC compressor each time you add refrigerant to your car. You need not wait for an AC clutch compressor malfunction before you do this, as it should be part of periodic maintenance.
In addition to oil levels, check for the quality of oil too. Always refer to manufacturer recommendations and never skimp on quality just to save a buck.
Remember, good-quality AC compressor oil is much cheaper than a brand-new AC compressor (view on Amazon).
4. Worn Belt
Interestingly, a worn serpentine belt is both a cause and an outcome of an AC clutch not engaging. It depends on what happens first — the AC compressor/clutch bearing seizure or the belt snapping.
If the AC clutch compressor seizes during operation, the resulting shock load will damage the belt. If the serpentine belt wears out first, it will cause clutch/compressor seizure.
Replacing the damaged serpentine belt is the obvious, easy fix to this dilemma. For added measure, replace the belt tensioner and idler pulley alongside the faulty belt.
5. Obstruction or Blockage
Any blockage in passageways or a component will naturally cause a system to malfunction. The same applies to a vehicle’s air conditioning system and AC compressor clutch. Clogged condensers, plugged suction lines, and filthy condenser coils are a few examples of what can cause your AC clutch compressor to stop working.
Symptoms vary depending on which component is stuffed. A dirty coil, for instance, would give off signs such as frost or reduced cooling capacity.
Meanwhile, a plugged condenser would set off warning lights on the dash or a burning smell from the vents. Either way, these issues should be promptly addressed, and the filthy components thoroughly cleaned.
6. ECM Judgement Calls
For most (if not all) modern-day cars, the ECM controls many internal vehicle systems and processes. And by this, we mean the AC clutch compressor is not exclusive to the ECM.
The latter also manages braking and steering components (among others) while constantly ensuring power is allocated to the system with the most urgent need.
That said, it would be helpful to know that your AC clutch compressor not engaging does not signify a problem all the time. If you pay mind to your car’s body language, you will notice your AC clutch does not engage during tight turns when the power steering pump (view on Amazon) requires more room to operate.
It also happens when you do sudden acceleration during take-off or your vehicle’s climate control feature is prioritized (for instance, when the in-cabin temperature needs to get turned down).
7. Poor Grounding
Weak or insufficient grounding prevents an AC clutch compressor from engaging — in the sense that it disrupts the electric flow and reduces the magnetic power necessary for creating a magnetic field required by the AC clutch compressor to function.
Refer to your owner’s manual for OEM-recommended figures, and use a multimeter like Fluke 1587 FC 2-in-1 Insulation Multimeter (view on Amazon) for testing.
8. Faulty Pressure Sensors
Pressure sensors have high and low-pressure switches on their respective side of the AC clutch compressor assembly. These tiny components monitor refrigerant pressure, check for pressure faults, and provide feedback to the A/C system in most modern vehicles.
They also work alongside evaporator temperature sensors in turning an AC clutch on or off.
Because of how vital their function is, these sensors need to be in perfect working condition all the time. However, this is not always the case.
Pressure spikes, electrical overloads, and mechanical damage (among others) can cause these pressure sensors to fail. And when they do, they create a low-pressure lockout — resulting in your AC clutch not engaging.
Unfortunately, there is no fix for faulty pressure sensors except for replacing the erring component.
Normally, a new AC pressure sensor costs between $60 and $80+ (parts and labor included but sans taxes and extraneous fees). But if it has already caused the A/C system and its surrounding environments complications, expenses will be higher.
9. Busted AC Clutch Compressor Relay or Fuse
This relay is how the ECM communicates with the AC clutch. If the relay or fuse has blown or failed, it would be highly improbable for your AC clutch compressor to engage since it is not receiving any commands from the ECM.
Moisture, short circuits, and corrosion are some factors that ultimately lead to an AC compressor clutch relay failure. However, these triggers are not exactly apparent.
The best way to detect a failing AC relay is by being aware of the following symptoms:
- Intermittent or inconsistent AC performance
- The AC unit is running, but no cool air
- The absence of a click when the compressor is turned on
- Rattling noise when switching on the AC
- AC compressor clutch not engaging
Depending on the symptoms observed and consequent findings, you may need to replace a part or the entire relay.
It may cost you between $540 and $1,400+ (including parts and labor) to outsource the task to a licensed mechanic. Otherwise, mechanically-inclined car owners need not spend over $700 and seven hours in their garage to sort out this issue.
10. Damaged Clutch Coil and Relay Diode
The AC clutch coil relay diode is a crucial aspect of the AC clutch compressor. It prevents electrical overload (amplified voltage powering the compressor from returning to the wire when the latter cycles off) and aids in the longevity of the AC compressor.
Conversely, a damaged one causes all kinds of interference when using your AC unit and eventually leads to your AC clutch compressor not engaging.
Conversely, the clutch coil permits the flow of electric current into the clutch compressor’s power circuit. If it is broken or split, electric current will not flow, and an electromagnetic field will not be generated. When this happens, the clutch will not engage, and the compressor will fail to spin.
11. Clutch-Hub-to-Pulley Air Gap
Compared to the other items in this list, this is a bit more technical and may not necessarily be within the comfort zone of less experienced car owners. Still, we need to cover the topic (no matter how briefly) because of its significance.
As mentioned earlier, the hub armature and pulley face are two of three sections of the AC clutch with an air gap in between. And a stringent requirement of this assembly is that air between these two parts is gauged properly.
The said air gap cannot be too wide that it results in slippage. Nor can it be too small that it causes the clutch to drag when disengaged.
The recommended AC clutch compressor air gap varies per manufacturer, so it is always a good idea to refer to your owner’s manual for specifics.
The important thing is to invest time in inspecting and correcting your AC clutch air gap. Otherwise, you risk premature clutch compressor failure resulting from clutch burning and poor compressor operation.
12. Current Overdraw
Also known as overloading, this incident is often attributed to a mechanical problem.
In the case of an AC compressor clutch, this could mean worn bearings or rods. However, it is also highly probable that a switching malfunction or amperage overdraw in the coil circuit could be causing it.
To test for switching or amperage anomalies, you will need to perform a current ramping test. This is a definitive test where “switching functions in the primary ignition circuit, shorting in the primary coil windings, and the total amperage being drawn through the primary coil circuit” are assessed. The results of this test are then shown in a graph and interpreted.
If everything in the AC clutch coil checks out, you should see a waveform slowly rising with no fluctuations in the graph. Any other result would mean the clutch compressor is drawing more than half of what its dedicated fuse says and is the reason behind the AC clutch not engaging.
13. Premature Part Failure
The AC clutch compressor assembly contains many parts vital to its function. Should one of these components go bad, there is a big chance the AC clutch will fail to engage.
We have covered many telltale signs throughout this guide. But when it comes to part failure, the most ominous indicator would be a compressor not spinning despite an engaged clutch. In this scenario, the clutch appears engaged, but there is no cold air.
The only way to address this situation is to change the AC clutch compressor itself.
Some vehicle owners choose to replace only the affected part (either the compressor or clutch section). However, the process for such tends to be more intricate, so I recommend replacing the AC clutch altogether.
14. Flawed Control Module
Finally, we have your engine’s ‘command center’ to blame. After all, your vehicle’s cooling system will not work without it.
Since the AC clutch compressor gets its operational commands from the control module, any mishaps with the latter would almost immediately result in AC clutch compressor malfunction (for instance, the AC clutch not engaging).
There are several ways to address an ECM problem, and resetting it is part of the process. Refer to my article on how to reset a car computer if you would like to know the steps.
How to Manually Engage an AC Clutch
Once troubleshooting measures have been exhausted and electrical problems have been ruled out, the only thing left to do is manually engage the AC clutch compressor. You best perform this via a jumper wire connected directly to the wire on the compressor.
Here are the steps to this process:
1. Locate the AC compressor clutch (usually in the engine bay near the compressor).
2. Inspect oil levels in your compressor and refill it as needed. Also, disconnect the clutch oil cable to prevent compressor damage while attempting to manually engage the AC clutch.
3. Check AC clutch pressure readings and discard overcharged refrigerant.
4. If the refrigerant is not overcharged, check levels and increase volume according to spec.
5. Determine if your AC compressor is not dead. If it is, recharge it before proceeding.
6. Uncouple the AC clutch compressor wiring pigtail (towards the front, behind the air conditioner)
7. Refer to the wiring diagram in your owner’s manual to identify power and ground wires relevant to the AC clutch.
8. Choose how you will apply power to the coil — by using a battery or Heavy-Duty Jumper Cables (view on Amazon).
9. If you opt for the latter, make jumper wires with an inline fuse embedded in its positive side.
10. Connect fused jumper wires between the clutch coil’s positive wire and positive battery post, with the remaining jumper wires between the clutch coil’s negative wire and negative battery post.
11. See if your AC clutch compressor engages (indicated by the sudden backward movement of the clutch plate, along with a click)
12. Record and assess your findings. An AC clutch engaging when jumped signifies an issue with the system’s control circuit. Otherwise, the entire AC clutch compressor assembly will need replacement.
Conclusion — AC Clutch Not Engaging
Once again, here are 14 of the most common culprits behind an AC clutch compressor not engaging:
- AC Switch Failure
- Insufficient Refrigerant Volume
- Low-Quality Oil
- Worn Belt
- Obstruction or Blockage
- ECM Judgement Calls
- Poor Grounding
- Faulty Pressure Sensors
- Busted AC Clutch Compressor Relay or Fuse
- Damaged Clutch Coil and Relay Diode
- Clutch-Hub-to-Pulley Air Gap
- Current Overdraw
- Premature Part Failure
- Flawed Control Module
I hope you find this post helpful — whether the need is immediate or in the far future.
If you are mechanically savvy, following this guide should enable you to diagnose and zero in on the problem source. But if it puts you out of your element, seeking help from a professional mechanic would be the better course of action.
Kris is an avid off-roader and outdoor enthusiast who loves to brave the elements and take on challenging terrain. He also enjoys sharing his passion and knowledge with others so that they, too, can appreciate the ride.
About Kris