AC compressor clutch not engaging: How to troubleshoot

The Ultimate Guide to a Compressor Clutch Not Engaging—and How to Fix It

When your car’s air conditioning stops blowing cold air, one of the first things I check is whether the compressor clutch is engaging. If you have an AC compressor clutch not engaging problem, the pulley will be spinning, but it won’t be connected to the compressor shaft because the clutch isn’t locking in. In this guide, I’ll walk you through the exact diagnostic process I use in the shop when a compressor clutch not engaging issue shows up.

4 Common Causes of Compressor Clutch Not Engaging

There’s a compressor disabling issue— Modern AC systems are far more complicated than their older counterparts. The HVAC head unit communicates with the ECM before it allows compressor engagement or allows refrigerant flow on a clutch-less compressor. For example, if your engine coolant sensor is reporting a high engine temperature, the HVAC head won’t supply ground to the AC clutch relay, and the compressor clutch won’t engage. What’s the reasoning? Because running the AC will add more heat to the radiator due to heat transfer from the condenser. If the engine is already too hot, why add more heat?

Here’s another: unstable idle. In this case, a dirty throttle body or carboned idle bypass valve can cause a low or unsteady idle. Turning on the AC compressor clutch will add more load to the engine, making the idle even more unstable, so the HVAC will lock out the compressor clutch relay.

Low Refrigerant Pressure— If the system is severely undercharged, the low-pressure cutoff switch won’t energize the AC clutch relay. Without that relay signal, your compressor clutch not engaging is simply the system protecting itself from damage.

Blown Fuse or Faulty Wiring— A short in the clutch coil windings or harness can blow the fuse protecting the clutch circuit. Inspect the fuse box for a blown fuse labeled “AC CLUTCH”—a clear sign your compressor clutch not engaging.

Defective Switch or Coil— A bad low-pressure switch or a burned-out clutch coil will fail to complete the circuit. Even with proper voltage, a worn coil can’t generate the magnetic field needed to pull in the clutch, leaving your compressor clutch not engaging despite all other systems working.

Professional Compressor Clutch Diagnosis Step 1: Know What Type of Compressor You Have

Late-model vehicles often use a clutchless variable displacement AC compressor. Instead of using a clutch, the belt-driven pulley on these compressors continuously spins the compressor shaft as long as the engine is running, even when the AC is turned off. When the AC isn’t on, or the HVAC head isn’t calling for AC, the wobble plate inside the compressor is in the 0° position, so the compressor isn’t compressing any refrigerant. In other words, the compressor is free-wheeling.

To learn more about clutchless AC compressors, see this post

How to tell if you have a clutch or clutch-less style AC compressor

Here are the key visual differences between a conventional clutch-style AC compressor and a clutch-less AC compress.

• The clutch mechanism is visible on the front of the compressor pulley.
•The clutch coil wire/connector is present.
•No control valve is present.

Clutch-less style AC compressor visual differences

There is no clutch mechanism on the front of the pulley
No clutch coil wire is present.
You see a control valve and a connector on the body of the compressor

Test to determine whether you have a clutch or clutch-less style compressor

1) Turn off the defrost setting.
2) Turn off the AC.
3) Start the engine.
4) If the center portion of the AC compressor clutch pulley is spinning, you probably have a clutchless AC compressor

Step 2) Next, check the clutch for power and ground

Some AC compressor connectors have a single terminal that provides power, and the compressor body provides the ground. Two-wire clutch connectors have a power and ground wire. To test:

• Disconnect the connector from the compressor.
• Using a multimeter set to DC volts, check for power and ground at the connector terminals.

Compressor clutch connector

If you see power but the clutch isn’t engaging, then the problem is either a bad AC clutch coil or an air gap issue. Skip to Step 7.

If you don’t see power, check a wiring diagram and test the AC clutch fuse and relay (Step 3). If both tests are good, check for the AC relay control coil ground. If the control coil isn’t getting grounded, it won’t move the contacts and power up the AC clutch.

If you don’t have power, move on to Step 3.

Step 3: There is no power at the AC compressor clutch connector. Use your scan tool to read live data

Here’s what you’re looking for in live data

AC request: Is it ON or OFF— If you’ve requested AC this should read ON. That confirms that the HVAC head switch is working and the head is requesting AC. If you’ve activated the AC switch, but the request shows OFF, the problem is in the HVAC head itself.

AC Clutch Relay: Is it ON or OFF— If the request shows ON but the relay shows off, you have a disabling issue going on, like high or low refrigerant pressure, high engine temperature, unsteady engine idle, etc.

If the engine temperature is normal but the AC Clutch reads OFF, start by checking the static pressure

The low-pressure switch prevents compressor operation when the refrigerant pressure falls below the specified psi for your AC system. Low refrigerant pressure indicates that the system has a significant leak and has most likely lost refrigerant oil. If you bypass the low-pressure switch and force the compressor to run, you can destroy the compressor due to a lack of oil.

The minimum low-pressure cut-off pressure varies among car manufacturers and depends on the type of refrigerant used and the system’s design. You can check the cutoff pressure for your vehicle by using a shop manual.

Start your diagnostic with a cold engine. Measure the air temperature in the engine compartment and then refer to the static pressure chart in this post to determine the pressure at that temperature. 

Here are some typical AC low-pressure switch values

WARNING: The Low-Pressure Cutout Switch prevents compressor operation at low refrigerant pressures. This is a critical safety device to prevent compressor damage due to inadequate oil flow to the compressor.

Usual AC low-pressure switch values for R-134a systems

Operates the compressor clutch at 34-psi, prevents the compressor clutch operation at less than 10–psi
Operates the compressor clutch at 47-psi, prevents compressor clutch operation at less than 25–psi
Operates the compressor clutch at 40-psi, prevents the compressor clutch operation at less than 32–psi
Operates the compressor clutch at 34-psi, prevents compressor clutch operation at less than 8–psi
Operates the compressor clutch at 47-psi, prevents the compressor clutch operation at less than 25–psi

Step 4: No power at the AC compressor clutch connector. Is the system pressure too high?

The High-Pressure Cutout Switch prevents system damage due to excessive pressure. It disables the compressor clutch when pressure exceeds the set point and allows it to re-engage when the pressure drops below the reset point. The high-pressure cutoff switch is mounted on or near the receiver dryer.
• Connect a manifold gauge to the high and low-pressure ports. AC is not run for at least one hour, and the engine and AC are turned off.• Record the high and low side pressure.
• Consult the R-134a temperature chart below to determine the STATIC pressure.
• If the STATIC pressure is above the high-pressure cutoff switch limits for your vehicle, the switch will not allow compressor clutch engagement.

R-134a high-pressure switch operating pressures

Prevents compressor clutch operation at pressures at or above 300 psi. Operates the compressor clutch at pressures of 260-psi and below.

Prevents compressor clutch operation at pressures at or above 275 psi.
Operates the compressor clutch at pressures of 260-psi and below.

Prevents compressor clutch operation at pressures at or above 275 psi. Operates the compressor clutch at pressures of 235-psi and below.

Step 5: Is the evaporator temp sensor reading too cold?

The evaporator temperature sensor reports to the PCM, which shuts down the compressor if the evaporator temperatures drop too low to prevent evaporator freeze-up. Not all vehicles have an evaporator temperature sensor, so check the shop manual to see if you have one, where it’s located, and what the readings should be.

Here are the most common causes of an AC compressor clutch not engaging:
The AC system is so low on refrigerant that the pressure is lower than the minimum required to operate the low-pressure switch, allowing power to flow to the compressor clutch coil.

• The AC system pressure is too high and above the high-pressure switch’s maximum allowable pressure.
• The AC compressor clutch fuse is blown
• The AC clutch relay isn’t working. The contacts are pitted/stuck, or the relay control coil isn’t getting power or ground
• The evaporator sensor is bad, disconnected, or giving faulty readings, which is causing the HVAC controller to stop compressor operation.• Engine temp is too high. If the coolant temp is too high, the HVAC controller or ECM will shut down the AC compressor
• Other engine-related trouble codes are stored. This varies by make and model, but some engine conditions will inhibit AC operation
• Bad ground or power to the AC clutch
• Bad AC clutch coil (short or open)
• Excessive AC clutch air gap

Step 6: There is no power at the compressor connector. Check the AC compressor clutch fuse, clutch relay, and ground.

In most vehicles, the low- and high-pressure switches control the engagement of the compressor clutch. In later model vehicles, the high- and low-pressure switches are inputs to the ECM, which controls the compressor clutch relay. In addition to pressure, the ECM takes other factors into account, such as engine temperature, stored trouble codes, and data from the evaporator temperature sensor. Consult a shop manual to determine which system your vehicle uses.

In a computer-controlled system, the compressor clutch relay is grounded by the computer.

In some vehicles, the compressor clutch relay is turned on and off by the low-pressure switch and the evaporator temperature sensor.

How to test the compressor clutch relay

Remove the relay from the socket. Refer to this relay diagram to determine which pins in the socket should have battery voltage when the key is in the RUN position and which should have ground when the high and low pressures are within limits.

If the power and ground check out in the relay socket, test the relay by swapping in a similarly sized relay. If you hear the relay click when activating the AC, move on to the next step to check the integrity of the wiring harness and ground.

Step 7: You have power at the AC clutch, but it won’t engage. Check the compressor clutch coil

If you have power and ground at the compressor clutch, check the compressor clutch coil for an open, resistance, short to ground, and short to power.

Set your meter to the ohms scale to check for coil resistance and connect both leads to the clutch coil terminals. If you don’t see a resistance value, there’s an open in the clutch coil, and it must be replaced.

To check for short to ground, connect one terminal lead to one clutch coil terminal and the other lead to a metal contact point on the compressor. Switch the lead to the other terminal and repeat. You should NOT see continuity during this test. If you do, the clutch coil has shorted to ground. If you find a short, replace the clutch coil.

Step 8: The AC clutch coil checks out, but it won’t engage.  Check the AC compressor clutch air gap

The gap between the clutch disc and the AC pulley must be a set distance. If the gap is too large, the clutch coil can’t pull in the clutch disc. If the gap is too small, the clutch disc may make noise when the AC is off. See this post for typical clutch air gap specifications.

Most common reasons why car AC won’t cool

©, 2018 Rick Muscoplat