AC compressor clutch not engaging
Diagnose AC compressor clutch not engaging
There could be several reasons why the AC compressor clutch is not engaging in your vehicle. Here are some common causes: 1) low refrigerant charge, 2) a blown fuse to the clutch coil, 3) bad low-pressure switch, or a bad clutch coil. I’ve compiled a step-by-step guide to help you diagnose the problem
Step 1) Are you sure you have a clutch-type compressor?
Late-model vehicles often use a clutch-less variable displacement AC compressor. Instead of using a clutch, the belt-driven pulley runs the compressor shaft the entire time the engine is running, even if 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 actually compressing any refrigerant.
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
or
• 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 connector on body of 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 clutch less AC compressor
Step 2 Does the AC compressor clutch have power and ground?
When the AC is turned on, and the high- and low-pressure switches see the correct pressures, the vehicle should provide power to the AC clutch coil. So, the first step is testing for power at the clutch electrical connector.
Some AC compressor connects have a single terminal that provides power. Others have two wires: one for power and one for ground. 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 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. Is static pressure high enough to activate the low-pressure switch?
The low-pressure switch prevents compressor operation when the pressure falls below the spec psi for your AC system. Low pressure is an indication 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 lack of oil.
The minimum low-pressure cut-off pressure varies with each car maker and is dependent on the type of refrigerant used and the design of the system. 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.
The low-pressure cutout switch allows the compressor clutch to engage once pressures rise above the set point. The low-pressure cutout switch is mounted on the low-pressure side of the compressor. Do NOT EVER bypass the low-pressure switch to get the compressor running so you can add refrigerant. AC refrigerant carries the oil to lubricate the compressor. Low or no refrigerant means you’ll have NO LUBRICATION entering the compressor. The bottom line; you can destroy the compressor simply by bypassing the low-pressure switch. That can turn a low refrigerant charge problem into a $2,000 repair.
Usual AC low-pressure switch values for R-134a systems
Operates compressor clutch at 34-psi, prevents compressor clutch operation at less than 10–psi
Operates compressor clutch at 47-psi, prevents compressor clutch operation at less than 25–psi
Operates compressor clutch at 40-psi, prevents compressor clutch operation at less than 32–psi
Operates compressor clutch at 34-psi, prevents compressor clutch operation at less than 8–psi
Operates compressor clutch at 47-psi, prevents compressor clutch operation at less than 25–psi
Step 4: No power at the AC compressor clutch connector. Is 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 the compressor clutch 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 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 compressor clutch at pressures 260-psi and below.
Prevents compressor clutch operation at pressures at or above 275–psi. Operates compressor clutch at pressures 260-psi and below.
Prevents compressor clutch operation at pressures at or above 275–psi. Operates compressor clutch at pressures 235-psi and below.
Step 5: Is the evaporator temp sensor reading too cold?
The evaporator temp sensor reports to the PCM and the PCM shuts down the compressor if the evaporator temps drop too low. It does that to prevent evaporator freeze-up. Not all vehicles have an evaporator temp 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 a refrigerant that the pressure is lower than the minimum required to operate the low-pressure switch and allow power to flow to the compressor clutch coil.• AC system pressure is too high and above the high-pressure switch’s maximum allowable pressure.
• 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 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 compressor clutch engagement. In later model vehicles, the high and low pressure switches are inputs to the ECM, and the ECM controls the compressor clutch relay. In addition to pressure, the ECM takes other factors into account, like engine temperature, stored trouble codes, and the data from the evaporator temp 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 open, resistance, short to ground and short to power.
To check for coil resistance, set your meter to the ohms scale 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
Posted on by Rick Muscoplat