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AC compressor clutch not engaging

Diagnose AC compressor clutch not engaging

I have about 50 AC repair articles on this site. If you don’t find what you’re looking for here, go to this page.

Here are the most common causes of an AC compressor clutch not engaging:

• AC system is so low on 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
• AC clutch relay isn’t working. The contacts are pitted/stuck or the relay control coil isn’t getting power or ground
• 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

How to test an AC compressor clutch low-pressure cutoff 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 pressures vary 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 see what the pressure should be 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 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

How to test AC compressor clutch high-pressure cutoff switch

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.

  1. 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.
  2. Record the high and low side pressure
  3. Consult the R-134a temperature chart below to determine the STATIC pressure
  4. 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.

If the low and high side pressures are above or below the set points, but the compressor clutch does not engage, check the compressor clutch fuse, 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.

compressor clutch wiring diagram

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

compressor clutch wiring diagram

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 AC compressor relaypins 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.

Next, check for power and ground at the compressor clutch connector

If the relay socket checks out, remove the electrical connector from

ac compressor clutch not engaging

Compressor clutch connector

the compressor clutch. Using a digital voltmeter check the terminals for full battery voltage and ground.

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.

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

Check the evaporator temp sensor

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.

©, 2018 Rick Muscoplat

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