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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 low on refrigerant
• AC system pressure is too high
• AC clutch fuse is blown
• AC clutch relay isn’t working
• Evaporator sensor is bad, disconnected or giving faulty readings
• Engine temp is too high
• Other engine related trouble codes are stored
• Bad ground to AC clutch
• Bad AC clutch coil (short or open)
• Excessive AC clutch air gap

For a fuller explanation of what causes an AC compressor clutch not engaging conditions, see this previous post. If you understand all the possible causes and want to learn how to diagnose a compressor clutch not engaging condition, this is the post for you

How to test AC compressor clutch low pressure cutoff switch

First, determine if there’s enough refrigerant pressure in the system to AC low pressure cutoff switchoverride the low pressure cut-off switch. Do that using your pressure gauge and the static pressure chart in this post.

The low pressure cutout switch prevents compressor operation when the pressure falls below the spec psi for your AC system. 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.

Here are some typical AC low pressure switch values

WARNING: The Low Pressure Cutout Switch prevents system damage due to low pressure or low refrigerant charge by disabling the AC compressor clutch when pressure drops below a certain preset pressure. The low pressure cutout switch allows the compressor clutch to engage once pressures rise above 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. 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

AC low pressure switch for R-12 systems

Operates compressor clutch at 34-psi, prevents compressor clutch operation at less than 10–psi
Operates compressor clutch at 15-psi, prevents compressor clutch operation at less than 4–psi

  1. Connect a manifold gauge to the high and low pressure ports. AC not run for at least one hour and engine and AC turned off.
  2.  Record the high and low side pressure
  3. Consult the R-134a temperature chart below to determine STATIC pressure
  4. If the STATIC pressure is above the low pressure cutoff switch limits for your vehicle, the switch should be allowing compressor clutch engagement.

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 set point and allows compressor clutch to re-engage when pressure drops below reset point. The high pressure cutoff switch is mounted on or near receiver dryer.

  1. Connect a manifold gauge to the high and low pressure ports. AC not run for at least one hour and engine and AC turned off.
  2. Record the high and low side pressure
  3. Consult the R-134a temperature chart below to determine 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 compressor clutch fuse, relay and ground

In most vehicles, the low and high pressure switches control compressor clutch engagement. In later model vehicles, the PCM controls the compressor clutch relay because it factors in other operating conditions such as engine temperature, other 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 checks 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 drops 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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