Tag: compressor relay

Auto AC system service

What is a clutch relay?

A relay is a device that allows you to control high amperage circuit from low amperage circuit.

Automotive switches and electronics are wired to relays since they aren’t designed to handle the arcing and high current demand required in some circuits.

How a relay works

Every relay has at least two circuits running through it: a low-amperage, or CONTROL circuit, and a high amperage, or load circuit.

When you energize a relay, you’re actually energizing the control circuit. This energizes an electromagnet in the relay, which opens or closes a set of contacts. The contacts are part of the load circuit. When the contacts close, they complete the high amperage circuit, allowing current to flow.

Different types of relays

Relays can be normally-open (NO), normally-closed (NC) or can have dual contacts.

Normally-open contact relays have one set of contacts that remain open when the relay control circuit is de-energized. When you energize the relay, the contacts close, completing the high amperage circuit.

Normally-closed contact relays work the opposite way: The contacts are closed when the relay is de-energized. Energizing the control circuit in the relay opens the contacts, shutting off the current in the load circuit.

Dual-contact relays have one set of normally-open contacts, and one set of normally-closed contacts. When the relay is de-energized, current flows through the normally-closed contacts. When you energize the relay, the normally-closed contacts open, and the normally-open contacts close. Now current flows through the normal-open contacts.


Most mini-relays have four or five terminals. The two small terminals(often specified as 85 and 86) are used to energize the coil that creates the magnetic attraction necessary to cause a connection between the high amperage switch contacts.Two other terminals make the high amperage-amperage power in (terminal 30) and power out (terminal 87) connection. Sometimes a fifth terminal (marked 87a) is used as a normally closed (NC) contact, as opposed to 87 being normally-open (NO), which closes when the relay energizes.

Solid state relays

Some manufacturers use solid-state relays in different areas of  the car. The operation of these devices is much like an electronic fuel injector. These components can be totally off or tally on, like regular relays. What differentiates them is their ability to be turned on to a certain percentage with a pulse-width modulated signal. The advantage of these devices is that they can control the current flow to the output device, resulting in a variable speed device.

A scan tool is necessary to troubleshoot these solid-state relays. The scan tool can retrieve trouble codes as well as perform output tests on these circuits during the troubleshooting process.

How do I know if my AC clutch relay is bad?

When testing these relays:

  1. Unplug the relay
  2. Measure for continuity between pins 30 and 87a.There should be less than 1ohm resistance.
  3. Measure for continuity between pins 30 and 87.There shouldn’t be any continuity between these terminals
  4. Apply power (12volts) to pin 86 and ground to pin 85.The relay should click.
  5. Measure for continuity between pins 30 and 87a.There shouldn’t be any continuity between these terminals.
  6. Measure for continuity between pins 30 and 87.There should be less than 1ohm resistance between these terminals.
What are the symptoms of a bad relay?

Signs of a Bad or Failing AC Compressor Relay

The AC compressor will not have power and the AC system will not function.

AC compressor intermittent operation causing weakened or intermittent cooling performance.

If you are experiencing issues with your AC system and suspect that your AC relay has either failed or is beginning to fail, we recommend having it diagnosed by a professional technician.