Electromechanical A/C Clutch Relay
Electromechanical relays are circuit protection and control devices used by the PCM and other sensitive circuits. They use an electromagnet (coil) that controls an arm that swings between two contacts.
Below is a common (SPST) single-pole, single-through relay. As low current passes through the coil, a magnetic field is created that draws the relay's arm towards the high current circuit. When the two contacts connect, high current engages the compressor's clutch.
A standard has been developed for the plug of a mechanical relay. The relay uses numbers 85 and 86 as the electromagnet control. This coil usually contains a diode to protect the contacts and other circuits from the sudden collapse of current when switched off. On some of these circuits, it's important not to reverse the 85 and 86 (coil) pins. The number 30 pin is connected to the high current source, and number 87a is the prong where the arm parks (off). The 87 pin activates the device (on)
Diagnosing a Sticking Relay
When a relay sticks closed, the compressor's clutch will stay engaged continuously. When the relay fails or sticks open, the compressor's clutch will not engage; this is common.