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Air conditioning sensors and switches

by Bob O'Neill and Bob Lincoln

Air Conditioner Cutout Relay

The air conditioner (A/C) cutout relay is designed to shut off the air conditioner compressor, by disengaging the A/C compressor clutch. Normally, when the air conditioner is active, the compressor users power from a fan belt to convert refrigerant (R12 or R134a, often referred to as Freon™) from a gas to a liquid. This takes a great deal of power. If the air conditioning compressor clutch is disengaged, the pulley will spin but the compressor won't operate, reducing drag on the engine. There are several normal conditions when the cutout relay is activated:

  • When the throttle position sensor indicates a wide open throttle (WOT) - in other words, when the gas pedal is floored - the logic module sends a signal to the A/C cutout relay to disengage the air conditioning (A/C) compressor clutch, so more of the engine's power can be used for acceleration.
  • The A/C cutout relay keeps the clutch disengaged for 10 seconds after the engine has started, partly to avoid stalling, and partly so the logic module can accurately set the target idle using the AIS motor.
  • If the idle drops below 500 rpm, the logic module disengages the A/C clutch to prevent the engine from stalling.

If the logic module sees any significant voltage when the circuit should be open (on), or does not see around 12 volts when the circuit is closed (or off), a code 33 is triggered. If the car doesn't have air conditioning, a code 33 may appear, especially on older vehicles; this can be ignored.

A/C Damped Pressure Switch

The air conditioning (A/C) damped pressure switch is used by the logic module to determine if the A/C clutch is engaged. This information is used to adjust the opening of the AIS motor to compensate for the extra load that the A/C compressor exerts on the engine. This adjustment adjusts the idle (typically to 1000 RPM) to insure proper A/C performance at idle.

On manual transmission cars, the AIS motor holds the idle speed up just a second longer than automatics, so that the compressor load will not pull the RPMs down too fast as you shift, and cause jerky shifting.

LINKS: A/C refrigerants
| Eliminating A/C odors

Sensors, Switches, and Other Systems | Main Repairs Page | EEKs

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