The pressure switch in the 1989 Mustang GT turns off the A/C compressor when refrigerant pressure in the low pressure side of the A/C system drops below 30 psi. This prevents freeze-up of the refrigerant in the accumulator/drier on the outlet side of the evaporator, resulting in the loss of cooling ability. If the switch fails the A/C compressor will not come on at all, or will not allow the compressor to cycle off when pressure drops in the system.
Tools Used: Wrench set
Replace the Air Conditioning Pressure Switch
Turn the engine off. Open the hood and disconnect the negative battery cable using a 13-mm wrench to loosen the terminal clamp bolt. Twist and pull up on the battery terminal until it slides off the battery. Locate the pressure switch on the Mustang's dryer, located in the right rear of the engine compartment, attached to the firewall.
Unplug the pressure switch's electrical connector by squeezing the connector clip and pulling up until it slides off the switch. Loosen the switch from the dryer using a 9/16-inch wrench, and quickly unscrew the old switch from the dryer to avoid losing refrigerant. There is a Schrader valve underneath that will seal the fitting as you unscrew the switch.
Replace the O-ring seal -- on the fitting the switch was screwed to -- with the new one that came with the new switch. Screw the new switch onto the dryer and tighten it snugly with the wrench. Plug the electrical connector for the Mustang's pressure switch into the new switch. Test drive the Mustang to make sure the system works properly.
Tips & Warnings
Wear safety glasses and work gloves when working around a running engine to prevent serious injuries.
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