Automotive Air Conditioner Pressure Switch Troubleshooting

by Robert Sylvus
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An automotive air conditioner's pressure switch cycles the vehicle's compressor on and off when the refrigerant pressure falls below an acceptable level. This protects the compressor from damage. Refrigerant does not wear out or evaporate: only leaking air conditioning systems have low refrigerant levels. Automotive air conditioning systems with low refrigerant levels must have the leak repaired before recharging the system. Most likely, a vehicle with an air conditioning system that will not cool has a low refrigerant level.

Step 1

Remove the air conditioner fuse. A good fuse, found in the fuse compartment, will not have any black burn marks. To test the fuse, turn a multi-meter to the ohms setting. Place a lead on each fuse terminal. The meter should show continuity. Replace the fuse.

Step 2

Install refrigerant gauges to the refrigerant service valve, found on the large refrigerant line in the engine compartment. Verify that the system has enough refrigerant, about 56 psi with the system in the "off" position. If not, repair the leak and add refrigerant.

Step 3

Disconnect the pressure switch electrical plug. The pressure switch, found on the same line as the service valve, has a set of low voltage wires connected to a plug. Lift the locking tab on the plug before pulling the plug free.

Step 4

Place the leads of the multi-meter, set to ohms, on the prongs found on the pressure switch. The meter should read continuity if the system has an acceptable refrigerant level. If not, replace the pressure switch.

Step 5

Turn the air conditioning system on. Check the voltage going to the plug with the multi-meter turned to DC-Volts. Place one lead against the vehicle frame and the other lead into the plug openings, one at a time. One opening should read between 12 and 14 volts. If not, troubleshoot the electrical system leading up to the pressure switch. Turn the system off.

Step 6

Insert a jumper wire into the two openings of the plug. Turn the air conditioning system on. If the compressor turns on, replace the pressure switch. If the compressor does not turn on, check the compressor relay. Turn the system off.

Step 7

Remove the jumper wire. Plug the switch in.

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