Here's how you generally check a GM AC low pressure switch:
1. Locate the Low Pressure Switch:
* Consult your vehicle's repair manual. The exact location varies depending on the year, make, and model of your GM vehicle. It's typically found on the AC refrigerant line, near the condenser or evaporator. It's usually a small switch with one or two wires connected to it.
2. Gather Your Tools:
* Multimeter: To test the switch's electrical continuity.
* Refrigerant Gauge Set (Optional but Recommended): To check the actual system pressure. This is highly recommended; a low pressure reading explains *why* the switch might be open.
* Wires and Jumper Wires (Optional): To bypass the switch (for testing purposes ONLY). This is only done after checking the system pressure is indeed low.
* Socket and Ratchet (if needed): To access the switch. Some switches are difficult to reach and require removal.
3. Test the Switch (Two main methods):
* Method 1: Using a Multimeter (Preferred Method):
* With the system OFF: Disconnect the electrical connector from the low pressure switch.
* Check for Continuity: Set your multimeter to the continuity setting (usually a diode symbol or a bell symbol). Touch the probes to the switch's terminals. A closed circuit (continuity) should indicate a good switch *if the system pressure is high enough*. No continuity means the switch is open (no signal is getting to the AC compressor). However, this test only tells you if the switch itself is working electrically – the low pressure in the system is why the circuit is open.
* Check for Open Circuit when system is pressured: If the system pressure is low, this is usually why the switch is "open". The switch should only close when the system pressure is within acceptable ranges. Using a refrigerant gauge set is vital here.
* Method 2: Bypassing the Switch (ONLY as a Diagnostic Test; Do Not Leave Bypassed):
* ONLY AFTER VERIFYING LOW SYSTEM PRESSURE with gauge set: This should only be done if you are *certain* the system is low on refrigerant and the switch is the suspected fault.
* Caution: Bypassing the switch will allow the compressor to run even if the pressure is dangerously low, which can severely damage the compressor. This is for diagnostic purposes only and should be undone immediately after testing.
* Disconnect: Disconnect the switch's connector.
* Bypass: Using jumper wires, carefully connect the two wires that were connected to the switch.
* Observe: Turn on the AC system. If the compressor engages, this points towards a faulty low-pressure switch. Immediately turn the AC off and disconnect the jumper wires. This only means your low pressure switch is defective, not the source of low pressure!
4. Interpreting the Results:
* Continuity and AC Works: The switch is likely good. The problem lies elsewhere in the AC system (refrigerant level, compressor, etc.).
* No Continuity and AC Doesn't Work: The switch may be faulty. But only if the pressure is acceptable. Using a refrigerant gauge set is vital!
* Bypassing the switch allows the AC to turn on (After confirming low pressure): The low-pressure switch is most likely faulty.
Important Considerations:
* Refrigerant Levels: Low refrigerant is the most common reason for a low pressure switch to open. Before replacing the switch, have a qualified technician check and recharge the system. Adding refrigerant without diagnosing a leak is a waste of money and potentially harmful to the environment.
* Professional Help: Automotive AC systems are complex. If you're not comfortable with the steps above, it's best to consult a professional AC technician. Improper handling of refrigerant can result in injury or environmental damage.
Remember, safety first! Always handle refrigerant with care and seek professional help when necessary.
