How To Test AC Pressure Switch With Multimeter

DANIEL FOSTER

As an electrical engineer from Stanford University with over 7 years of experience using multimeters.

When your A/C isn’t cooling as it should, it might be time for an A/C pressure switch test. Understanding how to test an AC pressure switch with a multimeter can save you time and money. In this guide, we’ll walk you through the steps of testing A/C pressure switch, ensuring you get your cooling back on track.

What is a Pressure Switch?

A pressure switch is a compact device commonly found in household equipment, vehicles, vending machines, and even large-scale industrial machinery. Its primary function is to monitor and control equipment, ranging from industrial welding machines to the heating and ventilation systems in our homes.

At its core, a pressure switch is composed of a pressure meter and a switch component. When the system reaches a specified pressure, the switch activates, either pausing or halting the electrical circuit it’s part of.

High Pressure vs Low Pressure Switches

While “high-pressure” and “low-pressure” switches are often used interchangeably, they serve distinct functions within an AC unit. Both are vital in regulating AC pressure and safeguarding the compressor from potential damage.

High-Pressure Switches: These are engineered to shield your AC from excessive pressures. If the pressure escalates beyond a safe threshold, these switches will deactivate the AC unit. Typically, the pressure on a high-pressure switch should lie between 250 and 350 psi. Deviations from this range might indicate a malfunctioning switch that requires replacement.

Low-Pressure Switches: Contrarily, low-pressure switches are designed to protect the AC from insufficient pressures. If the pressure dips too low, the switch will turn off the AC unit. The optimal pressure range for a low-pressure switch is between 90 and 110 psi. Pressures outside this bracket might suggest a faulty switch.

Both switches collaborate to ensure the safety and efficiency of your AC system. The high-pressure switch comes into play when pressures soar, while the low-pressure switch intervenes when they plummet. Each has its advantages and drawbacks.

High-pressure switches, for instance, are robust and can endure elevated pressures but come with a heftier price tag. Low-pressure switches are more affordable but might need to be sturdier to manage higher forces, contingent on their intended use and environment.

Where Is The Ac Pressure Switch Located?

Pressure switches are on the air conditioner’s pipes. There are two sides to the air conditioner: a low-pressure side and a high-pressure side. The low-pressure switch is on the smaller pipe. The high-pressure switch is on the larger pipe. The high-pressure switch is after the compressor. The low-pressure switch is before the compressor. Most of these switches are in the engine area. Check your car’s manual to find the exact location.

Common Issues with Pressure Switches

Pressure switches, like all components, are susceptible to wear and tear and potential issues. Some prevalent problems include:

  • Diaphragm Deterioration: Some pressure switches have diaphragms that degrade over time, affecting their functionality.
  • Wear and Tear: Regular usage can lead to cracks, broken components, and other damages.

For those primarily dealing with AC systems, a recurrent issue stemming from a defective pressure switch is the AC unit frequently shutting off, attempting to restart, or failing to operate altogether. 

How to Test AC Pressure Switch with a Multimeter

If you suspect that your AC isn’t functioning correctly due to a faulty low-pressure switch, here’s a step-by-step guide on how to test it using a multimeter.

Step 1: Assemble the Necessary Tools

Before diving into the testing process, ensure you have the right tools on hand. The essential tools you’ll need include:

  • 5/16″ nut driver
  • 1/4″ nut driver
  • Screwdriver
  • Multimeter 

Being proficient with a multimeter will significantly simplify the testing process.

Step 2: Locate the AC Low-Pressure Switch

Begin by identifying the location of the low-pressure switch on your AC unit. Typically, this switch is situated near the compressor, specifically on the suction line. To access it, open the furnace using the appropriate nut driver or screwdriver. Once located, the next step is to test its functionality.

Step 3: Diagnose the Low-Pressure Switch Using Ohms

Safety first! Ensure you disconnect the power at the service switch.

Set your multimeter to the Ohms setting, usually represented by the Greek letter omega (Ω). After setting up the multimeter, detach the two wires connected to the low-pressure switch and seal them off. Power on the service switch again.

As the combustion process begins (indicated by the draft inducer), place the multimeter’s two leads on the switch’s terminals. The switch should make contact without any resistance. If your multimeter beeps, it’s a good sign, indicating the switch is functioning correctly. However, if there’s no beep, it might be time to consider replacing the switch.

Is Your AC’s Low-Pressure Switch Faulty?

The AC’s low-pressure switch is a crucial component that ensures the system operates within safe parameters. But how can you tell if it’s malfunctioning? Here are some telltale signs that might indicate a faulty low-pressure switch:

1. Inconsistent Pressure Readings:

If the pressure readings on the switch deviate from the ideal range, it’s a clear indication that the switch might malfunction and may need replacement.

2. Compressor Issues:

The compressor plays a pivotal role in circulating the refrigerant through the AC unit, facilitating the cooling process. A malfunctioning low-pressure switch can lead to compressor damage, as it fails to regulate the pressure effectively. If your AC isn’t cooling your space efficiently, the low-pressure switch could be the culprit.

3. AC Unit’s Erratic Behavior:

One of the most evident signs of a faulty low-pressure switch is when the AC unit frequently cycles on and off. Such erratic behavior suggests that the switch isn’t regulating the pressure as it should.

4. Unusual Noises:

A well-functioning AC unit operates quietly. However, if you start hearing strange or loud noises emanating from your AC, it could be due to a malfunctioning low-pressure switch. Such noises often arise when the compressor overheats due to unregulated pressure.

5. Continuity Test Failures:

Using a multimeter, if you don’t get a reading of 0 ohms when checking for continuity, it strongly indicates that the low-pressure switch might be faulty.

If you’ve observed any of these signs and have tried troubleshooting without success, it’s advisable to seek professional assistance. 

Conclusion

Over time, these switches can face issues like diaphragm deterioration and general wear and tear, leading to AC malfunctions. Testing the low-pressure switch with a multimeter can help diagnose its functionality.

Signs of a faulty low-pressure switch include inconsistent pressure readings, compressor problems, erratic AC behavior, unusual noises, and continuity test failures. If you notice any of these signs, it’s essential to address the issue promptly, either through DIY troubleshooting or by seeking professional help.

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