Key Takeaway
An AC pressure switch is commonly found in car air conditioning systems. Its job is to monitor refrigerant pressure and protect the AC system from low or high pressure. To check it, turn off the engine and locate the switch — usually near the AC compressor or on the refrigerant line. Disconnect the electrical plug and use a multimeter to test continuity. At normal pressure, the switch should show connection (continuity). If the pressure is too low or too high, it may be open. You can also run the AC system and monitor whether the compressor turns ON and OFF at the correct times. If the compressor doesn’t run or keeps shutting off, the switch may be stuck or broken. Be careful — refrigerant systems are pressurized. If needed, ask a technician to inspect or replace the switch. A working AC pressure switch helps your air conditioning stay efficient and safe.
Function of AC Pressure Switch in Cooling Systems
AC pressure switches are critical for regulating refrigerant pressure in cooling systems like air conditioners and chillers. They monitor suction and discharge pressure to ensure safe compressor operation. If pressure drops too low, the switch prevents freezing; if it goes too high, it avoids overloading or damage. In cars, split AC units, and HVAC systems, these switches act as automatic protection—disconnecting power when pressure exceeds safe limits. Some systems have both high and low-pressure switches. Testing involves using a manifold gauge set and verifying switch continuity. A faulty AC pressure switch can cause the compressor to remain off, reducing cooling performance. Accurate pressure control ensures safety, efficiency, and prolonged compressor life.
Signs of a Faulty AC Switch (Compressor Won’t Start)
Here’s a common scenario: AC is on, fans are spinning, but there’s no cold air. The compressor just won’t kick in. That’s a red flag.
And many times, the AC pressure switch is the reason.
What are the signs?
Compressor doesn’t turn on even though all other components seem to work fine
Intermittent cooling – works fine sometimes, fails the next minute
AC cuts out in hot weather (high pressure builds up quickly)
Check engine light in vehicles (yes, car AC pressure switches can cause this too)
No voltage signal from the pressure switch to the compressor clutch
If you’ve already confirmed that the power supply is okay, refrigerant levels are within range, and the wiring isn’t damaged – your next suspect is the Pressure switch.
It could be stuck open or closed. And if it’s stuck, it won’t send the right signal. Which means, the compressor stays off no matter what.
We’ve seen systems where the only fault was a tiny, faulty pressure switch. Replacing it brought the cooling back to life in minutes. It’s a small part with a big impact.
Safe Testing with an AC Manifold Gauge
So, how do you test the switch properly? Step one is using the AC manifold gauge. This is your eyes into the pressure levels.
Here’s the safe, step-by-step method:
Turn off the system first. Always. Safety first.
Connect your manifold gauge set to the high- and low-pressure ports.
Turn the AC back on and observe the readings.
Now, this is what you’re looking for:
Low pressure should be around 25–45 psi when running
High pressure might range from 200–300 psi depending on ambient temperature and system
If the low side pressure is too low, and the low-pressure switch has cut the compressor off, that’s working as intended. Similarly, high pressure exceeding safe limits may have triggered the high-pressure switch.
If pressures are normal but the compressor still doesn’t turn on, then the switch may be faulty.
You can also use a multimeter to test if voltage is flowing through the switch. If the circuit is open when it shouldn’t be — or closed when it should be open — you’ve got your culprit.
Using a Jumper to Bypass and Verify Response
This one needs a warning: Only do this if you understand electrical systems. And even then, do it only for quick testing, not as a long-term solution.
Let’s say the AC system isn’t starting, and your pressure readings are within range. The switch might be stuck. That’s when we do a jumper test.
Here’s how:
Turn off the ignition and system
Locate the pressure switch harness connector
Use a wire to create a bridge between the two terminals
Turn the AC system on again
If the compressor clutch engages, it means the rest of the system is fine, and the pressure switch is the problem.
Important note: This is just to verify the fault. Never leave a jumper wire in place permanently. It bypasses a safety system that protects the compressor from pressure-related damage.
It’s a quick, effective test that we use in garages and workshops all the time. But always remember to fix the root cause.
Common Pressure Values in Car and Split AC Systems
If you’re new, knowing the normal pressure values gives you a strong head start.
Car AC Systems:
Low Side: 25–45 psi
High Side: 200–300 psi (can go higher in extreme heat)
Split AC (Home/Commercial Units):
Low Side: 60–80 psi depending on refrigerant (R22, R32, R410a)
High Side: 200–350 psi typically
These are ballpark figures. You’ll always get the exact specs from the manufacturer’s service manual.
Too low pressure usually means a refrigerant leak or undercharged system. Too high pressure could indicate a clogged condenser, overcharging, or fan failure.
The pressure switch is designed to respond to these ranges. For example:
A low-pressure switch might open below 25 psi
A high-pressure switch may cut off at 425 psi
Each AC type, especially in industrial or automotive systems, has its calibrated pressure switch for maximum safety.
So, if your readings fall outside the norm, and the system shuts down, the pressure switch might be doing exactly what it’s supposed to do.
Conclusion
In air conditioning systems—whether for cars, homes, or offices—pressure switches play a vital role. They help maintain correct refrigerant pressure and protect the compressor from damage. If pressure drops too low, the switch cuts power to avoid system freezing. If pressure rises too high, the switch prevents overheating or rupture. A faulty pressure switch can result in no cooling, compressor failure, or continuous cycling. To test an AC pressure switch, use an AC manifold gauge and multimeter. Some technicians use a jumper wire to bypass the switch briefly, confirming if the compressor starts. However, this should only be done temporarily and with caution. Pressure values for activation vary between systems (e.g., 25–45 PSI for low-pressure cutout). Always refer to your system manual. Without an accurate pressure switch, your AC system may run inefficiently or dangerously. Ensuring proper pressure switching keeps your AC system reliable, safe, and energy-efficient all year round.