An air compressor pressure switch is a vital component that controls the shut-off and turn-on pressure of the compressor. A faulty or failing pressure switch can cause a variety of problems and lead to inefficient and irregular compressor operation. Being able to recognize the symptoms of a bad pressure switch can help diagnose issues early and prevent more extensive damage to the air compressor system.
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What is the Function of an Air Compressor Pressure Switch?
The pressure switch acts as the “brain” of an air compressor, continually monitoring the system air pressure and giving commands to start and stop the compressor when needed. It has two critical jobs:
- Telling the compressor motor to start when the tank pressure drops to a pre-set lower limit
- Telling the motor to stop when the tank pressure reaches the higher cut-out pressure
This automated on/off cycling maintains the air tank pressure between a pre-set maximum and minimum pressure range. The pressure settings are adjustable with a screw or knob on most pressure switches.
The switch assembly contains a bourdon tube that senses tank pressure on one side and atmospheric pressure on the other. When the differential between the two reaches a certain threshold, it trips an internal switch that turns the compressor on or off. Pressure switches also include an unloader mechanism that relieves head pressure in the compressor pump during start-up to reduce wear and tear.
What are the Common Symptoms of a Failing Pressure Switch?
Here are some of the most common signs that indicate issues with a faulty or failing air compressor pressure switch:
1. Short Cycling
Short cycling refers to when the compressor turns on and off rapidly and frequently. With a properly working switch, the compressor should only cycle on occasionally when the tank pressure drops. Frequent on/off cycling indicates the pressure settings are off and need adjustment or the switch is faulty.
2. Air Leaks from Unloader Port
Most pressure switches have a small unloader port that releases air for a few seconds when the compressor first starts. This relieves head pressure. If you notice a constant air leak from this port even when the compressor is running, it likely indicates a problem with the unloader mechanism or switch.
3. Air Tank Loses Pressure Quickly
If the air compressor cannot maintain pressure and you have to refill the tank frequently, the pressure switch may be failing to shut off the motor when the tank reaches the cut-out pressure. This allows air to continue escaping from the tank until the pressure drops well below the cut-in setting.
4. Compressor Won’t Shut Off
In severe cases, the pressure switch may fail to shut off the compressor even after maximum tank pressure is reached. The motor will run continuously with the compressor pumping air until the safety relief valve vents air to prevent over-pressurization and tank rupture.
5. Compressor Won’t Start
On the other end of the spectrum, a faulty pressure switch may fail to turn on the compressor even as tank pressure drops well below the cut-in setting. This can prevent the compressor from maintaining adequate pressure to power pneumatic tools.
6. Pressure Fluctuations
Erratic pressure fluctuations from the compressor cycling too rapidly can also indicate issues with a malfunctioning or miscalibrated switch. The optimal pressure range is usually only a 10-15 psi difference between cut-in and cut-out.
7. Noisy Operation
If the air compressor is excessively noisy when starting up or turning off, the switch may be allowing pressurized air to escape rapidly. This can create hammering noises from vibration. Poor contact inside the switch can also cause chattering noises.
8. Loose Wiring
Physical signs like loose or damaged wiring connected to the pressure switch often hint at electrical issues affecting switch operation. Even if wires look intact, broken connections inside the switch enclosure can intermittently disrupt signals.
What Causes a Pressure Switch to Go Bad?
There are several possible causes for pressure switch malfunction or failure in an air compressor:
Normal Wear and Tear
The constant cycling on and off wears down switch components like bellows, valves, and contacts. Electrical and mechanical failure is bound to occur eventually after prolonged use.
Dirty Contacts
Dust or moisture inside the switch can coat electrical contacts and prevent signals from properly triggering the switch. This commonly causes short cycling.
Improper Pressure Settings
If pressure settings are set too close together, it can cause rapid cycling and wear on the switch. Settings too far apart lead to large tank pressure fluctuations.
Faulty Unloader
A damaged unloader mechanism will continue to vent pressurized air when the compressor starts. This air loss may prevent the tank from ever reaching cut-out pressure.
Defective Switch Assembly
Factory defects or damage to bourdon tubes, microswitches, or wiring in the switch assembly itself can lead to inaccurate pressure sensing and cause strange cycling behavior.
Pressure Spikes
Spikes in tank pressure from uneven compression can shorten switch life by stressing internal components.
Power Surges
Power fluctuations or electrical spikes can damage switch wiring and delicate internal parts.
Incorrect Voltage
Running the compressor motor on the wrong voltage for the pressure switch rating can burn out electrical contacts.
How to Test a Compressor Pressure Switch
You can perform a few simple manual tests to determine if a pressure switch is bad or not functioning properly before replacing it:
Visual Inspection
Check all wirings and connections to the pressure switch for signs of looseness, damage, or burns. Cracked switch casings may indicate internal damage.
Listen for Buzzing or Chattering
Turn on the air compressor and listen beside the pressure switch with the cover removed. Buzzing or rapid chattering indicates faulty internal contacts.
Check Pressure Settings
Use a tire gauge to measure cut-in and cut-out pressures by manually restarting the air compressor. Pressure settings grossly higher or lower than manufacturer specs indicate switch issues.
Leak Down Test
Isolate the compressor tank from the switch by closing valves. Shut off the compressor and monitor the pressure gauge. A sharp leak down indicates a faulty check valve or unloader.
Control Voltage Test
Use a multimeter to check for power across the pressure switch terminals during start-up and shut-off cycles. No control voltage at the switch points to wiring problems.
Pressure Switch Continuity Test
Use a multimeter to check for continuity across the closed switch contacts with pressure below cut-in and across the open contacts above cut-out pressure. Improper or no continuity confirms a bad switch.
How to Replace a Faulty Pressure Switch
Replacing a defective pressure switch in an air compressor is a straightforward repair you can do yourself by following these steps:
Turn Off and Disconnect Power
Always shut down, unplug, and disconnect the air compressor before starting any repairs.
Release Air Pressure
Bleed off all pressure from the air tank and lines.
Remove Cover
Take off the metal or plastic cover housing the pressure switch using appropriate hand tools.
Disconnect Wires and Tubing
Remove any wiring from the terminals and small air lines connected to switch ports.
Unscrew the Pressure Switch
Unscrew the pressure switch assembly from the compressor housing using an adjustable wrench or appropriate sized wrench.
Install New Pressure Switch
Screw in the replacement pressure switch and tighten securely.
Reconnect Wires and Tubing
Attach air tubes to the matching ports on the new switch. Connect wires to the correct terminals, following wire colors and any schematic.
Adjust Pressure Settings
Turn the adjustment screws or knobs to dial in the factory recommended cut-in and cut-out pressures.
Test Operation
Power up the compressor and allow the tank to fill to check that the new switch cycles on and off as expected.
Replace Cover
Reattach the pressure switch cover once you have confirmed normal operation.
Conclusion
Issues with air compressor pressure switches tend to develop over time after years of cycling on and off. But noticing symptoms like short cycling, air leaks, or pressure fluctuations early on can allow for pressure switch replacement before catastrophic switch failure. Performing some quick diagnostic tests helps determine if the switch alone is malfunctioning or if there are also unloader issues. Following good maintenance procedures and adjusting pressure settings accurately maximizes the lifespan of your air compressor’s pressure switch.