The loading solenoid controls the air supply to the inlet valve. When it fails, the compressor runs but won't pump air.
What you'll see
Before you blame the solenoid, check that the controller is actually sending a load signal. Press load/unload on the panel -- does anything change? If the controller itself is stuck in idle or showing an alarm, the solenoid might be perfectly fine. Also check if there's control air available at all -- a leaking minimum pressure valve or plugged bypass hole in the inlet valve can mimic a solenoid failure. See: No Electrical Signal to Solenoid, Control Air Line Blocked, No Control Air Supply.
Could also be:
How to diagnose
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Listen for the click
Press load/unload on the controller and listen at the solenoid. You should hear a clear 'click' when it switches. No click? Either the solenoid coil is burned or it's not getting electrical power. If it clicks but the compressor still won't load, the solenoid might be dirty inside -- air can't pass through even though the coil works.Result: No click = electrical or coil problem. Click but no loading = dirty solenoid or downstream issue. -
Check for magnetism with a screwdriver
Hold a metal screwdriver near the solenoid body while pressing load. A working solenoid becomes magnetic when energized -- you'll feel the pull. Simple trick, no tools needed beyond a screwdriver.Result: No magnetic pull when loaded = coil is dead or no power reaching it. -
Measure voltage at the solenoid
Get a multimeter and check across the solenoid terminals. You should see voltage (usually 24V AC or DC) when the controller is in load mode, and 0V in unload. On Atlas Copco machines with Electronicon 1 controller, the solenoid Y1 connects at X104, pin 1 and 3. If voltage is present but the solenoid doesn't click, the coil is burned through.Result: Voltage present + no click = bad coil. No voltage = problem is upstream (controller, wiring, pressure switch). -
Check control air output
Disconnect the control air line going from the solenoid to the inlet valve. Load/unload the machine and check if compressed air comes out of the solenoid's outlet port. Be careful -- this removes the control function. The compressor might build pressure uncontrolled until the safety valve blows. Keep your hand ready on the stop button.Result: No air from solenoid outlet = solenoid is blocked or not switching. Air comes out = solenoid is fine, problem is the inlet valve or the line to it.
How to fix it
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Clean the solenoid
Remove the solenoid, take it apart, and clean all internal parts. Dirt buildup inside is one of the most common causes -- the coil works, it clicks, but no air passes through. Clean it out and you're often back in business.
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Replace the coil (if burned)
If the coil is burned through (no magnetism, no click despite having power), you can often replace just the coil rather than the whole solenoid assembly. Check with your supplier -- coils are much cheaper than complete solenoid valves.
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Replace the complete solenoid
If cleaning and a new coil don't fix it, replace the whole thing. When installing, pay attention to the flow direction marking on the body. Installing it backwards will reverse the load/unload behavior -- the compressor loads when it should unload and vice versa.
Don't just replace the solenoid without checking why it failed. A burned coil can indicate an electrical problem (short circuit, wrong voltage) that will kill the new one too. Also: be very careful when opening the inlet valve assembly. Many models have a very powerful compressed spring inside that can pop in your face if opened incorrectly. Always follow the manufacturer's procedure.
Multimeter for voltage testing. Replacement solenoid coil or complete solenoid valve -- get the right one for your brand and model. Common solenoid voltages: 24V AC, 24V DC, 12V DC. On newer machines the solenoid is bolted directly onto the inlet valve. On older machines it's separate, connected by flexible hoses.
Always lock out and tag out the compressor before working on the solenoid or inlet valve. The solenoid operates at 24V AC or DC on most machines -- low voltage, but still worth respecting.
This issue can also cause
- Safety Valve Blows / Too High Pressure Safety valve opens or pressure exceeds setpoint, the compressor isn't unloading properly or there's a restrict...