The solenoid valve is fine, but it never receives the electrical signal to open. The problem is upstream -- controller, wiring, or pressure switch.

What you'll see

Motor runs, no air is produced. Looks identical to a solenoid failure -- but when you check the solenoid, it turns out to be perfectly fine. No voltage at the solenoid terminals when the controller says 'loaded'. The solenoid never clicks because it never gets the signal to do so.
Before you assume this is the problem

First confirm the solenoid itself is OK -- apply 24V directly to the solenoid to bypass the controller. If it clicks and control air flows, the solenoid is fine and the problem is electrical/control side. If it doesn't work even with direct power, you have a bad solenoid (see: Solenoid Valve Failure).

Could also be:

See all causes of compressor won't load →

How to diagnose

  1. Bypass test -- apply power directly to solenoid

    Apply 24VDC (or whatever your solenoid voltage is) directly to the solenoid terminals, bypassing the controller entirely. If the compressor loads, the solenoid and pneumatics are fine -- the problem is 100% on the electrical/control side. This is the single most useful test you can do.
    Result: Compressor loads with direct power = controller, wiring, or pressure switch problem.

  2. Check the pressure switch

    The pressure switch tells the controller when to load and unload. Listen for a click from the pressure switch when system pressure drops below the load setpoint. Check with a multimeter -- does it close the circuit at the right pressure? A bad pressure switch sends the wrong signal (or no signal) to the controller, which then never tells the solenoid to open.
    Result: No click from pressure switch at correct pressure = bad switch. Switch OK = problem is between switch and solenoid.

  3. Check voltage at controller output

    Find the controller connector for the solenoid output. On Atlas Copco Electronicon 1, that's X104, pin 1 and 3 for solenoid Y1. Is there voltage here when the controller is in load mode? If yes, the wire between the controller and the solenoid is broken or has a loose connection. If no voltage at the controller output, the controller itself or an internal relay has failed.
    Result: Voltage at controller connector but not at solenoid = broken wire. No voltage at controller = bad relay or controller fault.

  4. Inspect wiring and connections

    Check the cable from the controller to the solenoid. Look for loose connections, corroded terminals, broken wires (vibration is a common cause), and blown fuses. On some machines, the solenoid circuit goes through an auxiliary contact on the main contactor -- check that contact too.
    Result: Damaged or loose wiring found = repair and retest.

  5. Check for controller alarm or fault

    Some controllers will refuse to send the load signal if there's an active alarm -- high temperature, motor protection, phase fault, etc. Check the controller display for any active faults. On some Kaeser Sigma controllers, the display may show 'idle' permanently if there's an internal fault preventing the load command from being issued.
    Result: Active alarm blocking load command = resolve the alarm first.

How to fix it

  1. Replace the pressure switch

    If the pressure switch is the culprit, replace it. Set the load and unload setpoints according to your system requirements. Make sure the new switch is rated for the correct pressure range.

  2. Repair wiring / connections

    Fix any loose, corroded, or broken connections. Replace damaged cables. Clean connector pins. On machines that vibrate heavily, consider using cable ties or clamps to prevent future wire fatigue.

  3. Replace controller relay or board

    If the internal relay in the controller has failed, you may need to replace the relay (if it's socketed) or the entire controller board. Before going this route, double-check all external causes -- a controller replacement is expensive and it's frustrating to find out afterwards that it was just a loose wire.

Common mistakes

Don't replace the solenoid when the solenoid is fine. Always check for voltage at the solenoid first. No voltage = the solenoid is not the problem. Also, don't overlook the auxiliary contactor contact -- on some machines, the solenoid only gets power after the motor starter has confirmed the motor is running. A bad auxiliary contact can block the load signal even though the controller is working correctly.

Parts & tools

Multimeter (essential). Possibly a replacement pressure switch, relay, or controller board depending on what you find. A 24V DC power supply or battery pack is handy for the bypass test.

Review safety precautions before starting →

Safety

You'll be working with live electrical connections while the compressor runs. Use proper insulated tools and a multimeter rated for the voltage you're measuring. Lock out before opening any junction boxes or controller panels.