The inlet valve stays open when it should close. The compressor keeps pumping air past the setpoint until the safety valve pops.

What you'll see

Pressure rises above the unload setpoint. The safety valve on the separator vessel pops, releasing a loud burst of air. The controller may show 'loaded' when it should show 'unloaded'. You can hear the solenoid vent air (it clicks and blows off) but the inlet valve doesn't physically close. This is the inverse of 'won't load' -- same components, opposite failure.
Before you assume this is the problem

If the safety valve pops only during high air demand (not during idle), the problem might be a clogged separator, not a stuck inlet valve. See: Separator Pressure Buildup. Also check the pressure switch/transducer -- a faulty one may not send the unload signal.

See all causes of safety valve blows / too high pressure →

How to diagnose

  1. Listen at the solenoid during unload

    When the controller commands unload, the solenoid should click and vent control air. If you hear the click and the blow-off, the electrical side is working. The problem is that the inlet valve piston is physically stuck open -- dirt, worn seals, or a broken return spring.
    Result: Solenoid vents but valve stays open = mechanical inlet valve problem.

  2. Close the outlet valve slowly

    Without airflow there's no pressure drop across the separator. If the compressor now reaches setpoint and properly unloads, the issue was actually the separator, not the inlet valve. If it still won't unload and pressure climbs, the inlet valve or control system is the problem.
    Result: Unloads with valve closed = separator issue. Still won't unload = inlet valve or controls.

  3. Disconnect solenoid to force unload

    Disconnecting the solenoid electrically should remove control air from the inlet valve, causing it to close by spring force. If the valve still doesn't close, the piston is mechanically seized.
    Result: Still open with solenoid disconnected = piston seized. Closes = solenoid was stuck energized.

How to fix it

  1. Overhaul the inlet valve

    Open, clean, replace all O-rings and seals. Check the return spring. Dirt and sludge buildup on the piston is the most common cause. Be careful of the powerful spring inside.

  2. Check/replace the pressure switch or transducer

    If the controller never sends the unload signal, the pressure sensing element may have failed. Check the pressure switch contacts or transducer output with a multimeter.

Common mistakes

Don't increase the safety valve setting. If a safety valve has popped several times, it can weaken and pop at a lower pressure than rated. Verify its actual pop pressure with a gauge -- you might need to replace it too.

Parts & tools

Inlet valve overhaul kit. Replacement pressure switch or transducer if faulty. Possibly a replacement safety valve if it has weakened from repeated popping.

Review safety precautions before starting →

Safety

Do not block or increase the setting of the safety valve. It's protecting the pressure vessel. If it pops, stop the compressor and investigate.

This issue can also cause

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