The compressor tries to restart with compressed air still trapped in the discharge pipe. The motor can't overcome this back pressure and trips the overload. This is the same root cause as 'won't start' due to unloader/check valve problems -- but here the motor has enough power to try and fail, rather than not starting at all.

What you'll see

The compressor runs fine on the first start from empty. It fills the tank, stops normally, and you hear the unloader blow-down. But when it tries to restart, the motor strains, draws high current, and trips the overload within seconds. If you wait a while (pressure leaks down) or manually open the drain valve before restart, it starts fine. The overload only trips on restarts, never on cold starts from empty.
Before you assume this is the problem

If the overload trips even on a cold start from an empty tank, the problem isn't trapped pressure -- it's something else causing high current (low voltage, mechanical binding, bad capacitor). See: Low Voltage, Mechanical Binding, Failed Capacitor.

See all causes of overload relay trips →

How to diagnose

  1. Listen for the blow-down when the compressor stops

    When the compressor reaches cut-out and stops, you should hear a brief 'psssht' -- the unloader venting trapped air from the discharge pipe. If this sound is missing, the unloader valve isn't working. If you hear the blow-down but the compressor still can't restart, the check valve may be leaking air back from the tank, refilling the discharge pipe after the blow-down.

    Result: No blow-down sound = unloader not working. Blow-down occurs but restart still fails = check valve leaking.

  2. Open the drain valve before restart as a test

    When the overload has tripped, open the tank drain valve to release all pressure. Reset the overload and start the compressor. If it starts fine now, the problem is confirmed: trapped pressure in the discharge pipe was preventing the restart. The unloader, check valve, or both need attention.

    Result: Starts fine after releasing pressure = unloader/check valve problem confirmed.

How to fix it

  1. Fix the unloader valve

    Check and repair or replace the unloader valve. The connecting tube may be disconnected or cracked, the plunger may be stuck, or the pressure switch mechanism that triggers the unloader may be broken. See the dedicated Won't Start > Unloader Valve page for detailed diagnosis and repair.

  2. Fix or replace the check valve

    If the unloader works (you hear the blow-down) but the discharge pipe refills from the tank, the check valve between the pump and tank is leaking backward. Clean or replace it. See the dedicated Won't Start > Check Valve Leaking page for details.

Common mistakes

Don't install a time-delay relay to 'give the pressure time to leak down' as a workaround. That's a band-aid that wastes compressed air and delays the restart. Fix the unloader and check valve properly. Also: on 3-phase compressors with star-delta starters, the reduced starting current in star mode may not be enough to overcome even small amounts of trapped pressure. The unloader system is even more critical on these machines.

Parts & tools

Replacement unloader valve or pressure switch. Replacement check valve. Connecting tubing. Same as the Won't Start cause pages for these components.

Review safety precautions before starting →

Safety

Don't keep resetting the overload and restarting. Each attempt overheats the motor winding. Fix the unloader/check valve first.

This issue can also cause

  • Compressor Won't Start Motor hums, trips the breaker, or nothing happens: usually unloader valve, check valve, or electrical problems...