Most common cause of a reciprocating compressor not starting, about 40% of cases. The unloader valve is supposed to release trapped air so the motor can start unloaded. When it fails, the motor tries to start against pressure and stalls or trips the breaker.

What you'll see

You flip the switch and the motor hums loudly but doesn't spin, or it starts to turn very slowly and then the thermal overload or breaker trips. Sometimes the motor just buzzes for a second and clicks off.

The key clue: if you open the drain valve on the tank to release all the air, and then the compressor starts fine, this is almost certainly an unloader problem. The motor isn't strong enough to push against the leftover pressure in the discharge pipe.
Before you assume this is the problem

If the compressor won't start even with an empty tank and zero pressure, the unloader isn't the problem. Check the motor, capacitor, and power supply instead. See: Bad Start Capacitor, Thermal Overload Tripped.

See all causes of compressor won't start →

How to diagnose

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

    When a reciprocating compressor reaches cut-out pressure and stops, you should hear a brief blast of air -- a 'psssht' sound lasting 1-2 seconds. That's the unloader valve venting the air trapped between the pump and the check valve. If you don't hear this blow-down, the unloader isn't working.

    Result: Clear pssht sound on shutdown = unloader working. No sound = unloader stuck or disconnected.

  2. Check the unloader valve tube and linkage

    On most small piston compressors, the unloader is built into the pressure switch or connected to it by a small copper or plastic tube. Check that this tube is still connected at both ends and not cracked or kinked. If the tube has come off, the pressure switch can't actuate the unloader. Also check that the little toggle pin or plunger on the pressure switch moves freely when the switch clicks off.

    Result: Tube connected and intact, plunger moves freely = connections OK. Tube off or cracked = reconnect or replace.

  3. Inspect the unloader valve itself

    The unloader valve sits on top of the pump head or between the pump and the check valve. Remove it and check for a stuck plunger, worn seal, or debris. The plunger should move freely. On some compressors, the unloader has a small disc or poppet that lifts to vent air -- make sure it's not corroded or stuck in the closed position. Blow through it to check if air passes.

    Result: Plunger moves freely, air passes when open = OK. Stuck, corroded, or sealed shut = replace.

  4. Test by manually relieving pressure

    As a quick test: pull the ring on the safety relief valve (or crack open the tank drain) just before the compressor tries to restart. If the motor starts fine with the pressure relieved, you've confirmed the unloader is the issue. The motor needs zero or near-zero back pressure to start.

    Result: Starts fine with pressure relieved = unloader problem confirmed.

How to fix it

  1. Clean or replace the unloader valve

    Remove the unloader valve, clean all parts with a solvent, and check the seal or o-ring. If the plunger is corroded or the seal is torn, replace the valve. They're inexpensive -- typically $15-$40. Make sure you get the right one for your compressor model. Some pressure switches have the unloader built in, so you'd replace the whole pressure switch.

  2. Replace the connecting tube

    If the tube between the pressure switch and unloader is cracked, kinked, or disconnected, replace it. Use the correct diameter tubing -- it's usually 1/4 inch or 6mm. Make sure both ends are secured. A loose tube means the signal never reaches the unloader.

  3. Check the pressure switch mechanism

    Some pressure switches have a mechanical linkage that physically pushes the unloader plunger when the switch trips. If this linkage is broken or misadjusted, the unloader won't open even though the switch itself works. Check that the toggle lever or pin engages the unloader when the switch clicks off.

Common mistakes

Don't keep trying to start the compressor when it's stalling -- each failed start heats the motor winding and can burn it out. If it won't start, stop and diagnose. Also: some people replace the whole pressure switch when it's just the unloader tube that fell off. Check the simple stuff first. And if you replace the unloader valve, make sure it's oriented correctly -- some have a specific flow direction.

Parts & tools

Replacement unloader valve or pressure switch (match model). Connecting tubing (1/4 inch or 6mm). Wrenches for removal. Thread sealant tape for fittings.

Review safety precautions before starting →

Safety

Disconnect power before working on the unloader valve or pressure switch. Bleed all pressure from the tank first. The discharge pipe and check valve area can be hot after running.

This issue can also cause

  • Overload Relay Trips Thermal overload or motor breaker keeps tripping: either an electrical issue or the motor is working too hard.