Something inside the pump is stiff, tight, or seized: worn bearings, a tight piston, rust from sitting idle, or a foreign object. The motor has to work harder to turn the pump, drawing excess current and tripping the overload.
What you'll see
If the overload only trips during restart (not cold start), it's likely a starting-against-pressure problem, not mechanical binding. If the motor runs fine with the belt removed, the binding is in the pump, not the motor. See: Starting Against Pressure.
How to diagnose
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Try to turn the pump by hand
With the compressor off, try to rotate the flywheel or pump pulley by hand. It should turn with moderate effort. If it's very stiff, won't turn at all, or has a tight spot during rotation, there's internal binding. On belt-driven compressors, remove the belt and try turning the pump and motor separately to isolate which is binding.
Result: Turns freely = no mechanical binding. Stiff or stuck = binding present. -
Listen for unusual sounds during turning
While slowly turning the pump by hand, listen for grinding, scraping, or clicking sounds. A grinding sound suggests bearing wear or foreign material. A scraping sound suggests piston-to-cylinder contact. A clicking sound could be a broken valve piece rattling around.
Result: Smooth, quiet rotation = internals OK. Grinding/scraping = wear or damage. -
Check motor current with a clamp meter
If the compressor will run, check the motor current with a clamp meter. Compare to the nameplate FLA (full load amps). Current significantly above FLA (with good voltage) indicates the motor is working harder than it should -- something is mechanically resisting the rotation.
Result: At or below FLA = motor load normal. Well above FLA = excessive mechanical load.
How to fix it
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Free a stuck compressor that sat idle
If the compressor sat unused and the piston rusted to the cylinder, you may be able to free it. Remove the head and pour penetrating oil into the cylinder. Let it soak overnight. Then try to gently rock the flywheel back and forth. Don't force it with a pipe wrench -- you'll break something. If it doesn't free up, the pump may need disassembly.
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Repair or replace the binding component
If bearings are worn, rebuild or replace the pump. If a valve piece fell into the cylinder, remove the head and extract the debris. If the piston is scored from running without oil, the cylinder and piston need replacing. The fix depends entirely on what's binding -- but don't keep running the compressor hoping it will 'loosen up.' It won't.
Don't add oil to the crankcase and assume the binding will fix itself. If bearings are worn or a piston is scored, oil won't restore the clearances. Also: if a compressor sat in a damp location for months or years, don't just plug it in and hope for the best. Turn it by hand first to check. A seized piston on a motor that's trying to start will burn out the motor winding in seconds. And the damage is often catastrophic -- the connecting rod can bend.
Penetrating oil (for stuck pistons). Clamp meter for current measurement. Wrenches for disassembly. Replacement bearings, pump assembly, or motor if damaged. Belt for isolation testing.
A mechanically binding compressor can stall and overheat the motor rapidly. Don't keep trying to start it -- each attempt heats the motor winding. Investigate the cause first.
This issue can also cause
- Unusual Noise / Knocking New or unusual sounds from the compressor: often caused by loose parts, worn bearings, or valve problems.