The motor's built-in thermal protection has tripped due to overheating. This is a safety feature, not a fault in itself. The real question is: why did the motor overheat? Could be as simple as a hot day, or as serious as a failing motor.
What you'll see
If the motor is cold and still won't start, the thermal overload isn't the issue. Check the pressure switch and power supply. If the motor hums but won't spin, see: Bad Start Capacitor. If the breaker trips instead of the thermal overload, see: Overload Relay Trips.
How to diagnose
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Check if the motor is hot
Touch the motor housing carefully. If it's too hot to hold your hand on for more than a few seconds, the thermal overload has likely tripped due to genuine overheating. Normal running temperature is warm to the touch but not uncomfortably hot.
Result: Very hot motor = thermal overload tripped legitimately. Cool motor that won't start = different problem. -
Find and press the reset button
Most compressor motors have a small red or black reset button on the motor housing. It's usually near the wiring junction box or on the end cap. Press it firmly until you feel it click. If there's no reset button, the overload may be the auto-reset type that resets itself once the motor cools down -- just wait.
Result: Button clicks and motor starts after cooling = thermal overload confirmed. -
Identify why the motor overheated
If the thermal overload keeps tripping, something is causing excessive heat. Check: Is the compressor in a hot, enclosed space with poor ventilation? Has the duty cycle been exceeded (many small compressors are rated for 50-60% duty cycle, not continuous)? Is the motor drawing higher current than rated (use a clamp meter)? Is the pump binding or stiff? Is there adequate cooling airflow over the motor and pump?
Result: Identify the root cause -- ventilation, duty cycle, mechanical, or electrical.
How to fix it
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Let the motor cool and reset
Wait 30-60 minutes for the motor to cool completely, then press the reset button. If the compressor starts and runs normally, it may have been a one-time event caused by heavy use or high ambient temperature. Monitor it closely for the next few cycles.
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Improve ventilation and reduce duty cycle
If it's a ventilation or duty cycle problem: move the compressor to a cooler, better-ventilated location. Don't enclose it in a closet or tight corner. If the compressor runs more than its rated duty cycle, you need a bigger compressor or a bigger tank. A larger tank means the pump runs less often because there's more stored air between cycles.
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Check motor current draw
Use a clamp meter to check the motor current during running. Compare it to the nameplate full load amps (FLA). If the current is above the FLA, something is making the motor work too hard -- could be a worn pump, a mechanical binding, or low voltage. If the motor draws high current from the start, the issue is likely mechanical.
The biggest mistake is repeatedly resetting the overload and restarting without finding the cause. Each overheat cycle damages the motor winding insulation. After enough cycles, the insulation breaks down and the motor burns out -- a much more expensive repair than fixing the root cause. Also: don't tape the reset button down or bypass the thermal overload. It's there to protect the motor and prevent a fire.
Clamp meter for checking motor current. Thermometer (optional). May need larger tank or compressor if duty cycle is the issue. Reset button location varies by motor brand.
The motor housing will be hot. Let it cool before touching. Don't keep resetting the thermal overload and restarting -- repeated overheating destroys motor windings. Find and fix the root cause.
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.