The overload relay itself is worn out and tripping at lower current than it should. After years of thermal cycling, the bimetallic strip inside becomes weaker and trips earlier. The compressor is fine, the relay is just too sensitive from age.

What you'll see

The overload trips intermittently, often after running fine for a while. The motor doesn't feel excessively hot. A clamp meter shows the current is at or below the nameplate FLA (full load amps). No unusual noises or mechanical problems. The overload seems to trip for no reason -- especially on warm days when it's already hot. You reset it and the compressor runs fine again for a while. This pattern of 'nuisance tripping' is classic for a worn relay.
Before you assume this is the problem

Before blaming the relay, verify that the current draw is actually normal. Use a clamp meter to measure amps during running. If the current is above nameplate FLA, the relay is doing its job -- the problem is elsewhere (voltage, mechanical binding, bad capacitor). See: Low Voltage, Mechanical Binding.

See all causes of overload relay trips →

How to diagnose

  1. Measure motor current with a clamp meter

    While the compressor runs, clamp the meter around one of the motor leads and read the amps. Compare to the nameplate FLA. If the current is at or below FLA and the relay still trips, the relay is likely worn or misadjusted. If current is above FLA, the relay is correctly protecting the motor and the high current is the real problem.

    Result: Current at/below FLA but relay trips = worn relay. Current above FLA = relay is correct, find the cause.

  2. Check the relay setting (if adjustable)

    Some overload relays have an adjustable current setting (a dial or slider). Check that it's set to match the motor's FLA. If someone turned it down, the relay trips at a lower current than the motor needs. Set it to match the nameplate.

    Result: Set correctly = relay may be worn. Set too low = adjust to nameplate FLA.

  3. Check for heat affecting the relay

    Overload relays use a bimetallic strip that bends with heat. If the relay is mounted in a hot location (near the motor or in an enclosed panel with no ventilation), ambient heat can cause it to trip at lower than rated current. Check if the relay location is unusually hot.

    Result: Cool location = relay condition issue. Hot location = relocate or ventilate.

How to fix it

  1. Replace the overload relay

    Match the replacement to the motor's FLA and the type of relay (manual reset vs. auto-reset, mounting style). Replace the relay with a new one rated for the correct current range. They're not expensive -- typically $15-$50 for small compressors. Don't use a higher-rated relay than the motor requires -- that defeats the protection purpose.

  2. Adjust if the relay has a current setting

    If the new relay is adjustable, set it to the motor's FLA (nameplate value). This gives the motor its full rated current before the relay trips. Don't set it higher than FLA 'for margin' -- the relay already has a built-in margin above the setpoint before it trips.

Common mistakes

The biggest mistake is bypassing the overload relay because it keeps tripping. Without the relay, the motor has no protection from overheating and can burn out -- turning a $30 relay replacement into a $500+ motor replacement. Also: don't replace a manual-reset relay with an auto-reset type (or vice versa) without understanding the difference. Auto-reset relays restart the compressor automatically after cooling, which can be dangerous if someone is working on it.

Parts & tools

Replacement overload relay (match FLA and type). Clamp meter for current measurement. Screwdriver for terminal connections.

Review safety precautions before starting →

Safety

Don't bypass the overload relay to 'solve' nuisance tripping. The relay protects the motor from burnout. Replace the relay rather than bypassing it.