Compressor Tripping on Internal Overload: How to Confirm It and Find the Cause

A compressor's internal overload is a thermal protector in the motor's common circuit. When the windings get too hot or draw too much current, it opens and the compressor stops. It resets on its own once the motor cools.

A compressor sitting on its overload looks dead. The contactor is pulled in, there's voltage at the terminals, and nothing happens. The overload isn't the problem; it's telling you something else is. If you reset it and walk away, you'll be back.

Safety: Shut off and lock out power before ohming terminals. A compressor that has been cycling on its overload can have a damaged terminal; don't stand over the terminal cover while it tries to start.

Confirm it's the overload

1. Feel or shoot the shell. A compressor on its overload is hot. The ACHR News suggests an infrared thermometer reading of a hot compressor points to an overload trip.
2. Clamp the common lead. Voltage at the terminals with no amps at all means the motor circuit is open. HVAC School's guidance: if you read no amps, give the overload time to reset.
3. Ohm the terminals (power off, wires off):

The R–S reading is the giveaway: the overload is in the common lead, so the two windings still read through each other.

How long it takes to reset

Longer than you think. Bryant's service manual for a Copeland-equipped rooftop unit says the overload may need up to 60 minutes or longer to reset. Friedrich's service manual says it can take anywhere from a few minutes to several hours, because heat built up inside the shell is slow to dissipate.

To speed it up, the ACHR News suggests running a garden hose over the top of the shell (not the terminals), or a wet towel for a slower cool-down. Some techs leave an ohmmeter on C–R in continuity mode so it beeps when the overload closes.

If the shell is cool and C–R and C–S are still open, the overload may be stuck open. That's a failed compressor.

Scrolls with temperature protection

Some Copeland scrolls also have Advanced Scroll Temperature Protection (ASTP). It's a disc that unloads the scrolls when the internal discharge temperature hits about 300°F (149°C). The motor keeps running but pumps nothing. Bryant's manual notes that if the compressor isn't stopped, it runs until the motor protector trips, which can take up to 90 minutes. A label on the compressor tells you if it has ASTP.

Why it overheated: the usual causes

A compressor trips its overload because the motor got too hot or drew too much current. Find which one.

Not enough cool suction gas

Most residential compressors are cooled by the suction gas returning from the evaporator. Anything that starves the compressor of suction gas lets it overheat.

• Low charge. Low suction pressure, high superheat. See how to find a refrigerant leak.
• A restriction. A plugged filter-drier or a TXV that isn't feeding gives the same starved low side. The ACHR News describes a case where a TXV starving the low side let a refrigerant-cooled compressor overheat and cycle on its overload until the run winding failed. See restricted liquid line and TXV troubleshooting.
• Very low indoor airflow in a frozen-coil situation can do it too, though floodback is the more common result there.

High head pressure

The motor has to work harder against high discharge pressure, and discharge temperature climbs.

• Dirty condenser coil. See how to clean a condenser coil.
• Condenser fan not running or running slow. See condenser fan not spinning.
• Overcharge or non-condensables after a repair.
• Recirculating discharge air (unit boxed in by shrubs or a fence).

Voltage problems

The ACHR News says voltage at the motor terminals, with the compressor running, should be within 10% of the motor rating. Low voltage raises motor current and heat; high voltage raises inrush at startup. Measure at the compressor terminals under load, not just at the disconnect, and look for loose or burned connections in between.

Capacitor and starting

A weak or failed run capacitor raises running current and heat, and can keep a compressor from starting at all, so it draws locked-rotor amps until the overload opens. The ACHR News lists a bad capacitor among the causes of excessive current. Test it against its rating; see how to test a run capacitor. A compressor that's mechanically tight may need a hard start kit, but treat that as a check, not a cure.

Short cycling

Each start heats the windings. A unit cycling on and off every few minutes, from a thermostat problem, a pressure switch or a missing time delay, can push the motor over the edge. See AC short cycling.

What to measure once it's running again

1. Running amps vs. the nameplate RLA. The ACHR News notes running current shouldn't exceed the manufacturer's rated full-load amps at heavy load. See AC nameplate RLA, LRA, MCA and MOCP.
2. Voltage at the compressor terminals while running.
3. Suction and liquid pressures, superheat and subcooling. See superheat vs. subcooling.
4. Condenser fan amps and rotation, and the condenser coil condition.
5. Run capacitor µF under the same visit.

If everything reads normal and it still trips, check the winding resistances and look for a shorted turn (resistance well below the manufacturer's figure) or a failing overload.

Common mistakes

• Condemning a hot compressor. Open C–R and C–S with a good R–S reading is an overload, not an open winding.
• Not waiting long enough. An hour or more isn't unusual.
• Hosing the terminals. Cool the top of the shell, not the electrical connections.
• Resetting it and leaving. Find the low charge, restriction, high head, voltage or capacitor problem first.
• Adding refrigerant to a restricted system. Low suction with a restriction looks like low charge. Check subcooling.

FAQ

Sources

• Bryant 569J service manual, compressor overload and ASTP
• Friedrich service manual, internal overload
• ACHR News, "Troubleshooting Single-Phase Residential Compressors"
• ACHR News, "How to Determine the Cause of Electrical Compressor Failure" (Ted Craig, 2021)
• ACHR News, systematic troubleshooting case (starved TXV, compressor cycling on overload)
• HVAC School, "Compressor Won't Run Diagnosis"