A hum followed by a click, then silence
The internal overload is opening because the motor cannot start and is heating. That is either a start component fault or a genuine mechanical or winding problem, and the tests distinguish them quickly.
The most expensive misdiagnosis in the trade
AC Compressor Repair in Alpha, NJ
- Winding and ground tests before any verdict
- Start components ruled out first
- EPA Section 608 recovery on every open system
- Written workmanship warranty
A compressor that will not start looks the same from the thermostat whether the compressor has genuinely failed or whether a twenty dollar capacitor has. The difference between those two findings can be several thousand dollars, and it takes about five minutes with a multimeter to establish which one you have.
That five minutes is skipped more often than most homeowners would believe. A humming outdoor unit and a hot house is enough for some companies to quote a replacement system on the spot, and the customer has no way to challenge it.
How a compressor is actually tested
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A hermetic compressor is a motor sealed inside a welded steel shell with a pump attached. It has three electrical terminals, common, start and run, and everything that determines whether it is alive can be measured at those terminals plus the components feeding them.
The tests are cheap, quick and conclusive. Skipping them is not a technical limitation, it is a commercial choice, and it is the single most common way homeowners are pushed into system replacements that were never necessary.
The sequence that separates the cheap fault from the expensive one
Incoming voltage is confirmed at the disconnect first, then across the load side of the contactor while the system is calling. A contactor with pitted or welded contacts will drop significant voltage across itself, starving the compressor of what it needs to start. The symptom is a hum and a failure to spin. The part is inexpensive and the compressor is entirely healthy.
The run capacitor is measured against its rated microfarad value with a meter, not assessed by whether the case has bulged. A capacitor can read well below tolerance while looking perfectly normal, and a compressor fed by a weak capacitor will draw locked rotor current, heat up and eventually fail for real.
Then the windings themselves. Resistance is measured between common and start, common and run, and start and run, and the readings must be consistent with each other and with the manufacturer values. An open winding shows as infinite resistance. A shorted turn shows as a reading well below what it should be.
Finally the insulation test: resistance from each terminal to the shell. A healthy compressor reads effectively open to ground. A compressor that reads continuity to ground has failed internally, and that is the one finding that genuinely condemns it. At that point the verdict is evidence based and you can see the reading yourself.
What happens when the compressor really has failed
A failed compressor rarely fails alone and it almost never fails without a reason. A burnout contaminates the whole refrigerant circuit with acid and debris, and fitting a new compressor into that system without addressing it guarantees a second failure, usually inside two seasons.
So a genuine compressor replacement means recovering the charge lawfully, flushing or replacing the affected components, fitting a suction line filter drier, correcting whatever caused the original failure, then evacuating to a documented vacuum and charging by weight. It also means an honest conversation about whether that work is worth doing at all on the system in question.
Age, refrigerant type, coil condition and whether the equipment was correctly sized in the first place all bear on that answer. On a system past its middle life with an obsolete refrigerant, the repair frequently costs more than a meaningful fraction of a replacement that would also fix the sizing error.
What kills compressors, and why it is almost never the compressor's fault
Compressors are built to run for well over a decade. When one fails early, something else in the system was doing the damage, and replacing the compressor without identifying that guarantees a repeat performance.
Low refrigerant charge is the most common killer. Refrigerant carries oil back to the compressor and cools the motor windings on its way through. A system running short does both jobs badly, so the motor runs hotter than designed and the moving parts run drier. A homeowner who has had refrigerant added twice has a leak that has been quietly shortening the compressor's life the entire time.
Restricted airflow across the indoor coil is the second. A loaded filter, a fouled evaporator or a duct system that was never able to carry the design airflow all lower the suction pressure and can allow liquid refrigerant to return to the compressor instead of vapor. Liquid does not compress, and a compressor repeatedly asked to try will eventually suffer mechanical damage.
A dirty condenser coil produces the opposite problem. The system cannot reject heat, head pressure climbs, discharge temperature rises and the oil begins to break down. High head pressure also raises current draw, which heats the windings further. A condenser buried in cottonwood seed or hemmed in by fencing and shrubbery is doing measurable damage every hour the system runs.
Short cycling from oversized equipment adds starts, and starts are where the wear happens. Locked rotor current at each start is several times running current, and a system that satisfies the thermostat in eight minutes accumulates those starts remarkably quickly across a season.
Finally, the electrical supply. Low or unbalanced voltage, a loose connection heating at a terminal, or a contactor that has been arcing for a season all stress the motor. That is why the diagnosis does not stop once a failed compressor is confirmed. Fitting new equipment into the conditions that destroyed the old one is not a repair, it is a deferral.
Test results and what they mean
| Measurement | Healthy result | What a failed result indicates |
|---|---|---|
| Voltage across contactor load side while calling | Full line voltage present | Pitted or welded contacts starving the compressor |
| Run capacitor microfarad value | Within the tolerance printed on the case | A weak capacitor forcing locked rotor current |
| Winding resistance, common to start and common to run | Consistent readings matching manufacturer data | Open winding or a shorted turn inside the motor |
| Insulation resistance, terminal to shell | Effectively open circuit | A grounded winding. The compressor is finished |
| Amp draw under load | At or below rated load amps | Mechanical binding, high head pressure or a failing winding |
| Suction and liquid pressures | Consistent with ambient and design | Valve failure inside the compressor, or a charge or metering fault |
How the work proceeds
01
Isolate and make safe
Power is removed at the disconnect and the capacitor is discharged before any terminal cover comes off.
02
Rule out the external electricals
Contactor, capacitor, control voltage and incoming supply are each measured, because any of them can mimic a dead compressor.
03
Test the motor itself
Winding resistances and an insulation test to the shell produce a verdict you can see rather than a claim you have to accept.
04
Establish why it failed
A compressor that has genuinely failed did so for a reason: overheating, liquid return, contamination or chronic high head pressure.
05
Quote both paths honestly
Where replacement is viable you get the repair figure and the equipment figure side by side, with the reasoning for each.
Symptoms that point toward the compressor
The breaker trips as soon as cooling is called
A hard short, frequently a winding grounded to the shell. Repeated resetting after this symptom risks the supply wiring as much as the equipment.
Cooling gradually weakened over a season
Internal valve wear reduces the compressor's ability to pump, so pressures look wrong in a characteristic way. This is a slow failure rather than a sudden one and it is often misread as low charge.
A loud metallic rattle from the outdoor unit
Mechanical damage inside the shell, or loose mounting hardware. The distinction matters, because one is a cheap fix and the other is terminal.
It failed once before and was recharged
A system that has needed refrigerant added more than once has a leak, and running a compressor short of charge starves it of the cooling and lubrication it depends on. The recharge shortened the life of the compressor that later failed.
The outdoor fan runs but the compressor never does
That separates the two motors immediately. The fan proving it has power means the supply and the contactor are at least partly healthy, which narrows the fault to the compressor circuit.
Cooling works for a while, then stops until it cools down
The internal overload is opening on temperature and resetting once the shell cools. That is a compressor running too hot, and the cause is usually outside the compressor itself.
Compressor questions
What Homeowners Ask Before Authorizing This Repair
This is the largest single component in a cooling system, so the questions below tend to arrive in a hurry and under pressure.
Ask for the readings. A grounded winding shows as continuity between a terminal and the compressor shell, and an open winding shows as infinite resistance between two terminals that should read a specific value. Both are numbers a technician can show you on a meter in front of you. A verdict delivered without any measurement, on the strength of a hum and a hot house, is not a diagnosis. Any competent technician will be entirely comfortable demonstrating it.
It turns on four things. The age of the equipment, because a compressor outlasting the coil and the cabinet is a poor investment. The refrigerant, because obsolete types are expensive and getting more so. The cause of failure, because contamination raises the cost and the risk. And whether the system was correctly sized, because a replacement is the only chance to fix that. As a rough guide, once the repair passes a meaningful fraction of a replacement on equipment past its middle life, replacement usually wins.
Only if the circuit is cleaned properly and the cause is corrected. A burnout leaves acid and debris throughout the system. Correct practice is recovery, flushing or component replacement, a suction line filter drier, a documented evacuation and a charge by weight. Skipping that is why replacement compressors sometimes fail within a season, and why the cheapest quote is frequently the most expensive outcome.
Frequently yes, and it is worth checking before anything is authorized. Compressors commonly carry a longer manufacturer term than the rest of the system, often conditional on registration at installation. The part may be covered while the labor, refrigerant and ancillary components are not. We check the serial number and tell you what the warranty position actually is rather than quoting as though there were none.
Yes, and it is the single most common cause of a compressor that hums and will not start. The capacitor provides the phase shift the motor needs to begin turning. When it weakens, the motor draws locked rotor current, heats, and the internal overload opens. Left in that state for long enough, the repeated thermal stress will eventually kill a compressor that was perfectly serviceable. That is why the cheap part is tested first, every time.
Fifteen years or more is entirely normal for one that has been correctly charged, kept clean and not asked to short cycle. Failures at eight or ten years are usually traceable to something specific rather than to bad luck: a leak that was topped up rather than repaired, a condenser coil nobody ever cleaned, an airflow restriction, or equipment oversized from the day it was installed. That is why the diagnosis looks past the compressor to the conditions around it. Knowing why the first one died is the only way to predict how long a replacement will last.
Also in the catalog
Work Commonly Linked to a Compressor Diagnosis
Compressor findings usually arrive alongside questions about the age and sizing of the wider system.
Before you authorize a replacement
Get the Compressor Tested Rather Than Assumed
The electrical tests that confirm or overturn a compressor verdict take minutes and cost a fraction of what the wrong decision costs.
You get the readings, the reasoning and both figures, then you decide.
- Contactor, capacitor and control voltage ruled out first
- Winding and insulation readings shown to you on the meter
- Warranty position checked against the serial number
- Recovery equipment on every sealed system job
- Repair and replacement figures presented side by side
- Flat rate quoted and approved before work begins