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Compressor-condenser unit diagnostic procedures:
How to diagnose an outdoor air conditioning or heat pump compressor/condenser unit that is not working, won't start, noisy, or humming. This article summarizes common HVAC compressor condenser unit problems and their diagnosis. We link to more detailed HVAC compressor repair procedures.
This article forms part of our series on how to diagnose an air conditioner or heat pump that is not cooling: this article explains how to diagnose and correct air conditioning problems like lost or reduced air conditioner cooling capacity, reduced or no cool air flow, reduced or no actual lowering of the air temperaturer, or an air conditioner that won't start.
If your air conditioner or heat pump won't start, we suggest beginning diagnostics indoors at the room thermostat. If you haven't done that you might want to go back to
see AIR CONDITIONER WON'T START before continuing here. Below we give some summary diagnostics for non-starting compressor motors at the outdoor condenser unit.
Compressor problems - Air Conditioner Compressor: problems such as an aging air conditioner compressor motor that is at or near the end of its life may be unable to properly compress the returning refrigerant gas to a sufficiently high pressure. A service technician will need to evaluate and test the system and if needed, replace the compressor.
Since this is a costly repair, be sure to ask why the compressor failed and to correct any underlying cause (such as low voltage).
Variations in line voltage can lead to improper compressor operation and loss of cooling output.
Silent compressor/condenser motors: relays on the control board for the compressor/condenser unit start the compressor motor and the condenser unit's cooling fan individually, but they may be related as we'll explain. Check the outdoor components in this order:
1. Is there power to the compressor/condenser unit? Check that the main circuit breaker or fuse is on, that the outdoor service switch is on. If the compressor/condenser circuit's main breaker is tripped or fuse blown the system may have a seized compressor motor or a shorted electrical wire.
Back at AIR CONDITIONER WON'T START we reminded you to check that the thermostat was calling for cooling. If you forgot to do that, go back and check that article.
2. Is the refrigerant pressure in the system at the pressure level equivalent for the ambient temperature at the condenser unit? Modern compressor units include both high and low refrigerant pressure safety cut-out switches that can shut down the equipment in abnormal conditions.
3. Is the line voltage delivered to the compressor/condenser unit normal? Occasionally low voltage will prevent equipment from operating or will shut it down. Low voltage can also mean motors have trouble starting or run at abnormal power levels.
4. Is electrical power delivered to the compressor & to the condenser fan? Look for burned, shorted, disconnected wires supplying these two motors.
5. Is voltage being delivered to the start relay - the compressor contactor? If you see control voltage (typically 24VAC) at the contactor and you have line voltage to the compressor/condenser unit but the relay does not pull in then probably the contactor is bad (or there is a blown fuse on the control circuit board). Next we give diagnostic steps for the following situations
If there is no control voltage at the compressor's start relay and there is electrical power to the unit and the thermostat is callling for cooling, then one of the safety cut-out switches we mentioned above may have tripped.
Check each of those safety relays for voltage on the relay output side. If there is no voltage then that relay has tripped (or failed).
Find and check the following:
Low pressure safety switch: if abnormally high pressures (typically over 55 psi) are found on the low-side of the compressor/condenser unit this switch will open to stop the system.
The low pressure switch normally will reset automatically when the low side pressure drops. If it does not re-set this switch has probably failed.
Condenser oil level safety switch: some compressor/condenser units (particularly larger and commercial systems) include a safety switch that monitors the oil level in the compressor motor.
Low compressor motor oil levels can shut down the system.
Watch out: most residential cooling systems use a hermetically sealed compressor motor; oil is not normally added tothese systems.
If you add oil to a commercial compressor/condenser system you need to check oil levels again after the system has returned to operation to be sure that oil levels are not too high - a condition caused when oil previously left in the piping system has returned to the compressor.
High pressure safety switch: The compressor/condenser over-pressure or "high pressure" relay safety switch.
A failed condenser unit fan motor or a damaged fan itself (motor spins but fan does not) can lead to compressor/condenser unit high temperatures and pressures.
If refrigerant pressrues in the system reach unsafe or abonormally high levels this relay will shut down the compressor motor. A clogged condenser or a failed thermostatic expansion valve can also cause abnormally high pressures in the system.
We first mentioned
at NOISES, COMPRESSOR CONDENSER that we had a field report from a reader who explained that a noisy outdoor compressor unit was, according to his HVAC service technician, traced to a blocked, clogged outdoor condensing coil. We speculate that perhaps the compressor was running hot and that correcting air flow across the condensing coil corrected that condition.
Compressor motor protection switch: some compressor systems include a protection module at the main power terminals for the compressor motor. This switch may have opened.
Voltage safety switch: The voltage monitoring or phase protector relay (not present on all equipment): transient low voltage can trip this relay. It may require manual reset.
Watch out: some electrical motor safety switches require a manual reset. For examples see this separate aritcle on
Check for voltage at the compressor motor itself. If there is no line voltage check the main breaker or fuse or the local service switch again.
Is the compressor motor humming?: If you have a hard-starting air conditioner compressor that "hums" but doesn't start, it may be possible to get more life out of the compressor motor with a simple starting capacitor (rather than a costly whole new compressor motor.)
Check that the fan blades will spin - with power off. If the fan motor won 't turn it's seized and needs to be replaced.
Check the fan relay switch: if the fan relay does not pull in and there is control voltage to the relay then the relay is probably bad. If there is no control voltage check the low voltage transformer and circuit wiring. Note: some systems incude a low ambient temperature control relay that will keep the system from running at low temperatures.
If the fan relay does pull in but the fan does not run, check the line voltage and wiring to be sure power is being delivered to the fan.
Check for voltage at the fan motor and go back to check circuit breaker or fuse if there is none
If spinning the fan will start the motor and it keeps running then the motor's start capacitor is probably bad.
If the fan motor has voltage but will not run at all then the motor may have failed or the motor's run capacitor may have failed.
(Aug 2, 2011) Raten said:
Outside fan doesnt turn on. What's wrong?
for an outside fan that does not turn on we give some diagnostic suggestions above in this
article COMPRESSOR / CONDENSER DIAGNOSTICS
Start by assuring that the unit has power and that your thermostat is calling for cooling.
Your service tech may then check the fan relay, wiring, and fan motor, or the motor start/run capacitor.
See CAPACITORS for HARD STARTING MOTORS
Carla said: My outside air unit will not turn off on its own. I have to manually turn the breaker off to turn it off. Does anyone know why or how I can fix it?
Carla if your A/C won't turn off it could be that the thermostat is set to a temperature that the system cannot reach - due to lost cooling or due to a setting below the capability of the system.
If your A/C won't turn off even if you set the thermostat to a temperature that is above the current room temp, then the thermostat or an A/C control board or switch is bad and needs replacement - in that case you need a service call from a trained HVAC Technician.
First check that the thermostat is not calling for cooling or turn it off completely. Also check for thermostat wires that are shorted togewther.
If the HVAC compressor motor will stop but the compressor / condenser fan is not working properly,
A compressor which appears to have lost cooling capacity can be diagnosed by a service technician who can connect the appropriate test gauges to the system. Lower than normal discharge pressure and higher than normal suction vacuum will indicate this problem.
But before assuming that something is wrong with the air conditioning compressor, some basic investigation is in order.
Unless there is an obvious indication of a compressor problem (noise, hard starting, compressor won't start), the service technician, to be thorough and economical, will inspect the system in an order, checking the easy and least-costly problems first, such as presence of electrical power, proper setting and operation of system controls, condition of filters, condition of duct work, operation of blower fans, before moving on to check the compressor itself by looking at the air conditioner operating temperatures, pressures, and current draw in Amps.
Is the outdoor compressor-fan unit (COMPRESSOR & CONDENSING COIL) running?
If not, be sure that the outdoor compressor/condenser unit service switch [photo] and the air conditioner indoor power switch, fuse, or circuit breaker are all in the "on" position.
Check the outside compressor condensing coil for damage or blockage.
A blocked condensing coil can cause the compressor to overheat, stop running, or become noisy and work poorly. A/C Compressor Problems
If you cannot find your air conditioner controls and switches see A/C - HEAT PUMP CONTROLS & SWITCHES - list of controls and switches found on air conditioners and heat pumps
Condensing unit fan diagnostics:
at FAN, COMPRESSOR / CONDENSER UNIT we explain the diagnosis of problems with the compressor/condenser fan and fan motor.
Also see ELECTRIC MOTOR DIAGNOSTIC GUIDE.
at BURNED-OUT COMPRESSOR we explain that if the condensing unit fan is not working the compressor itself may shut down or even be ruined by overpressure or over temperature.
Also see NOISES, COMPRESSOR CONDENSER where some noise problems are traced to the cooling fan .
The cooling capacity of an air conditioning equipment refers to the ability of the compressor/condenser (usually outside) and the air handler/evaporator (usually inside) to deliver cool air to the occupied space.
Briefly, the compressor/condenser draws refrigerant gas from the building air handler, compresses it and cools it back to a liquid refrigerant, and the air handler/evaporator coil permits liquid refrigerant to evaporate inside a cooling coil, across which the fan blows building air to cool and dehumidify it.
combination of this equipment has a cooling capacity, usually rated in BTUh or thousands of BTU's of cooling capacity per hour,
documented on equipment data tags discussed
at RATED COOLING CAPACITY.
Also see COOLING RULES OF THUMB to guesstimate how many tons or BTUs of cooling a building needs
A longer than normal on cycle combined with little or no cool air conditioner output could be due to
What about the opposite problem: too-short compressor on-cycles?
Here is a brief summary of some diagnostic tips for short cycling HVAC compressors:
(June 8, 2012) Renee said:
My AC unit during the hot and humid months here in FL, will not run without some help from me. Lets say it is 80 degrees and you set the thermostat at 75, it will run until temp drops about 3 degrees, then shuts off.
You then need to shut the unit off via the thermostat, for approximately 20-30 mins turn it back on in order for the air to start blowing again. I tried changing out the thermostat but this has not helped. What could be the problem?
Renee, often when shutting off the A/C returns the delivery of cool air we trace that problem to frost formation on the cooling coil.
Click on FROST BUILD-UP on AIR CONDITIONER COILS check for, diagnose, & repair that problem. But first, make sure that your air filter is not debris clogged.
Short cycling air conditioner compressor diagnosis: I have the same problem as Mathew: my A/C compressor cycles on and of every ~10 seconds. I is about 5yr old. What can we do to fix this system. Please help. Thanks!
Short cycling air conditioner problems: Our air conditioning compressor cycles on and of every few seconds or minutes. What can we do to fix this systems not even a year old
(mod) said to Joe and Matthew (re questions just above):
But if someone is turning the system on and off quickly, the compressor may have a hard time re-starting against the pressure on its outlet side. A starter capacitor addition or replacement might fix the problem. If your A/C compressor is showing this symptom but works OK if you leave it shut off for 30 minutes or longer, that may be the trouble.
We have also see or a damaged compressor internal refrigerant valve causing high head pressures;
In sum, you need a service call from a professional to correctly diagnose and repair the problem. Ask the service tech what she/he found and let us know - what you find will help other readers.
If your air conditioner or heat pump has the opposite problem, staying on too long,
see LONG-ON CYCLING AC COMPRESSOR.
Low amperage draw: unlike a high-amp current draw which indicates that the compressor is danged internally in a way that its piston(s) is(are) tight in the cylinder, a low-amp current draw, if below normal, may confirm internal wear on the compressor parts, and would support the diagnosis that the compressor is worn and inefficient.
Where there are no gauge ports to actually measure compressor low side and high side vacuum and pressure, this simple electrical test is a useful first step.
See ELECTRIC MOTOR DIAGNOSTIC GUIDE for how-to details.
If you have the opposite concern, that is the A/C compressor is turning on and off too frequently,
see SHORT CYCLING AC COMPRESSOR.
Refrigerant line pressure readings which are abnormal (probably too low) on the high pressure side (compressor output) or on the low pressure side (compressor input or suction line) can indicate a problem with the compressor's ability to develop normal operating pressure ranges and thus will affect the cooling capacity of the air conditioning system.
For more details on air conditioner refrigerant line pressures and how they are examined
Details about air conditioning compressor functions, how air conditioner compressors work, what goes wrong with air conditioner compressors, and how to measure and diagnose air conditioner compressors are provided
Several readers have described a step of cooling down an overheated air conditioning or heat pump compressor motor to get it running again producing cool air indoors. One reader used a garden hose to spray the outdoor unit (after all it's intended to tolerate being rained-on). Another used a watering can. A third, buckets of water.
All three reported that this step got their cooling system cooling again when before either the compressor had stopped running entirely or it was running but the system was not producing cool air. But for several reasons this is not an effective durable repair:
While we wait for more comments from air conditioning service technicians and experts (CONTACT us), here are some interpretations of what may be going on:
Contactor relay T1
Red wire to dual start/run capacitor C or COM terminal
Red wire to Compressor’s R run terminal
Red wire to Fan motor’s R run terminal
Contactor Relay T2
Black wire to Compressor’s C or COM terminal
Black wire to Fan motor’s C or COM terminal
Contactor Relay L1 - black line voltage in
Contactor Relay L2 - red line voltage in
Contactor Green wire connects grounds contactor & other components to system ground
Contactor Relay low voltage wires (24VAC) to the magnet that operates the relay
Start-Run Capacitor FAN terminal yellow or brown wire to Fan motor’s S terminal
Start-Run Capacitor C or COM terminal red wire to Compressor/Condenser Contactor Relay T1
Start-Run Capacitor H or HERM terminal purple wire to Compressor’s S or Start Terminal
Continue reading at COMPRESSOR / CONDENSER REPAIR - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see COMPRESSOR / CONDENSER DIAGNOSTIC FAQs questions and answers posted originally at this page
Or see BURNED-OUT COMPRESSOR
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