Q&A on hard-starting HVAC compressor motors:

These questions & answers about hard-starting air conditioner or heat pump compressor motors help diagnose why the motor may hesitage, be slow starting, or may continue to burn out the starting capacitor.

This air conditioning repair article series explains procedures for the diagnosis and repair of hard-starting air conditioning compressors.

FAQs on A/C Compressor Hard Start or Intermittent Run or Stutter

These questions and answers about HVAC compressor motors that have trouble starting were posted originally

at HARD STARTING COMPRESSOR MOTORS

Photo: a motor starting capacitor, a common repair effort for hard starting AC compressor motors. [Click to enlarge any image]

On 2018-10-14 by (mod) -

Often a freezing oil is due to low refrigerant, or to something simpler like reduced or no air flow due to a dirty filter or blower not running.

Search this site for or go to FROST BUILD-UP on AIR CONDITIONER COILS to see details

On 2018-10-14 by Shannon

My AC outside unit comes on but the inside freezes up almost immediately..

I can feel cool air coming through the vents but the blower fan isn’t turning.. I can hear the motor humming, I believe..

I’ve checked the 3 amp fuse in the connection and it is still good.. the squirrel cage fan turns freely because I can spin it with no effort with the power shut off at the main breaker panel.. the fan will not come on in either cooling or heating mode even when the thermostat is switched into the ON position

On 2018-06-10 1 by (mod) -

Sounds like a shorted wire or control, not a capacitor problem.

On 2018-06-09 by Earl

Purchased an OLD Freidrich 36,000 BTU window air conditioner. Seller fired it up and ran perfect (for 15 minutes before buying it), very cold air flow and relatively quiet for it's age.

Transported it right side up (not to upset fluids inside), got it home and placed it in the window, fired it up, ran for 5 minutes or so, then something growled and compressor kicked off sending out a smell like something electrical was burning. I shut the unit down and unplugged it immediately. Pulled unit out of window enough to open and expose electrical components and found nothing visually.

Leaving all still exposed, I plugged it back in, fired it back up. Fan/motor starts and runs good, but the compressor (after calling for it to start) does absolutely nothing. No click.., no hum..,

nothing.., as if the wires were not even hooked up to it. I tested all wires and have proper voltage to the compressor. Does the start/run capacitor have anything to do with the compressor? Why would a smooth starting and running compressor just stop like that?

On 2017-08-15 by Jose

@Heather,

What was the resolution ?

On 2017-07-30 by Scott

I am the one who has the 220 Frigidaire unit is the capacitor inside the motor or the big one that I see outside the control box

On 2017-07-30 by Scott

I have a 220 window unit Frigidaire 18000 18500 the fan motor will not start the fan up if I take my finger and start it it takes off do I need to change the run capacitor and where is it

On 2017-07-20 by (mod) -

Glad to help, let us know what repair is required.

On 2017-07-20 by Heather

, thank you so much for your response I have yet another tech Coming hopefully today! It is miserable!

On 2017-07-20 by (mod) -

Heather,

A service tech changes of the capacitor as an inexpensive effort to get a compressor unit running.

However if the motor has essentially failed it will simply blow out replacement capacitor.

So I suspect that the compressor motor in yr unit is overheating or seizing or otherwise failing.

Your service tech can testing confirm this for example by measuring the current draw in amps when the compressor tries to start.

On 2017-07-20 by Heather

We have had two repair men out from different companies in less then a month and both changed out the capacitor and then two days ago the unit blows cold air for maybe 2 mins and then stops completely then does it again 15 mins or so later. Any ideas?

On 2017-07-20 by (mod) repairmen changed out the capacitor but two days later the AC stops working

Heather,

A service tech changes of the capacitor as an inexpensive effort to get a compressor unit running.

However if the motor has essentially failed it will simply blow out replacement capacitor.

So I suspect that the compressor motor in yr unit is overheating or seizing or otherwise failing.

Your service tech can testing confirm this for example by measuring the current draw in amps when the compressor tries to start.

On 2017-07-20 by Heather

We have had two repair men out from different companies in less then a month and both changed out the capacitor and then two days ago the unit blows cold air for maybe 2 mins and then stops completely then does it again 15 mins or so later. Any ideas?

On 2017-07-19 by Doc

THANK YOU

On 2017-07-19 by (mod) Roof Top RV AC DUO THERM used to run but now won't

Look first for a bad capillary tube or thermostatic expansion valve.

On 2017-07-19 by Doc

A Roof Top RV AC DUO THERM unit was given to me.Guess unit is about 25 years old.Used to work when removed from last RV. There are 3 speeds for fan that work. 3 speeds for AC.If switch is put on AC the compressor kicks on runs for about 5 minutes,

Pipes are getting hot and cold then you hear presser building up compressor slow down and stop trying to start fan compressor thumps and jumps around before thermal shut down.

30 seconds go by and fan will run air is cool, but not cold. then it will run for 15 min. before starting agian. I have put a new hard start compasitour on unit.

On 2017-06-06 10:55:00.318961 by (mod) repaired fan but now compressor and fan won't start

Justin

I can't say, but I'd start by tracing power to the fan and to the compressor.
All switches are on, right?

It's not an FPE Stab-Lok or Bulldog PUshmatic electrical panel powering the equipment, right?
You didn't short a control board, right?

On 2017-06-06 by Justin

Inside fan on my240volt air-conditioning went bad. I changed it,n put new plug. now the compressor n fan won't start. What did I do wrong

On 2017-05-29 by (mod) AC compressor takes 10-15 minutes to start

Ankit,

What are the symptoms? If they are the symptoms described in the article above then that's the best to diagnosis of your compressor problem.

On 2017-05-29 by Ankit Patel

Why My split ac is taking 10 to 15 min to start Plz help

On 2017-05-11 by (mod) re: compressor has blown 4 capacitors in a year

I would try looking for an excessive current draw or an intermittent short such as a winding short when a motor is spinning.

On 2017-05-10 by Scott

3 ton straight ac,fixed orifice. The unit has blown 4 capacitors in 1 year. Both sides of the dual run capacitor read zero with no leaks or puffing out. I cant see a strart relay and none is indicated by the schematic. The unit runs strong untill it repeatedly blows the capacitor.

On 2017-04-09 by (mod)

James:

Several problems can cause the quick-off A/C compressor problem you describe, such as
- a failing control relay or circuit board
- a seized compressor motor

- a compressor trying to start against high head pressure due to a stuck or failed thermostatic expansion valve TEV or cap tube

On 2017-04-09 by (mod) ac compressor comes on for like 3 seconds then shuts off

Rahul

If this is a new problem that is occurring on a window AC system that's been in service for some time then I suspect the compressor motor is having trouble starting - it may be failing; the unit could be repairable if the problem is only the need for a new start/run capacitor but I'm not optimistic

On 2017-04-09 by James

My ac compressor comes on for like 3 seconds then shuts off ? Any advise will be thankfull

On 2017-04-09 by Rahul

When I start my window AC the voltage of my house gets low

On 2016-07-21 by (mod) Trane BTB 718A 1984 compressor diagnosis

Ted

The unit may have other internal safety controls that shut it down due to overheating, abnormal gas pressures, or another malfunction; your service tech will check those controls.

Try cooling down the system with a garden hose to see if that makes a difference.

On 2016-07-20 by Ted ABQ

A/C Unit: TRANE
MODEL#: BTB 718A 100A1
Manufacture Date: 7/84

Hello:
This hard-working unit is located at our apartment in Albuquerque, NM.

For the second consecutive day, the compressor and fan for this unit simply shut off between 4-5P.

The 2 30-AMP circuit breakers for this unit were NOT tripped. I am able to restart the A/C by resetting the two circuit breakers, which starts up both fan and compressor, which then run normally in accordance to thermostat settings.

These are the first occurrences of this condition since we started renting this unit on November, 2014. In your opinion, what is the probable cause for this unit shutting off in the middle of the afternoon?

On 2016-07-02 23:48:05.357085 by (mod) still having compressor problems after added a capacitor

sounds like a failing compressor.

On 2016-07-01 by Stephanie Evans

Fan outside and I side is blowing put start up capita tor on and still having problems with the compressor kicking on.

When it finally kicks on with the help of the garden hose it will run for hours and then kick off and about 20 minutes later it start the process all 9ver again. By the way it's Lennox. I do here the compressor string to start.

On 2016-06-18 by 1stgma

My compressor sounds like it's trying to start but then stops and never comes on fulli?

On 2016-06-17 by (mod) over-sized compressor not only wastes energy, but it won't dehumidify the home

An over-sized compressor not only wastes energy, but it won't dehumidify the home adequately as it'll cool down too fast.

On 2016-06-17 by Dd

You bring up the main issue really! Our unit is indeed to big for our home and it has caused us $$$$ over the past 6 years.

When sitting at closing our builder off handedly said, " they delivered the wrong Ac unit to your house...it's bigger and better for the same amount" something to that affect.
How were we to know.

A year later he up sized all the duc work in the house because of issues with the house not cooling...thought all was well but at that point we were on our own.
Many hvac techs later and they all suggest we buy a brand new smaller unit at around 5k....

On 2016-06-17 by (mod) As soon as the house reaches just 1 deg below temp setting the Ac will kick back on and flip the breaker

If the compressor itself is working normally and is the properly-sized unit for the home then indeed what you describe suggests a problem elsewhere. I'd give your service manager a call and ask for diagnostic help from their senior, very experienced technician.

Wacth out: while one-step over-fusing on AC compressor circuits is usually permitted (check the data tag on your unit), upping from 15 to 40A is improper, unsafe, indicates lack of expertise and worst, is just asking for a house fire.

Your tech will probably check the thermostat settings, wiring, controls, and relays in the system. Indeed the suggestion of some of your comments is that the compressor is trying to re-start too soon and is trying to start against a high head pressure: a condition thats a problem for many compressor units.

Something as simple as a sticking TEV (themostatic expansion valve) can cause failure of the head pressure to drop after the compressor shuts off.

On 2016-06-17 by DD

Hi,

Long issue time issue with our hvac! Doing the same old thing for 6 years....no solution but told to just buy a new unit$

Had the compressor replaced last year$$$$ (unit is only 6 years old...new house). Same issue that caused the first compressor to fail is still present.

Unit will kick on and cool home just fine. It will reach the desired temp then cycle off. As soon as the house reaches just 1* below temp setting the Ac will kick back on and flip the breaker:/ it does have a 5 min start delay but this doesn't seem to help:/ form the breaker flipping

We had the breaker fused uped to a 40amp from a 25amp last year...but it didn't help.

I have figured out that if I run and set the thermostats up 10* after the Ac kicks off from cooling the house...the Ac doesn't kick on after the 1* and give my Ac time to rest....a few hours it will not trip the breaker.

This method works every time.
This is my life everyday...babysitting the Ac :/
Help

On 2016-06-16 by (mod) our compressor will cut off and start right up and then cut off and start right up

This sounds like a control or wiring problem, possibly a bad sensor or relay. I'd get it fixed lest you damage the compressor.

On 2016-06-16 by Donna

Hi
we have notice that our air compressor will cut off and start right up and then cut off and start right up. It seems like it does it the longer it runs. We have had coolant added and we have replaced our inside thermostat. However we are still having the same problem . What should we try next?

On 2016-04-02 by Anonymous

noisy start up

On 2016-03-28 by (mod)

OM

Click to enlarge any image to see a sharper, bigger version. We use small image files to speed page loads for mobile readers

On 2016-03-28 by om ji

Bad photo

On 2016-03-10 by krkotrdwararao

Good

On 2015-12-25 by (mod) Sunbeam portable AC wiring diagram

Nicki,

As I don't have that specific diagram try going right to b the Sunbeam service center at Sunbeam .com as they offer manuals for their products. Let me know how that works for you.

On 2015-12-24 by Nicki

Can i please have a picture of the wire connections to the compressor of a portable aircon. Make is sunbeam. Model nr spa1000. Thanx

On 2015-09-02 by (mod) if the compressor does not run the system won't produce cool air

Ingrid

Indeed if the compressor does not run the system won't produce cool air. The problem could be a failed compressor motor (expensive) or perhaps a bad control or relay. You'll need to call your A/C service company for diagnosis and repair.

Some diagnostics for an A/.C comporessor that won't start at all are found at

https://inspectapedia.com/aircond/Air_Conditioner_Wont_Start.php AIR CONDITIONER WON'T START

On 2015-09-02 by Ingrid

Yes hi, I have a carrier heat pump. When turning on the AC the out side unit fan comes on but the compressor does not. the compressor tries to kick on but then quits. The air blowing out of the vents isnt cold

On 2015-07-29 by (mod) water cooled compressor to start and then shut off on high head pressure immediately

Perhaps a stuck or failed reed valve in the compressor.

On 2015-07-28 16:18:23.984145 by jim

what would cause a water cooled compressor to start and than shut off on high head pressure immediately with no type off water control valve

On 2015-07-17 by (mod) the condensing coil did have a leak, the system had zero coolant

Sounds like multiple problems:

1. a refrigerant leak that needs to be found and fixed
2. needing a proper refrigerant charge
3. need to repair or replace burned, damaged electrical components: not just wires but possibly relays, control boards, whatever is damaged and leading to overcurrent that burns up the wire.

On 2015-07-14 by Chris N

Just to clarify, the condensing coil did have a leak, the system had zero coolant, and the yellow wire between the relay switch and going into the unit bundle (not yellow between the capacitor and relay) was fried and appeared to have melted the brown wire between the capacitor and unit bundle.

Does this mean there is problem with the capacitor or since its the yellow wire is there possibly an issue with the condenser itself, or even would this have been a result of some other malfunctioning issue with the unit??? Your help is greatly appreciated!

I have an external a/c unit that had lost total cooling function (presumably from a leak in the condensing coil), but on inspection of the outside unit we found a yellow wire that was charred and sparking. It was not directly connected to the capacitor but we are trying to consider what could be the culprit. Any thoughts?

Question: Could Loose Electrical Wiring (or a bad A/C start relay) Explain a Hard-Starting Air Conditioner or Heat Pump Compressor Motor?

I had a tech come out and install a hard-start kit to keep a 4-year-old 3-1/2 ton Carrier compressor from tripping the circuit breaker. The compressor failed this summer. A second tech was suspicious that a compressor less than five years old would be tripping the breaker.

He discovered a loose connection in the cut-out switch. Would the hard-start kit have been necessary had the loose connection been tightened prior to it's installation? I'm wondering if the failure could have been avoided with proper diagnosis by the first tech

In 2009 tech # one had come to respond to my problem of a 4-year-old compressor kicking the circuit breaker. Without even looking at the connections in the shut-off box adjacent to the unit, he went straight into control panel, checked compressor current draw at start-up and installed a hard-start kit

. I thought it odd that a compressor less than five years old would be drawing too much current. Had no problems in 2010 but last month the circuit breaker started kicking off again late in the day.

Tech #one returned, checked coolant pressures and return temp, added some freon and started to pack up. I asked if he wasn't going to check the current draws.

He reluctantly removed the control cover, checked draw and voltages and pronounced the unit was running perfectly before leaving with my $168 check.

When the circuit breaker kicked off again the next day, I started to suspect the breaker might be worn out. Called an electrician a few days later.

He replaced the breaker switch, again without checking the connections at the shut-off box, and within eight hours my compressor had died - coils shorted to ground - confirmed by tech # one who quoted me a price to replace the unit.

Now I called a contractor I knew asking if he knew any good AC techs. He referred me to tech # 2 who, in my first call, was immediately suspicious that my original problem may have been in the wiring, not the unit. His reasoning was that a compressor less than 5 years old shouldn't have been giving me problems to begin with.

He came over to confirm my compressor was shorted out. He then did what no other tech had done since I owned the house - popped the cover off the shut-off box and tightened the leads at the terminals - black wire loose, white wire "very loose."

My question is whether that may have been my problem all along and would a hard-start kit have temporarily relieved the symptom -tripped circuit breaker- without curing the problem.

Reply:

Tx Red: you and your second HVACR Tech have come across a useful diagnostic step: check for loose, or even burnt or arced wiring connections. I can't be sure of the answer to your question, as time has passed and we can't know for sure what conditions the first tech saw. A bad relay can cause compressor start problems, and I pose that its possible that arced, burned, or loose connections at the relay could make it misbehave.

Your thesis that a loose wire could have been misdiagnosed as a hard-starting A/C or heat pump compressor motor is reasonable but not, IMHO, absolutely proven by events.

A compressor start relay (or most other switches) that are loosely wired will typically experience arcing at the loose connection (an event that can, by the way, when the circuit is active or the switch closed, draw higher amps than normal due to the resistance at the point of arcing and corrosion.)

Adding a start-capacitor is adding a device that gives a big voltage surge just during motor start-up.

A voltage surge that normally is overcoming inertia in a still motor might also overcome a start relay contactor resistance, caused by damaged or burned connecting points, or by a loose connection that was in effect acting in that same fashion.

The start capacitor is delivering voltage and pushing the motor into rotation.

So without being an E.E. but with some experience around this topic, my reply is "could be" but not that we can say for sure.

Also FYI even a brand new A/C compressor could become hard-starting due to other system problems that cause internal damage to the device or even by (perhaps less likely) a manufacturing defect, events at the property, even general power delivery conditions such as abnormally low voltage. To research the topic you'd look for events that can damage the start winding on an electric motor.

Finally, it's not a surprise that the first tech didn't check nor re-check for loose wiring but I agree it would have been a useful step. Often service techs, contractors, and some investigators become habituated to a short cut of trying the "fix" that usually works successfully and fast.

IF the original start relay or contacts that were found to have been loose is still installed, one could investigate further by inspecting that device for evidence of arc burning or damage.

Question: how often to replace the hard start capacitor kit?

(May 24, 2014) Rich said:
How often should Hard Start kit be replaced? What is contained in such a kit? What is an estimated cost of the kit?

Reply:

Rich: in my opinion, once. If the kit is not working then the compressor motor probably needs replacement. An exception might be if a power surge or other electrical or mechanical event damaged the capacitor.

A hard start capacitor kit should not have to be regularly replaced on HVAC equipment. If the capacitor keeps failing I suspect a wiring error or a motor that is failing solid and needs replacement.

Question: how do I fix my AC if I can't afford service

(May 27, 2014) Kevin Bresee said:
Hello,
I have a Rheem 2ton central air unit. My dog months ago ate wires outside and blew the board, thermostat, and some fuses.

I had all fixed accept I just got a new thermostat and checked wiring and my problem is that the heater had worked but now that its hot the AC turns on all systems go but then it keeps shutting off and the new board is suppose to have a diagnostic light but tells me nothing.

Cold air comes out of registers, a little slower, I can see the blower motor spinning in unit/air handler and outside compressor is spinning and staying on even when the blower motor shuts off.

I looked at the only capacitor inside where blower motor is and it is not swollen or anything but unable to check voltage. Please help 98+ temps lately and cant afford service!

Reply:

A homeowner can perform basic maintenance and checks to keep an AC or heat pump system working, such as checking and replacing dirty air filters. A homeowner with more skill can check that power is being delivered to the equipment.

...

Continue reading at HARD STARTING COMPRESSOR MOTORS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• COMPRESSOR / CONDENSER REPAIR - home
  • BURNED-OUT COMPRESSOR
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  • COMPRESSOR / CONDENSER DIAGNOSTICS
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  • COMPRESSOR MOTOR INSULATION
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  • NOISES, HVAC SYSTEM DIAGNOSIS - home
  • REFRIGERANT FLOODBACK, LIQUID SLUGGING
  • TIGHT or SEIZED AC COMPRESSORS
• ELECTRIC MOTOR DIAGNOSTIC GUIDE tests to diagnose electric motors

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HARD STARTING COMPRESSOR MOTOR FAQs-2 at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to AIR CONDITIONING & HEAT PUMPS

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