Photograph of  This new compressor was placed directly against
a brick wall. One third of its condenser coil cooling ability was blocked.Air Conditioner / Heat Pump FAQs
Fix Air Conditioners & Heat Pumps Set#5

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Air Conditioning & Heat Pump FAQs set#5

Questions & answers about fixing problems with air conditioners & heat pumps can help troubleshoot various operation and control problems like a blower that won't stop or lost cooling.

Fifth set of A/C & heat pump questions & answers help diagnose and repair or adjust air conditioning & heat pump systems.

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Air Conditioning & Heat Pump Troubleshooting FAQs-1

Flex duct in a horrible routing of excess lengths (C) Daniel FriedmanThese questions & answers about diagnosing trouble with air conditioners and head pumps were originally posted at AIR CONDITIONING & HEAT PUMP SYSTEMS

A good place to look for diagnostic procedures is DIAGNOSTIC GUIDES A/C / HEAT PUMP.

Also see this series of A/C & Heat Pump Diagnostic FAQs sorted by major topic area are given at AIR CONDITIONING & HEAT PUMP SYSTEM FAQs LIST

On 2011-08-06 by (mod) -

Becky: if your fan has stopped working, take a look at the diagnostic article link found by searching this site for BLOWER FAN OPERATION & TESTING

On 2011-08-06 by (mod) - when a compressor trips the CB immediately on turn-on

TN, when a compressor trips the CB immediately on turn-on, there is a short somewhere, or the motor is shorting out or drawing very high amps at startup. That's what your tech found and it's consistent with a seized compressor motor. Your ductwork indoors at the air handler doesn't sound like a source of compressors blowing a circuit breaker.

And what the tech said about R22 no longer in use is quite correct. You'd either go to R410A which works similarly to the old refrigerant but not precisely the same, or ...

On replacing a seized-up compressor, if the new compressor is matched to the rest of the system one wouldn't necessarily think about changing rest of the system components.

However because the new compressor will also use a new refrigerant, the installer may want to compatible key components throughout: the condensing coil, evaporator coil, and refrigerant metering device (TEV), possibly other controls all to work properly with the new refrigerant.

By the time you pay for the labor to R&R coils indoors and outside as well as other parts, you can understand why the HVAC company wants to install everything new - it may be less labor, faster installation, and perhaps even more profitable.

On 2011-08-06 by TN-Goose

Need some help in deciding what to do with seized-up compressor (replace just the compressor and the coil at the air handler or replace the entire condensor/compressor unit along with new coils in the air handler). Current system is a 3-ton 10SEER unit with Copelant scroll compressor & R22 refrigerant

If I replace just the compressor & coils (to equivalent parts), but charge it with R410A refrigerant, would it still be compatible with the existing condensor? Is it typical to replace the TXV (or TEV) valve when replacing the air handler coils? Contractors are pushing towards replacing the entire condenser/compressor & coils (at a significantly higher cost).

On 2011-08-06 by Becky

My fan on the ac unit stop working so we bought abother fan for it and it turned on and then stop what can I do

On 2011-08-06 TN-Goose

Had Service tech check out my 3-ton compressor, which has been tripping the breaker immediatly upon turning ON. Tech checked the resistnace of compressor terminals and noted that it has a short-circuit. Says the cause of compressor short-circuit can be caused by rapid power ON-OFF from storms, and/or undersized duct diameter in the blower of the air handler in the attic.

My duct size is 14" on one side and 16" on the other side of the air handler. Is there any truth in 14" diameter duct causing a 3-ton scroll compressor to seize-up/short-out? Tech also it's illegal to recharge with R22 refrigerant; must use R410A. Is this true?


On 2011-08-06 by (mod) - what if my indoor unit has higher BTU than my compressor?

Alan, if the numbers are close the system may be adequately balanced; I'd write down the data tag information from both the compressor/condenser unit and the air handler unit, then give the manufacturer's customer support line a call to ask their opinion. Let us know the specifics of what you read on the data tags and what the manufacturer says.

On 2011-08-06 by Alan

hello i need your consult. what if my indoor unit has higher BTU than my compressor? i suspect the installer has fixed the wrong unit

On 2011-08-05 by (mod) - what "too low" means on the cooling coil

Peter, I'm not sure what "too low" means on the cooling coil - maybe mounted so that it does not receive enough airflow?

If that were the problem I'd think the system would always have performed poorly. In any case, since being low on refrigerant can cause coil icing, as can a dirty filter, I'd check and fix those two things before doing something more costly.

Assuming air flow is not blocked by a dirty filter or crimped ductwork, ask a tech to check for a refrigerant leak and be sure the system is properly charged with refrigerant and see what happens.

On 2011-08-04 by Peter T.

Hi, I bought an older home, a/c was working great when I viewed the home and it was very cold. The lines iced over in the first few days after taking possession and had a tech out who said the coil was too low and needed to be raised. I had another friend look at it who noticed the a/c only had 50% of its freon/gas and need to be recharged. He tells me the coil is fine but the company wants $1100 to reinstall the a/c and recharge.
I dont know what I should do? One guy says one thing and another says something else... help please.

On 2011-08-03 by (mod) - compressor is tripping the circuit breaker

Eric if the compressor is tripping the circuit breaker it may be at end of life - but a service tech should check the system before replacing anything.

On 2011-08-03 by Eric Fisher

HELP! My home is ancient and small (-1000sqft), it is wood with little or no exterior wall insulation, the a/c is 20+ yrs old. The compressor/condenser unit has begun to trip its breakers in the heat of the day when it gets to 105+ degrees and when it is running it is nearly impossible to get the house below 85F. Am I short on coolant?

On 2011-08-03 by (mod) -

Dean I'd start by reviewing the LOST COOLING diagnostic steps (use the search box to find that article)

There is a variety of possible causes of the A/C system blowing warm air - you need first to see if all the equipment is even running. If the compressor is not running you won't have any cooling.

Because you recently received service I'd first look at that bill to see what was done. If refrigerant was added, you may have a refrigerant leak;

On 2011-08-03 by Dean

In June, I received service for refrigerant. Two weeks ago it was well over 100 degrees and my air started to feel warm on the first and basement level. I noticed the thermostat was going over the set temperature, like it was struggling to keep up. So now it is warm in the house but it is trying to give cool air but not cool enough. The fam and motor seems to be runnng but I noticed yesterday the air above the unit was cool. What could be problem. Thanks

On 2011-08-02 by (mod) - the temperature drop is more accurately measured right at the cooling coil -

Eric the temperature drop is more accurately measured right at the cooling coil - that tells us what the A/C core components are doing and ignores for starters the effects of duct length, insulation, leaks. Right at the cooling coil I expect to see a temperature drop from air into the coil to air out of the coil around 15 to 22 degF.

If you are seeing just 10 degrees difference in air temp when measuring far away from the cooling coil, say at the return air inlet and at the closest (to the air handler) supply air outlet, your measurement is, compared to the one I describe above, ambiguous since we don't know about duct insulation, run length, crimps, bends, leaks, etc.

By separating out those factors we know what to focus on in improving system performance.

On 2011-08-02 by Eric

A/C contractor said system is running as best as it can, but a energy audit found return vent temp of 80deg, and register vent temps of 70deg. The closest vent is a mere 6 ft away from the coils. Energy Auditor says the drop should be in the neighborhood of 20-25 degrees, not 10 degrees. Who is right/wrong? I don't know if the A/C contractor is being deceptive because he is contracted by the landlord, and not by me.

On 2011-08-02 by (mod) - outdoor unit abnormally quiet

Just a guess, Carl but if the outside unit is "running" but more quiet than usual I wonder if just the condenser fan is running (you'd see the blades spinning) but not the compressor motor? If so it could be a bad start/run capacitor, start relay, or compressor motor.

On 2011-08-02 by Carl

Ok my outside unit's fan is running, but seems quieter than usual. The air coming out of the vents is not cool, and the lights periodically dim while it is running. The power going to the unit is good, could it be a capacitor on the compressor?

On 2011-08-02 by (mod) - yes the compressor/condenser fan will still run in a system short on refrigerant charge

Eddie, in most residential systems, yes the compressor/condenser fan will still run in a system short on refrigerant charge

IN some HVAC systems there may be pressure sensing switches that can shut the system down if abnormally high or low refrigerant pressures are measured. In that case the whole system should shut off.

On 2011-08-02 by Eddie

Will a compressor fan outside still run if low on freon?

On 2011-08-01 by (mod) -

Lost in Amps - take a look at the Q&A I've posted above about the effects of voltage variation or loading on electric motor current or motor efficiency

On 2011-08-01 by lost in amps

got one for you. i put an ammeter on my air handler and it read 8.25 amps, I removed the filters and it went up to 9.75 lmao at the situation the amperage should have gone down. what gives here

On 2011-08-01 by (mod) - turn off a compressor that hums but doesn't start

Kenzie: turn off your system promptly and call an HVAC service tech. A humming motor sounds like one that is unable to start. I'd leave it off for electrical and fire safety. The "fix" may be as simple as a switch or start capacitor, or another system control like a relay or valve, or it may be something more costly like a motor or compressor.

Beth a fan that is running intermittently may be a problem with a control in the air handler, a failing motor capacitor on the fan motor, failing fan motor, or something easy like a loose fan pulley or blade.

On 2011-07-31 by Kenzie

My air is blowing well indoors, great flow, but not cool. Outside, it is just sitting 'humming', the fan is not working. please HELP!

On 2011-07-31 by Beth

Air conditioner seems to be working, fan is blowing but doesn't sound the same. Fan will stop for a second and goes on again. Air is somewhat cool coming into the house but is not cooling. What gives?

On 2011-07-28 by (mod) -

Chad: if your A/C unit seems to run normally except when it's very hot out, and if by "not running" you mean electrically the compressor/condenser does not start, I have to suspect that a motor is overheating. YOu can experiment by shading the unit with a beach umbrella (don't block airflow) or even spraying with a garden hose as a diagnostic. Overheating can be a sign of a motor at or near end of life.

Mary it sounds as if your new A/C system is having trouble starting. Could be a bad start relay, bad start/run capacitor, or something else we haven't thought of. Let us know what the service tech finds.

On 2011-07-28 by Chad

My ac unit is not running during the hottest time of the day but runs perfectly during the morning and evening. It is an outside unit, does anyone have any ideas?

On 2011-07-28 by Mary

HELP! I have a completely new system. The installer has been here twice. There is a problem. The unit when it cuts on after sitting for awhile has a slow poulsing continuing humming vibrating which goes on then ceases then begins again it just now continued through this cycle as long as it ran which was about fifteen minutes. What is happening?

On 2011-07-26 by (mod) - rotation speeds of cooling fans, amperage draw of the compressor, and system refrigerant pressures

Dennis, did the diagnostic steps confirm rotation speeds of cooling fans, amperage draw of the compressor, and system refrigerant pressures? A bad capacitor or control board that causes a fan to run backwards or slowly could perhaps be involved.

On 2011-07-26 by Dennis - storm damage

Sorry, i guess i wasn't clear. My unit was running and cooling well before we had a storm.

After the storm it is still running and cooling, but not as well. All the normal checks and issues have been checked and the tech even says the refrigerant and pressures show normal, the filters are new, coils are clean and remember almost immediately after the storm brownout it lost cooling power.

Even though the tech says everything looks fine (pressures, voltages, filters, coolant, etc) it is just not putting out air as cold as it was, showing about a 13 degree drop between inlet and outlet temperatures. The compressor is cycling but it just acts like it is not cooling enough. Anyway, thanks for the help.

On 2011-07-25 by (mod) -


a number of problems could cause an air conditionre to not cool so well - low refrigerant for example. If your tech replaced the compressor he would have had a reason, such as damage from the electrical storm, since that is quite an expensive repair. Or the first time it was under warranty and profited the contractor but the second time it's not under warranty.

But you and I can't know that the two cases are identical with so little diagnostic data. First it would be appropriate to correctly diagnose the cause of poor cooling - it could be something trivial like a dirty air filter, or it could be moisture or dirt interfering with a thermostatic expansion valve, or if the electrical storm actually damaged the equpment, perhaps a control circuit board.

On 2011-07-25 by (mod) - if you see a smoking electrical component you should shut down that equipment immediately


Watch out: if you see a smoking electrical component you should shut down that equipment immediately as there is risk of an electrical fire. Sounds as if a motor or wiring has burned up.

On 2011-07-25 by Dennis

I have a PAyne 3.5 ton unit on my house. It was replaced about 3 years ago. LAst year we had a severe electrical storm and immediately after I noticed that the unit runs all day and does not cool well. A service tech says that everything looks fine but he replaced the compressor anyway and the unit began to cool better. This year it has done the same thing after a storm but the company will not replace the compressor because the gauges show it is working. How can this be?

On 2011-07-25 by elizabeth

my a/c was working good until yesterday... it is an older unit, maybe even the original from our home. the fan does not work and it appears to smoke from the unit.... most likely this needs to be replaced right!?

On 2011-07-24 by Sid

My A/C is an older unit, that until yesterday, 07/23, was working fine. Then from about mid-afternoon, it started blowing outside (HOT-Oklahoma City) air. The condensor/compressor both appear OK, and blower fan is working OK. No breaker/fuse problems, and the condensate draining is OK. Low refridgerant?

On 2011-07-24 by (mod) - no cool air is coming out

TFN: re no cool air is coming out, just warm air and buzzing circuit breaker, I worry that you have an overcurrent condition and the circuit breaker is not tripping; especially if your brand is FPE - Federal Pacific Electric, possibly Zinsco or Challenger, these problems have been reported more often. The risk is an electrical fire; the underlying cause could be a failing motor drawing high current. Of course it could be something lucky like just an individual defective breaker.

On 2011-07-24 by (mod) - if your A/C system keeps tripping the breaker it sounds like a bad compressor motor

TN Goose

I agree that if your A/C system keeps tripping the breaker it sounds like a bad compressor motor; a tech will check that by looking for abnormally high amps draw.

On 2011-07-24 by (mod) - humming coming from the inside air conditioning air handler


humming coming from the inside air conditioning air handler could be a bad blower motor or motor bearing.

On 2011-07-24 by (mod) - intermittent A/C cooling problems


re intermittent A/C cooling problems - when you describe a rather short cycle of cool air then warm air, and the blower is operating continually and the compressor/condenser is running, I suspect a problem with the refrigerant metering device - the thermostatic expansion valve - that could be clogging from debris or freezing up from moisture in the system. If it were a longer cycle of cool air then air flow reduction, I'd look more at a coil icing problem.

On 2011-07-23 by bg

My a/c is an outside unit, with compressor and fan contained. The fan won't spin, and i don't know how to check if it's the fan motor or the compressor. I've isolated the problem to be between the "contractor" (relay between the thermostat and the compressor/fan). That is, everything works between the house and the relay. Any idea how to check if the motor is ' bad' or the compressor is 'bad' (or both)?

On 2011-07-23 by TFN

The unit is running and all switches are set to on and cool but no cool air is coming out, just warm air. There is an intermittent buzzing sound, lasting a second or two, coming from the circuit breaker that happens about every four or five minutes. When that occurs, the fan at the top of the outdoor unit stops running momentarily and then starts up again. We have turned off the unit and flipped the circuits in the breaker box for safety reasons already but wonder what it is we could be dealing with. Thanks.

On 2011-07-22 by TN-Goose

Chris W: Mine did the same thing and after I replaced the capacitor and the contactor, it's still tripping the breaker. Anyway, after much troubleshooting, it's beginning to look like the compressor motor locked-up or shorted, thus tripping the circuit breaker. By the way, does your outside condenser fan rotate at about half-speed when the indoor thermostat is switched to OFF?

On 2011-07-21 by Tami

What do you do, if your central air is making a humming noise (not coming from the compressor outside, but from the unit inside)? I noticed that nothing comes out of the airducts when I turned it on for a short moment. Thankyou.

On 2011-07-21 by Scott

Intermittent Cooling

Normally when the outdoor compressor unit is running, the air coming out of the vents is about 20 degrees below room temp, and the insulted outdoor copper freon line gets cold.

Now while the outdoor compressor is running, it cools as described above for a short period and then the vent air temp heats up to about 5 degrees below room temp, and the outdoor freon line does not feel cold. Air flow seems the same in both conditions.

I see no frost or ice in the blower unit. Water drains from the condensate drain during both modes of operation. What might be causing this intermittent cooling problem?

On 2011-07-20 by (mod) - what if the air handler door won't latch shut?

Chris if you are referring to a door on the air handler of your air conditioning system, there is often a blower door safety switch that will turn the system OFF if someone opens the door to the blower compartment. If someone tampered with the switch or bent it or moved it that could be a problem that interferes with keeping the door shut properly.

If the switch is intact and present the system should NOT run when the door is open and should run when the door is in place and latched. Did someone mis-wire or tamper with the switch?

On 2011-07-20 by Chris M

3 x 8 steel door wont latch if the air conditioner is running what is wrong ?

On 2011-07-20 by (mod) - If the A/C unit is off but the fan keeps running,

If the A/C unit is off but the fan keeps running,

If you mean the outdoor fan on the compressor/condenser unit never stops, then it sounds like a fan control problem - call a service tech.

If you mean the indoor blower fan in your air handler never stops, first see if your thermostat switch is set to fan ON instead of fan AUTO. That would keep the fan running. A second switch on air handlers that also provide heating, not just ocoling, may be a white manual fan switch found inside the air handler on the fan limit control. On a Honeywell unit if someone pulls the white knob out the fan will run continuously.


On 2011-07-20 by renee

a/c unit is off but the fans wont shut off

On 2011-07-20 by (mod) - if your home is not cooling

Chetan if your home is not cooling, you will want to start by going through the simple diagnostic steps found beginning at the end of this article

On 2011-07-19 by chetan

my home ac no cooling pls give me advice.

On 2011-07-19 by (mod) -

Chris W

If your A/C system trips a breaker at startup then there is most likely a short circuit or a failing compressor motor that is drawing high amps. Leave the system off (for fire safety) and call a service tech.

On 2011-07-19 17:15:41.901244 by (mod) -

Nancy and others:
If your indoor air handler fan won't run and the thermostat is calling for cooling and the outside compressor is starting up normally, check for lost power or power off at the indoor unit; check for a fan motor reset button popped out; check for a blower fan driven by a pulley and belt with a lost, loose, or broken fan belt;

On 2011-07-19 17:08:23.335499 by Chris W

What would cause the circuit breaker to trip to the off position when I try to start my AC? My blower fan works in the furnace so I'm able to circulate air, but when I try to start the AC, the circuit breaker trips after 1-2 seconds after I reset it. I have replaced the filter, cleaned the coils, and vacuumed out the debris in the bottom of the compressor/coil housing outside, and my thermostat is set correctly. Any help would be greatly appreciated!

On 2011-07-19 15:57:35.805792 by (mod) -

Ron please see our new article linked at page left, titled FAN, COMPRESSOR/CONDENSER UNIT

about the condenser/compressor fan and its motor.

If your fan motor won't start but is humming it may need a new start/run capacitor. Check with the power off to be sure that the fan motor rotor is not already seized - just see if it turns readily. And be sure nothing is jamming the fan blades.

On 2011-07-19 03:16:52.047101 by Ron

Quick question: My condenser Fan motor doesn't kick on. It was getting really Hot. I thought the motor was bad so i replaced it. Now it Hums but still does not kick on. Could the in line Capasitor affect the motor in this way?

On 2011-07-18 23:13:34.633519 by (mod) -

Nancy if the outside compressor/condenser runs and the inside A/C blower won't, after checking for power at the blower unit (air handler) (and there may be more than one off switch, including a condensate drip tray overflow switch), anyway after checking that there's power and all of the switches are on I'd check that the blower fan motor has not gone off on reset (or that the motor is running but a belt is broken inside the fan unit if yours is a pulley -driven unit.

On 2011-07-18 23:12:00.603435 by (mod) -

Dave T: thanks for the nice comments; sorry but I just don't know enough to debug those error codes, though a service tech familiar with your particular brand/model might have more to say. It's worth another call to the manufacturer.

On 2011-07-18 23:10:56.417346 by (mod) -

Patrick: I wouldn't replace a compressor unit just based on those pressure readings, as other controls could be at fault such as a thermostatic expansion valve; also at very high outdoor temps the compressor/condenser may just not be keeping up. A diagnosis of bad A/C compressor would look not just at its output pressure but also the amps it's drawing.

Finally, high pressure on the LOW side after the compressor has been running for a while would certainly make it worth looking at that refrigerant metering device.

On 2011-07-18 by Nancy

Our central AC's blower motor is not coming on. Condensor turns on and I can see it's fan is running and one line starts ice on it. Please help

On 2011-07-17 by tom

why does my ac inside blower goes off and on every 5 seconds

On 2011-07-14 by Dave T - E4 error code

This is an incredible resource, so first up thanks for putting it up, mad props.
I'm not a technician or anything, just a consumer. Our split type AC has been acting up the past couple of years.

First the problem was an E4 error code, which has been occurring for over 4 years now. When you leave the AC in normal cooling mode (approx 23-25deg C) sometimes the AC shows E4 then shuts down. There's no consistent way to monitor as there are days where it just runs fine the whole day, sometimes, it runs for 40-50mins then E4 comes on then shuts it down.

We 'remedied' the problem by leaving the AC in 'dehumidifying' mode where the AC shuts down automatically in set intervals and turns back on with cooling.

Last week, we had our AC cleaned as well as the drain pipe de-scaled/flushed. The technician called the manufacturer about the E4 problem and the support told him it's a thermostat sensor issue. They gave him a part that sticks into the evaporator coils in the room unit.

After that repair/cleaning, the E4 problem went away and the AC was quiet and cool.

However, less than a week after the repair, a new error code appeared (E5 this time) and a new weird symptom came up.

The AC would turn on normally, but after 5 seconds or so, the blower will turn on to its maximum setting without cold air, then shuts down with an E5 code. With this new code, there's no 'remedy' like before as it just keeps the AC off.

We just spent a couple of hundred bucks in repair and cleaning and not sure what to do at this point, is the problem due to the age of the unit (replacement imminent) or is it something less serious? We need to inform our landlord ASAP if replacement is needed as it's really hot here in Singapore during summers.

The AC brand is Fujiair model FC9 I believe. It's quite old, I suppose, it's no longer listed on their website. I'm sure it's at least 5 years old since we've been living here with the same AC for that long.

Thanks a lot.

On 2011-07-14 by mann

ac does not start

On 2011-07-14 by PatrickBumgarner

I have two heat pumps on the property that i work at. Outside air temp. has been about 95 degrees, The high side pressure is about 190 to 200 psig & the low side is about 110 to 120 or higher. I have already replaced the compressor on another unit doing tha same thing. before i replace these two i just wanted a second opinion that it is the compressor that is going bad.

On 2011-07-13 by (mod) - electric motor overload switch that automatically shuts off the system

Thanks Dee. Mike: Dee is referring to an electric motor overload switch that automatically shuts off the system - more often found on fan motors at the compressor/condenser unit or in the air handler blower compartment on that fan.

On many A/C compressors and some other electric motors such as well pumps, there is an internal overload switch that will reset itself automatically once the unit cools down.

A bad overload switch (if there is a replaceable unit) may be worth a try but more often in my experience if a motor keeps tripping on overload there is a more basic problem at work causing that overheating.

On 2011-07-13 by dee

mike- sounds like compressor overload, most units can be replaced- the overload is generaly under 50 bucks.

On 2011-07-13 by (mod) - "test" for an overheating compressor/condenser unit by wetting down that outside component with a garden hose,


people "test" for an overheating compressor/condenser unit by wetting down that outside component with a garden hose, on the premise that the system is designed to be exposed to rainfall without damage. But if the unit is overheating I'd check for

- blockage of air flow by location too close to a building wall or fence or shrubs
- dirt or debris inside the condenser unit
- dust and debris on the condenser coil fins (big payback from keeping the coil clean)

and last and worst
- a failing, overheating compressor motor that is drawing high amps.

TN-Goose: specifying voltage levels and wiring for individual control circuit boards is, sorry to say, beyond my competence and scope of such a general discussion. Other than checking for normal voltage levels (120V or 240V or 24V depending on the component), more detailed control circuit analysis surely depends on the air conditioner model.

In my experience, (and what Hilliard taught at A/C school) most electrical problems on A/C systems are in compressors, relays, and overloads.

On 2011-07-13 by mike

my outside unit turns on and will run for about 15 minutes and then stops. i let it cool down for a while and it will work again but only for 15 minutes. i live in georgia and its been 100+ degrees here for a week or more. any info would be great

On 2011-07-13 by TN-Goose

DanJoe: By "yellow control wire", I meant the wire coming off the indoor thermostat. So, when you say "control board or control switch", are you referring to the indoor thermostat components? Could you describe which voltage levels to check and what voltage to expect? Thank you.

On 2011-07-12 by (mod) -

Sounds more and more like a shorted wire or control board or control switch; be sure to check the voltage levels.

On 2011-07-12 by TN-Goose

By the way, I disconnected the yellow control wire from the coil terminal, but the fan still ran at about half-speed. I guess that was expected since the indoor thermostat was switched OFF to begin with.

Didn't do anything weird; electrical connections are the same as it was before the troubles began. While the fan was running at about half-speed (with the indoor thermostat still switched OFF), I checked the voltage across the contactor with my multimeter.

One probe tip on the supply-power terminal, and the other on the terminal where the fan & compressor wires are connected. It read 126 VAC, however, the contactor was not closed (still floating up)!!! Somehow, power is being sent to the fan as if the contactor is closed. Also, I checked the resistance between the contactor's coil terminals, and it read 37.5 ohms.

Somewhere I read that if the resistance is significantly higher than 20 ohms across the coil terminals, the the contactor is OPEN and it should be replaced. If the contactor is OPEN, how is it transmitting power to the fan? Any thoughts on what could be the problem? Could it be related to wire damage in the coil of the contactor?

On 2011-07-11 by (mod) - Fan running half speed:

Fan running half speed: Don't most fan motors run at 120V?

Jeffrey: did you check the voltages?

Did you check the start and run capacitor for the fan motor?

On 2011-07-11 by jeffrey

I installed a new split system in a friends house. Well when it runs the fan motor on condensor does not look like it is running fast enough. And about 30 minutes after running compressor starts to make a weird loud sound the compressor goes into a lock out mode.

On 2011-07-11 by Fan running half speed

Sound like you have a partial short to ground allowing the fan to run on 110 volts. Or did someone do what my neighbor did, come off one leg of the ac power and used the ac ground as the neutral for his deck power. When you do weird stuff, weirder stuff usually happens.

On 2011-07-10 by (mod) -

Some fan motors are designed to run at more than one speed, but I haven't seen that on a condenser unit. I wonder if you've got a low voltage condition>?

On 2011-07-10 by TN-Goose

I googled and learned how to troubleshoot the contactor. However, it's still very strange why the condenser fan would run at about half-speed while the indoor thermostat is switched OFF. I checked the contactor while the fan is running and it is not closed (it clearly has a gap between the spring-loaded contact and the bottom portion, so the circuit is open).

However, the fan kicks into full speed when the indoor thermostat is switched ON (contactor plunger activates and closes the circuit), but a split-second later, the breaker would trip and shut-off the condenser/compressor unit. Maybe it's compressor seizing up, but what could be the reason for the fan to stay running at half-speed?

On 2011-07-09 by Anonymous

I was wrong about contactor. I used another (my father -in-law had identical replacement) and still had problem. I realized I should not have been checking the voltage to ground. I should have connected from one top connector to the other top connector (which both went to wire into the house).

That was 0 V -- so no connection -- due to a failing sensor that opened the loop. I bypassed both sensors and when the "cool" turned on there was a 24V circuit and the contactor engaged. The two sensors are hooked together with wires. So I took them a part and tested each directly, both with my continuity checker and with by jumping (alligator clip connector) directly to the contactor. (I never disconnect the brown/common side of contactor).

The TEV sensor works. The other sensor (yellow wires) does not work. It is a sensor on the larger diameter refrigerant suction line.

Well I don't know if is a sensor problem or if the wires are bad or if the sensor is good and simply is doing its job. I am still trying to find information on this sensor.

On 2011-07-09 by ted

i have and outside air conditioner that is not getting real cool. the suction line is cold but the liquid line going to the ouside unit is not hot. also the head pressure is running over 300 pounds. everything is running great. just will not get below 78 degrees.


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