AC Unit does not receive 24 volts on outside unit


First, you will need a Voltage meter. Next go to the unit. If you have a split system meaning that you have an air handler and a condenser you will need to go to the air handler. If you have a package unit both the air handler and the condenser are combined in one unit. Next pull the disconnect to the unit or air handler. Take off the cover where the electrical components are stored. You will need either a 5/16" or 1/4" driver or socket wrench for that. Next find the transformer if you are unsure of what that looks like google 120 to 24v transformer and look at the images. Now reengage the disconnect. If your working in the air handler there may be a push button switch which disconnects power to the unit whenever the cover is removed. If this is the case put a piece of tape on it to hold it down. Now on a transformer there are four leads (at least on most) two on the primary side and two on the secondary side. Put your volt meter on the primary side you should have either 120,...

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Hi, this relay is your contactor that you are pushing in to make it run. You are not getting 24 volts to the 2 small wires that terminate on the side of this contactor. These 2 wires are low voltage and are controlled from the thermostat and indoor unit. You either have a faulty thermostat, or you may have blown the fuse on the indoor unit, if yours has one. Remove the panel on the indoor unit to see if you have a circuit board.See if you have a fuse that is just like a automotive plug in fuse that's plugged into the board. See if it is blown. If not, is the indoor unit running, its just the outdoor unit that will not come on? If the indoor unit comes on and runs, but the outdoor unit does not come on, you more then likely have a time delay relay that is mounted on the outdoor unit that is faulty and will not send 24 volts to the delay.It has failed, and that is why it will only start when you push the contacts down on the contactor. Some are a small black square box with 2 to 3...

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Problem: Why I am not getting 24 volts to the contactor? Where does the 24 volts come from?

Answer: The 24 volts comes from a low voltage transformer. Please see the pictures of the two transformers below. The transformer has a primary (high voltage side) and a secondary (low voltage side). The high voltage side takes 110 to 240 volts AC and transforms the voltage to 24 to 28 volts AC. If the contactor is not getting the 24 volts to the contactor coil then it could be a time delay relay (if the unit is equipped), thermostat problem, transformer, safety control (low on refrigerate) or wiring problem. I would like to suggest that you turn the power off to the unit and check all your connections to make sure they are good and tight. You can turn the power back on and set the thermostat so the system is calling for cooling (AC ON). You could test the contactor with a volt meter to see if you are getting 24 to 28 volts AC to the contactor 24 volt coil. If you are not getting any...

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Outside of your home there is a large fan that is in charge of moving heat away from the condenser coils. Over time, these fans just wear out and die. If they quit working they can cause the compressor to overheat and shut down.It’s easy to tell if your fan motor is bad (or is going bad.) Just watch it work for a while. If it barely spins, or does not spin at all, there is a good chance it need to be replaced. If you’re not sure this is your problem, and the fan is not turning, you can bring it to your local parts store and they’ll be happy to check it for you


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Outside AC unit not running? Perhaps the fan inside the furnace unit runs, but doesn't blow cool air?

Here are a few of things you can do before calling an AC repair company.

Make sure the thermostat is set to COOL

A repair person will check this first, so you might as well, right?

Ensure that your thermostat is set to COOL and then set the temperature setting at least 5 degrees below the indoor temperature.

Look for a blown fuse or tripped circuit breaker

If there was a power surge, your air conditioner might have blown a fuse or tripped a circuit breaker.

Look for this problem in two places:

Your main electrical panel -- On old homes this is a 'fuse box' while newer homes have a circuit panel. If it's a fuse box, look for a blown fuse (replace that thing!) On a circuit breaker, look for a tripped circuit and flip it to "off" and then back to "on". At the shutoff box near the AC -- Some models of air conditioners have a fuse at...
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by Bob

I have a Carrier central air unit (model 38ed048300)manufactured in 1983. It has been serviced several times over the years. When I set the thermostat switch inside my home to "cool" the furnace blower comes on and everything appears to be ok at the furnace.

The fan on the external unit does not come on at all. I checked the two cartridge fuses (50A) in the fuse box near the AC, and they checked good with an ohmeter. When I had the fuse block out, I checked the voltage at the fuse box and there was about 25v AC across each of the fuse terminals. This voltage was there even with the thermostat switch inside the house set to "Off".

I also checked the main circuit breaker box in my garage to see if any breakers were tripped but none were. I even turned each circuit breaker off and then back on. It made no difference.

When I look inside (through the vent holes) my AC unit, everything appears to look relatively new....

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Before you make that call, try this basic air conditioner troubleshooting guide. This guide will help you to diagnose problems with your typical ducted split type air conditioner. A typical repairman will cost you about one hundred dollars per hour. With a few household tools and a little bit of knowledge, you can do it yourself and save that money. (You provide the tools and we will help with the knowledge part.)

You should ensure that the routine maintenance has been performed on the unit. If the air filter has not been changed in the past month, check to see if it is dirty.



Is there air coming out of the floor or ceiling registers?

If there is not, then, check to see if the fan on the outside unit is running. If the fan on the outside unit is not running either, check to make sure that the...

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I've recently replaced my A/C condenser motor. I idiotically installed the capacitor with a piece of banding from work instead of a proper bracket. Well, when I removed the top grille to switch the directional jumpers on the condenser motor to the proper direction (had them wrong at first), I accidentally pulled the wires too much and apparently dislodged the capacitor from it's "bracket".

So, it slid down and shorted out against one of the compressor's capacitor terminals and the case of the unit.

I discovered this when the house was not cooling down 2 days later. When I took off the panel and discovered the fan's capacitor had went bad , I ordered a new one (with a proper bracket) and a new compressor capacitor just too play it safe and in hopes this would fix it.

I installed both today and switched on the unit. The inside blower started, but nothing outside. So I got out my multimeter and started probing. I am getting 24V at the contactor and it is pressing in....

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How to Test Your Air Conditioner Capacitor

Are you having air conditioning problems in your home and suspect it may be an air conditioner capacitor problem? Below I have laid out a step-by-step guide on how to test your AC capacitor to see if it is good or if you have a capacitor problem.

First – before I go any further I need to lay out this disclaimer! WARNING – This procedure involves a risk of shock – do not attempt this unless you are familiar with high-voltage safety.

Ok, here are the tools you will need:

Screw Driver with an insulated handle Insulated Gloves Meter with micro farads or meter with resistance (ohms)


If you are unfamiliar with high voltage in your outdoor unit, do not attempt this procedure!!!!! Remove the disconnect or turn off the circuit breaker this should turn off the 240 volts to the outside unit Turn your thermostat down below room temperature and select cooling (this will still send 24 volts to the outside unit but...
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HVAC - Outside Compressor or Fan Motor Not Running

Shared Knowledge

"Heating, Ventilation and Air Conditioning (HVAC)"

"Outside Compressor or Fan Motor Not Starting or Running"

26 April 2011

Updated: 7 August 2012

Summary: the most common causes for the outside compressor and fan not running when they should is either: a seized fan or a blown run capacitor. Both the fan and run capacitor are do-it-yourself (DIY) replace if you are careful.


HVAC has 240, death, alternating current (AC) voltage.

Do not attempt any do-it-yourself (DIY) repairs on HVAC if you have never worked with or around high voltage.

Always, double and triple check that circuits are dead. This means using a voltmeter.

Do not trust a circuit breaker.

Do not trust a quick disconnect.

Do not work on HVAC when you are tired or too hot or pressured.

Yes, it can get hot inside your home...

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What are your symptoms?

We will assume that before we begin any of the steps below, we have already checked that the air filter is clean, batteries in the thermostat are good and the settings are correct (cool - auto - 75 or low enough to come on).

Unit does nothing no matter where I set the thermostat. Unit comes on (makes noise) but air never comes out of the vents. All of a sudden, unit started blowing warm air. It seems as if the air just got warmer and warmer. There is water dripping or running out over my window or a place it never has before. My power bill is too high. Unit stopped working I turned it off for an hour, turned it back on and it worked.

Unit does nothing no matter where I set the thermostat

Ok, now we will start by turning the thermostat too cool, auto and 60 * now assuming you have a split system, go outside to the unit. Bring along a 5/16 inch nut driver, 1/4 inch nut driver, voltmeter (I prefer a fieldpiece), take the cover off the unit...

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How Air Conditioners Work & When They Don't Work


To post a message or a question visit my message board

Are you sweating and want to fix or at least learn about air conditioning, either home or car then you are at the right place.

If your looking for AUTO air conditioning help go here.

A/C trouble shooting tips

How to read and understand A/C gauges

If you know that the problem is electrial ie. fan motor not running or everything isnt running then go here to my a/c electric page.

The main thing to remember is that all freon(refrigerants) work the same way no matter what is cooled, as in ice machines,de-humidifiers,freezers,coolers for flowers,ect.

They all have a compressor, evaporator(cold coil), condensor(hot coil), expansion valve, and some way to remove the heat such as fans,water,buried...

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Heating, Ventilation, and Air Conditioning (HVAC) contractors are your go-to professionals for air conditioning repair, installing central air conditioning, and heat pump installation. If your system is more than 5 years old, air conditioning repair is a task worth looking into. Even if you don't feel a noticeable difference in your system's performance, calling a professional for air conditioning repair is often a good idea to make sure that you'll continue to have efficient operation in the future. If your system is more than 15 years old, air conditioning repair is probably a less desirable choice than replacement. Installing central air conditioning is not going to be cheap, but because today's equipment is so much more efficient than anything available 15 years ago, it is nearly always the case that getting rid of your old system and putting in a new one will bring you noticeable savings on your utility bills. Installing central air conditioning is a good choice for many...

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