Ask Me Help Desk - Air Conditioning

You should have 5 wires running to the outside unit. 2 big heavy, insulated ones plus a ground from the breaker outside by the unit, and a couple of little insulated ones coming out from the furnace. With the thermostat calling for cooling, the big ones should have 240 V between them and the little ones 24 V. If both pairs of wires are hot, check the other wires connected to the rely they go to. If they aren't hot, the relay is bad. Fairly cheap and easy to pick up at an electrical or HVAC supply.

If either pair lacks power, follow it back and find where the problem is.

To do simple checks like this you do need some tools. A test light, a meter, or a voltage detector might be the best place to start with. I came across the niftiest gadget for trouble shooting, a voltage detector. They work through the insulation of wires. There are several brands. I have a GB Instruments GVD-505A, less than $15 at Home Depot. Touch it to a hot wire, and the end glows red. Find the doodad that lights it on one side, and not the other, and you have the culprit. You do not have to open up housings and expose electrical contacts. You are looking at where your hand is, not where the meter is. Most people are capable of doing repairs and will get it going and not get hurt if they use a little sense. The voltage detector makes it even easier.

During the summer of 2005 I was using my central air conditioning averyday, and when it got cold during November of 2005 I started to use the central heating. At this time I would rap tarp around my AC unit that is exposed out side from getting dirty. What I notice is that when I took the tarp out and turned on the AC unit I notice that the AC outside was not turning on. I first check the main circuit breaker but did not find anything that trip the breaker. I also look at the breaker that is near to the AC unit and saw nothing wrong.
Can you suggest what might be the next step to look at.

Got to thinking about that breaker. Usually there isn't one outside by the unit. Code requires a disconnect, just a switch. Often it includes fuses. You might shut it off, open it, and check for a blown fuse.

Also, breakers may not look like they are tripped. Turn it all the way to off, and then back to on.

Yes the box that is near to the AC outside is a off and on switch. There is a 240volts power coming form the switch and then I followed the line to the AC unit of the transformer there is also 240volts going in the transformer but going out for 24volts there in no signs of 24volts at all. Do you think this is the problem why the AC condenser unit outside my house is not working at all because the transformer is not outputting 24volts at the other end.

Quote:

Originally Posted by jayrol

Yes the box that is near to the AC outside is a off and on switch. There is a 240volts power coming form the switch and then i followed the line to the AC unit of the transformer there is also 240volts going in the transformer but going out for 24volts there in no signs of 24volts at all. do you think this is the problem why the AC condenser unit outside my house is not working at all because the transformer is not outputting 24volts at the other end.

If I understand you correctly, it sounds like the relay is not closing. Either a wire has come loose or the coil on the relay is bad.

Have you ever done any electrical work?

You can manually activate the relay to see if the coil is bad but you must use a non-metallic object to push it in. This can cause an electrical shock to you if you do not know what you are doing. Stay safe.

If you don't have 24 volts at the coil outside, when the thermostat is calling for A/C, check why. The relay is not working because the coil lacks power.

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan.

Go into the furnace and check for 24 v at the terminals the wires to the outside unit connect to, Y & B. If so, the wiring out to the condenser is bad. If not check for power between the blue wire and red. If it has power, the thermostat or wiring to is is bad. Go to the thermostat and check.

If no power at the R terminal, either the transformer is bad, or a fuse is blown. Check for 120 volts on one side of transformer and 24 volts on the other side. If you need more details, post back.

I am confuse when you said relay and coil, again I was Able to use a voltmeter to check if there was any voltage coming from the switch that is located near the AC condenser. I was a ble to get 240V from the off/on switch no prblem threre, and I folloewd the line to the ac and the wire connects to a Transformer. I checked that connection on the input of the transformer and also has 240V, and then I checked the output of the transformer Igot no Voltage reading on my voltmeter. The output reading should be 24V but I get 0V. The transformer also has coil to reduce the voltage from 240V to 24V. From my understanding when 240V is going into the input of the transformer the output should receive 24V. So when I measured the output for any voltage source I found no volts at all 0V.
The relay would only activate when there power coming from the transformer that produces 24V.

OK, maybe I needed to read your post before a little more carefully. If the transformer is hot on one side, and not the other, it is bad. It needs to send 24 volts to the thermostat to be switched to the coil of the relay to operate. No 24 volts, and the relay doesn't close. May need to hit an electrical or HVAC supply for a 240/24 transformer. Most systems use 120 volt.