Air Conditioning - Is transformer problem?

I called an HVAC guy to come to take a look at the problem. He failed his promises and did not show up for three times. "That is enough," I then said to my self. I lost a week for waiting this guy. So, I decided to cancel this guy and called another Tech. This new Tech did not even show up or call to notify us whether he could make it or not. A day later I called in and he said he was sorry. He got stuck at one house and forgot about it. What's a ...sad experience. I then decided to go on internet to search for the cause of the problem.

Firs of all, this is a great website, which helps me to understand how an AC unit works. And secondly, I thank you all for contributing your problem info. and knowledge toward diagnosing and fixing it.


Here is the problem:

When calling to cool, my AC unit does not start at all; nothing happens; neither hum nor click sound at all. However, my heater unit is still working fine when called for heat and the hot air comes out ok. I read many threads and performed several checks as follows:

1) If I manually press the contactor button, both fan and condensor will start immediately.
2) At the AC unit outside the house, I measured the voltage comming in the contactor from the two small wires Brown and Yellow, nothing there. It seems no voltage at all. It makes me think that was the reason the contactor did not get pulled in. I bought a new contactor but it did not help. The AC unit only starts if the contactor is manually pushed in.
3) Checked and measured the voltage at the TStat. There are five wires Blue, White, Green, Yellow and Red. When the Blue and White wires are measured together, there voltage is present. There is no voltage at the Green, Yellow, and Red wires. So, that was the reason why when called to cool, nothing happenned.
4) Checked the transformer in the furnace area and found that there is no voltage at the Green, Red, and Yellow wires, which I think they wire up to the TStat upstair. Other two wires coming from the heater at the furnace to the TStat have voltage. So, this explains why when called to heat, the heater unit will work immediately.

That's all I've checked so far. My heater is ok and the hot air comes fine. The fan motor must be functioning correctly, right? Based upon the findings, I must come to a conclusion that the transformer at the furnace must be burned out or gone bad. Is my conclusion correct?

My AC unit's been down quite awhile already. Please help me to knock this problem down. let me know if you need more information.

Thank you

Joe

have you changed any tee stats in home before this happen

No thing has been changed. All of a sudden, it does not work any more.

You did not mention how exactly you checked for voltage at the thermostat or at the unit. Assuming you have a average AC voltage multimeter, do it this way and report back. Set the thermostat to cool and bring the arm down until it clicks and you know it is calling for cooling , now take your multimeter and go to the terminal board on the unit itself and take a reading between Y R and G you should have no reading at all in any combination since all of these should be 24 volts, now take a reading from each one (Y R and G) to a known ground and you should get 24 volts each time. Now turn the thermostat to the off position and run thru the same procedure, now you should have 24 volts between R and Y and 24 volts between R and G and 0 volts between G and Y and 24 volts from R to ground and 0 volts from Y and G to ground. If this checks out your transformer is Ok and your thermostat is telling the unit to come on and working as it is supposed to. If evertything is working up to that point you probably have a safety switch that is not allowing a relay to pull in to start the unit. Have the charge level checked by a licensed tech next.

Thanks, DNT1, I performed the procedures you suggested and the result is that there is no voltage at wires either at tstat or transformer, where the expected voltage is 24. However, there is 120V coming into the transformer.

Please let me know if need more information.

Is this, by any chance, an oil fired or old gas furnace? It is unusual to see 5 wires except on oil or old gas furnaces. To what terminal on the thermostat is each wire connected?

many of the stats out there have a 24V return leg (C). if you look at the transformer and it has 120 going in, it should have 24V comming out on the secondary....usually a blue and a yellow wire. If its dead, get a Mars 50VA transformer with a reset. OH!, look at the transformer, it might have one either a small white rocker or a red stick like comming out of it...and reset it... if you can only get it without the CB then get an inline fuse holder and a 3 amp pack of 5 fuses (in the electrical fuse section of HD), and install inline on either leg of the transformer secondary in the event it blows again it will cost only a fuse and protect the transformer. These go out during lightning storms quite a bit.

Bravo, bravo!!! I found what the problem is. The transformer was not dead. For some reason, the L1 got loose and of course there was no power coming out. BTW, the transfomer model is R8239A1052.

The problem was resolved and the AC started right away after I had the L1 connected.