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Outside unit wont go on ( central air conditioner)

When we turn on thermostat it clicks like its going on but unit outside does not go on.We replaced the contactor and when we hold down the two button things it turns on but as soon as we let go it goes off..What could it be

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Hi, yes when you hold down the contactor with a screw driver it will send high voltage to the unit and start up. What is happening is, on the lower sides of the contactor will be a 24volt coil, a wire to each side. You are not getting voltage to this coil for it to close the contactor on its own. If this system is central air with gas heat, you will have 2- small thermostat wires that tie to the 2- wires on the contactor coil. If its not gas fired, it will be a heat-pump with several wires coming from the indoor unit and thermostat. Either way, the 2 wires you need to worry about is the red and white or red and yellow for cooling coming out and connecting to the coil of the contactor, probably a blue and brown wire to each side. These are what closes the contacts on this. By using your finger it tells me you have high voltage there, that's good. If the indoor unit is running, the problem is either in the thermostat or the wires that come back from the outdoor unit to the indoor unit are not getting 24 volts. So, if the indoor unit runs, that tells me the transformer is good. Check the thermostat wire at the outdoor unit real well to see if it may have a break in the wires. If it looks good, go to the indoor unit and remove the control panel. There may be a panel switch on it that kills power to the indoor unit when removed and you may need to tape it down while you check things out. Just have it running when you pull the panel and if all stops, you will no.If it has a circuit board, look for a auto type plug in fuse 3 to 5 amp that may have blown. If you had a meter you could check the terminals at the 2 wires that head outside to start the unit for 24volts. If all looks good there, no broken or loose wires, go to the thermostat and remove the cover to expose the wiring. If its not a heat-pump and gas fired, you will have these terminals, RC, RH, with a red jumper wire across them, a G green for fan, Y yellow for cooling and a W white for heat.To see if the thermostat is to blame, take a piece of insulated wire and touch and hold it to either of the Red terminals and the yellow terminal. Have someone outside to watch the outdoor unit. If the stat is at fault, the contactor will close on its own when you jump the 2 terminals out. Make sure the indoor panel is in place first. I am almost certain this will be your problem since the indoor unit runs. If its a heat-pump, you will have more terminals at the stat. Still jump across the Red which is 24 volts to Yellow which is cooling. If the outdoor unit starts, replace the stat! This will solve the problem. Its just a matter of finding why and where you have lost the 24volt control voltage to the contactor coil to energize it, pretty easy if you follow my steps. It has to either be a broken thermostat wire at the indoor/outdoor/ or thermostat, or thermostat is faulty not closing R to Y to bring on the outdoor unit. Please keep me posted as I would like to know the out come. I am confident you can find the problem now that you know it is a 24 volt control problem not energizing the contactor coil. I will be waiting to hear back from you.
Sincerely,
Shastalaker7

Posted on Jun 04, 2010

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