Compressor unit won't start, possible lack of 24V signal

[Pianoman2]

I have a Nordyne S3QA-048Ksplit system air conditioner unit. I have installed a new fan (in the outside unit), a new contactor, a new fan capacitor, and had the run capacitor checked (it was fine). I also replace the wall thermostat. I have no voltage tester, but everything I was able to check with a continuity tester worked fine. When I set the thermostat to cool, I got a closed circuit from the red wire to the Yellow and Green wires at the furnace control box, which seemed to indicate the appropriate leads would be hot when the thermostat was set to cool and the fan on auto. When I checked the heat setting the circuit showed closed from the red wire to the white wire. The grey wire, I presume, did not light up due to the furnace controller not allowing the main indoor fan to come on without the heat section reaching appropriate temperature. I performed the above continuity testing with the furnace power off, of course, simply to check for breaks in the thermostat wiring from the wall thermostat to the furnace control box. I then checked continuity in the thermostat wiring circuit to the compressor unit by disconnecting and shorting the red and white thermostat leads at the compressor end and testing for closed circut to compressor unit and return, and it lit up. So I do not have any discontinuity switch or wiring issues to this point. After restoring power to the indoor and outdoor units, the heater section worked, and the indoor fan ran. The indoor fan ran when the fan switch was set to on. When the fan was set to cool the indoor fan would run but the compressor and outdoor fan would not come on, no buzzes. no nothing. Yes, I did reconnect the outdoor fuse. I also jumped the contactor by depressing the spring loaded switches operated by the 24v inputs from the anti-short cycle time, and the compressor and fan came on. Would it be advisable to bypast the ASCT and connect the Yellow and black wire (from the Yellow and RH wall unit sends which make a pitstop at the furnace control box) so see if the unit starts or do you have a better idea or a more likely culprit? I have no way of checking for the presence of 24 volts at the coil connections when the unit is on, no VOM available.

Thanks.

Sorry for the long winded explamation but though it was necessary to include all the changes and symptoms. I suspect the ASCT, but is there a better place to rule out first??

Praying for enlightenment>>>Pianoman2

[Robert]

Hi,

IF you can look at the ASCT and see the two wires that would be the contacts when timer closes, tie them together and see if unit runs.


Be SURE it is those two wires to each side of the open contacts.


The yellow from wall t-stat to furnace control box, then whatever it connects to to go to outside condensing unit ASCT.

THAT wire and other side of contact wire to bypass.


IF it runs, leave that IF and only IF you do not foresee any storms or possible power interruptions until you can get a new ASCT.


IF power is interrupted, turn t-stat OFF immediately for 5 minutes.


IF you leave home, turn t-stat off while gone.


IF your unit stops and starts too quickly without the ASCT, you can lose a compressor at the blink of an eye.


We have the ASCT if needed and can give ordering info.






IF you tie those two wires together and still no go, follow the two wires (24vac) back under home and as far as you can looking for any breaks in them.


You have 24vac circuit or the indoor blower would not run in fan on position and your outside components work, so you either have a common bad ASCT or a break in the wiring.


After you're done, buy yourself a cheap , but useable multi-meter at Lowe's, Home depot or Radio Shack.

All homeowners need one handy.



Thanks,
Robert

[Pianoman2]

Many thanks, kind sir. Discovered also a couple of things I did not know-apparently some newer digital thermostats come with a device called a "compressor lock" which I am told will take care of the compressor protection issue if I choose not to replace the asct, which did turn out to be bad. Also in reviewing the compressor in general I discovered that the fan needs to be reversed to run in residential applications, a fact that went unnoticed until the unit actually ran. Got some help from a local HVAC guy as well, who actually spotted the reversed fan. As it turns out I also needed some Freon. Lots of work, still needed professional assistance, but cut down on a lot of expenses and made the whole ordeal affordable. Again, muchas gracias, amigo!!!

[Robert]

Hi,

You're very welcome, happy to help and glad you got it going again.


Yep, the compressor lock/delay timer on those t-stats is same as the ASCT.


Some folks use both to have backup, but not needed except as extra insurance.


Enjoy the cool air.


Thanks,
Robert