Thread: Central AC not working walk through

Had a weird issue where the AC just stopped working.
Figured I’d share the steps for those, like me, that hate paying others to be observant where I can be.

So, furnace blower is running but ac unit outside is not.
Right there you can rule out condensate overflow or bad signal coil in the furnace.
Power down everything an hr, to allow a reset and thaw, just in case.
Check main house breakers for anything popped and reset.

Power up furnace and select cool on thermostat.
No change. Inside blower comes on, outside nothing.
Rules out frozen evaporator

Pull thermostat off wall and remove the red, yellow and green wires. Connect all three together.
No change, inner furnace on, AC unit off.
This rules out the thermostat. Put it back on and turn everything back on to demand.

Now you need a meter. Go out and verify you’ve got 240 vac at your ac unit disconnect across the poles, and 120 vac from each leg to ground.

pull out disconnect and move inside ac unit.
Check capacitor capacitance is as per nameplate. If so it ain’t that.
Check wiring at contact for broken bent wires.

Now power on disconnect and carefully test for 240 at input to contactor.
Coil should pull in when unit calls for cool, so you should have 24 vac across the middle of the contactor.



In my case everything to this point checked out, but ac unit did not start.
I didn’t have 240 on the top poles of my contact even though the unit did pull in.
Tear the contactor apart, voiding warranty!! Oh no!!

Dead bug there, but that’s not to bad, dig in deeper.

That’s some arcing burn evidence and the residue of something.

Earwig bbq anyone.
Lil shit walked through the contact at the wrong moment and got smashed inside. Then became an insulator across the pole.
Cleaned it up with sandpaper, and put everything back together.

System fired right up..
Just a lil share..
Cheers men.
Hope it helps one of you out

Great info for those who have a little clue about VAC. Thanks, Mike. One other thing to check before you break out a meter is...
Most AC units outside are wired to an outlet that is outside. These outlets also have built in breakers. So be sure to check it too before taking the unit apart and getting out the meter.
Its more of a quick disconnect then a breaker but take a good look.

Originally Posted by Gasman

Great info for those who have a little clue about VAC. Thanks, Mike. One other thing to check before you break out a meter is...
Most AC units outside are wired to an outlet that is outside. These outlets also have built in breakers. So be sure to check it too before taking the unit apart and getting out the meter.
Its more of a quick disconnect then a breaker but take a good look.

Yep, mines gotta fuse. Though, I did have a capacitor go bad, once.

The bulge and burn marks made it easy finding the problem.

Replaced it for mere penny's, compared to the price of called a specialist.

Good points gents..
Also figured I might as well add that if everything checks out, and your contact works too, to check the compressor itself use your meter, on resistance setting to check the three leads going to the compressor itself.
With all three disconnected from the circuit you should have 3 different ohm readings between the wires.
Ie, between 1&2 you get 1.5 ohms
2&3 you get 2 ohms
1&3 you get 3.5 ohms, means it’s a good compressor.
So the total on one leg should equal, about, the sum of the other two legs,, rule of thumb. Anything open or far off these numbers means bad windings in the compressor after all.. at that point call a specialist..
Cheers.

Bulging caps are a good way to tell for sure. But be careful playing with caps. They store power and can bite. Badly! Enough to kill you in some cases.

You know I can drill a well 10,000 feet down, then turn and go horizontal another 6,000 feet into a seam 100’ wide and stop my bit within 10’ of my target. I can build repair or even fix bikes online for people. My HVAC system I keep that dude on speed dial. Your the man Mike

Bugs are strangely attracted to contacts. I have found relays in RV gensets completely packed with tiny dead ants who crawled in one by one through a tiny vent hole..
Go figure!