As several other posts on here, our outside AC unit is not turning on. I've been learning how these things work and trying to diagnose but so far haven't tracked down the source of the problem - looking for a little guidance.

Symptoms: Hot house. AC unit outside non-responsive at all. When I turn the temperature down and it should kick on, I do not hear any buzz from compressor and the fan does not kick on outside. Inside the house, air circulation does start to happen, although it doesn't seem to blow very strong out of vents...I'm not sure if it was always that weak or if it used to blow harder...you have to be <1ft away to even feel any movement from vent.

What I've tried:
* No tripped circuit breakers
* Replaced capacitor outside (went with exact same model as what was in there)
* Tested voltage at thermostat - there are 24VAC between R-G, 28VAC between R-Y and 28VAC between R-W.
* When I set it to cold, the inside furnace and blower does click and kick on (but again, doesn't blow too strong, but maybe that was always the case)
* When I set to hot, we get some hot air coming out
* Looking at the board on inside unit, it has two indicator lights - they appear to flash green slowly, which based on unit should mean everything is OK
* Stick test - I can rotate the fan outside with a stick. It moves freely and doesn't take much pressure to move it - it doesn't spin very long after I poke it (the upstairs AC unit spins more after an initial poke), but it does spin.
* Looked for fuses outside - I don't think we have any. We have a circuit breaker outside rather than a disconnect. I didn't see any fuses within the AC unit outside or anything.

 

Since you apparently have a voltmeter, I would suggest that you do the following.

First, turn off the power to the outside unit, and remove the access cover (the one you removed to change the capacitor). Turn power back on, set the thermostat for cooling, and check the following:

1) Carefully check the voltage across the wires at the top & bottom of the contactor. You should have ~240VAC across the wires at one end of the contactor. Since the outside unit isn't turning on, you probably won't read anything across the wires at the other end.

2) Carefully measure the voltage across the wires on the sides of the contactor. Ideally, you should measure ~24VAC. If you measure 0 VAC, check the voltage across the two control lines where they come into the outside unit. The wires will be relatively small diameter wires, since they carry 24VAC. Don't confuse them with the 240VAC coming into the unit (larger diameter wires).

Let us know what you find.

 

Simply push the contactor button and see if that starts the unit..

 

Making progress finally!

So, pushing the contactor button seems to get it started (at least the fan kicks on, I didn't hold to see if it actually started cooling).

Taking the measurements and I do get 240VAC across the top pair and 0 on the bottom as you suggested. However, I don't appear to be getting anything on the side wires. I'm guessing the 24VAC that should show up there is supposed to bridge the 240 across and make things turn on (and should stay a steady 24VAC for as long as thermostat set to cool).

So, it may not be an issue with the contactor but wiring or maybe back at controller board or something? I put the probes directly on the metal clasps that slide onto the contactor from the 24VAC supply to make sure it was the wire having issues and not something else on contactor.

 

Yes, the contactor is actually a relay. When the thermostat calls for cooling, it sends 24VAC to the contactor which pulls in the contacts (what you pushed in to make it turn on). There are several possible reasons why you don't have 24VAC at the contactor. You could have a break in the wire going from the furnace to the outside unit, or your outside unit may have a high or low pressure switch which is open.

You should check for 24VAC where the low voltage wires come into the outside unit. They should be connected to the internal wiring using wire nuts. If you don't have 24VAC coming into the outside unit (when thermostat is calling for cooling), you could have a broken wire from the house to the unit, or your furnace control board could be bad. If you don't have 24VAC coming into the outside unit, check to see if you have it where the wire connects to the furnace control board.

 

I checked outside and assuming I checked the right wires (the only two small ones coming in and only with nuts) it looks like only 4VAC is coming in (I did not undo the nuts but can if that makes a difference, I just stuck the probes inside the nut where some copper was visible and think I still had a good connection for testing).

I tried to trace back to the furnace but am getting lost at this point. If I am not mistaken, all wires (the 240VAC along with the 24VAC) come to the outside unit in a single bundle wrapped in foam and tape. Does that sound accurate? If so, inside it all goes into the top of the furnace which appears to be more sealed off than the rest (no removable panels) - there are some screws but it looks like there is silicone seal as well that I may need to cut through if I have to take that part of furnace apart to continue following the foam bundle.

The bottom of the furnace is more accessible, where the control board is. However, it's a jumble of wires and I'm not sure which ones ultimately end up as the 24VAC that should appear outside. Is there something I could look for on the board that would guide me towards the right wires? It was a pair of red/white wires outside, but unfortunately there are several of these visible inside.

Is there supposed to be a direct run for this pair essentially from the board to outside, or is there something in between? We also have an aprilaire 550 humidifier, which I'm not sure affects the configuration or not. It does have a pair of red/white wires that branch off to it from the board, but that may not mean anything. I'm also not even sure if that humidifier part works, it looks fairly old and I've never messed with it (was here when we bought the home a few years ago).

I also didn't see anything coming from top of furnace into the bottom of furnace to try and trace from there. It looks like there could be some wires behind the blower (or what I think is the blower) in the back of furnace, possibly getting routed to the top half. It's just buried deep in the furnace so hard to track down and I would prefer to not have to start taking things apart to follow if possible.

 

Superman, the wire going to the outside unit should be a direct run (single insulated "wire" with multiple conductors). It may be bundled with the 240VAC wires (depends upon installer preference). The outdoor wire should connect to the control board in the lower portion of the furnace. Can you follow the wire where it comes into the house from outside and see where it connects to the furnace? Also, if you give us the make/model of your furnace, we may be able to find some installation info online. Or, if you have a copy of the installation manual, post a picture of the wiring diagram.

 

I will have to try and track down model of furnace when I get home. There is a wiring diagram on the inside of panel, but I don't have the manuals. The outside unit I believe is a Lennox H26 series (which specific, I'm not sure...it just says H26)

Following the run is the hard part. I'm pretty sure it's all packaged together in a foam wrapping (there is one big bundle going from the furnace to outside wall where outside circuit breaker is), but I didn't cut the foam off to verify. On the furnace, this is the part that runs into the top of the unit that doesn't seem as accessible, which is why seeing where the individual wires actually terminate inside the furnace is difficult (for me at least). All you can visually see (after removing front panels) is the bundle coming into sealed top of furnace, and then a bunch of wires around the control board - how the wires at the bottom get to the top of the furnace where the bundle comes in is unclear - probably hidden behind blower / etc in back of furnace. But, I figured, if on the control board there is something I can test for 24VAC, then I know the issue should be between that part of control board that relays 24VAC and getting outside.

 

Usually, on the control board, Y and C wires go to outside, check the voltage between control board Y and C terminals (while the T-stat is calling for COOL).
and you should see 24V.

 

Making progress again!

Indeed it looks like the C and Y wires were correct. They measured 24VAC and just so happen to have a pair of wires running to them that align with outside (others use same colors). Following that pair back and it's actually not in the foam bundle I thought, but was hidden away alongside it and runs outside.

So I double checked outside - I took the wire nuts off this time and measured directly - it read ~24VAC when coming into the unit outside. Put nuts back on and still reads 0 at the contactor.

It looks like the only thing in between where they enter and the contactor is one of the incoming wires connects to a sensor (or something) on the copper pipe feeding the compressor. I'm assuming it is a sensor for low refrigerant, and thus that may be my problem. Is this an accurate assumption?

 

It sounds like you have a pressure switch lockout.

If you replaced the capacitor that serves the outdoor fan, you may have already corrected the cause of the high pressure switch trip.
(Herm, F and C on a dual cap would indicate that it would also serve the outdoor fan)

Does your pressure switch have a reset button on it?

 

I don't believe there is a button. A little googling and it sounds like it is a automatic pressure switch (Texas Instruments PS80). It's the silver/black item with the two yellow wires running to it.

As for capacitor, there are no guarantees that was original problem but if a bad capacitor can cause the switch to fail (and this one isn't resetting automatically), maybe. I just tried that because it was one of the common fixes I found online and pretty cheap and easy to change out myself. The original didn't look really bulged or anything but wasn't brand new looking either (at least had a bunch of rust on the pins).

Also, is it a concern that the large pipe feeding the compressor looks nearly solid black in the picture? I just noticed that when uploading these pictures. Not sure if it is indicative of anything or not.

 

My picture and your diagram have both a high pressure switch and a low pressure switch. Post a picture from further back. The high pressure switch is toward the left if it is present.

 

Thanks! I went out to snap a few more photos but saw the other thing you must be referring to not shown in the picture, and it had a button on it. Pushed the magic button and it all started working!

So I'm guessing that was the high pressure switch with the button, and the Texas Instruments PS80 shown in previous photos is a low pressure switch or something else? Actually referring to the photo, the close up of the PS80 also shows two blue wires running to the bottom of unit - those actually connect to the switch with the button which is sort of visible in photo - it's black with a gold button on top.

What causes these things to need that reset button pushed? Since you mentioned it may have been related to me changing the capacitor - is it that the capacitor may have gone out causing the fan to stop running or something, and then the compressor wasn't cooling properly so the switch kicked in? Just trying to understand for future reference.

Thanks again everyone though. House is back to cool and has been working since yesterday afternoon without shutting off again, so maybe it's here to stay.

 

If the condenser fan isn't running, the condenser coil doesn't cool as well (no airflow through it), so the pressure runs higher. It reaches a level where the high pressure switch opens up to prevent damage to the system. It has a reset button to reset the switch after the problem (what caused the abnomally high pressure) is corrected.

 

Yep, If the old cap was bad it could have caused the fan to stop which would trip the high pressure switch.

The picture that you posted was the low pressure switch.

Looks like Bob called it in post 2.

I am glad that we were were able to help you get cold air again.