HVAC help request - Central A/C troubleshooting

We've not needed the A/C much this year, but had it on most of the day a couple days ago. In the afternoon, the outdoor unit seemed a bit noisier than usual, then a bit afterward I noticed the lights inside flickering a bit when it cycled - about every 10-minutes. An hour or so later and the house seemed warmer, and after a bit more time I realized the A/C definitely wasn't cooling.

I should add that we just bought the house in January, and although the unit is around 10-years old it passed the inspection before we closed...not that it all means much, other than that we've not used it enough to really get a feel for what's "normal".

My extent of troubleshooting so far has been to confirm the fan on the compressor is running, no breakers were blown, and that the thermostat seems to still be controlling the operation - adjusting the temp above ambient, or switching it to heat or off, will cycle off the compressor. What else can I test/try before calling in a pro?

In reply to petegossett:

Can you tell if the compressor is running? (Not the condenser fan but the compressor itself). Is the blower in the house running? (air blowing out the vents)

I'm presuming it isn't, based on symptoms, but not really sure how to tell? The indoor blower is working fine.

Oh, and can one of our admins move this to Off Topic please?

In reply to petegossett:

Is this a heatpump of an air conditioner? I'm confused by you saying that its not cooling, but the compressor runs when on heat? If its a heatpump and it puts out heat and not cold it could be a stuck reversing valve.

Do any of the copper lines feel cool to the touch when its running? You want to make sure the compressor is actually running, not just the condenser fan. The lights flickering indicate a high amp draw when the compressor starts, you may wish to put a hard start capacitor kit on it.

You can hear the compressor when it kicks on. They have a very distinct sound. Kind of a deep humming/buzzing sound. Hard to describe.

If the compressor is not coming on it could be a few things, could be a simple burnt or disconnected wire. Could be a bad start/run capacitor. Could be a bad compressor.

If the condenser fan and indoor blower are coming on that tells me control wiring and stat are ok. Contactor is probably ok.

I would start by turning the ac on and turning the temp down. Go outside and see if the compressor is running. If not, pull the disconnect. Open the cover and look for any burnt or disconnected wires. Follow the wiring from the contactor to the capacitor to the compressor. Burnt wires usually happen at the connectors where they are plugged into varius components. Look at the capacitor, is the top puffed up? If so it is probably bad. If not puffy it could still be bad. You need a tester to be sure.

If you dont see any burnt or disconnected wires you could try replacing the cap yourself. The old cap will have a value on it, will look something like 45/5 or 30/10, etc. You can get them on amazon cheap. Dont touch the terminals.

If no wiring problems and new cap does not fix then you have other problems that will require a hvac guy.

^Thats a good post! Do that!

I only know about house AC because of knowing about car ac and piecing it together, but yeah your first step is seeing if the compressor is running, and if it isn't then tracing the wiring back looking for problems. One 'weird' thing about house AC though is that unlike a car AC, the compressor will still run even if all the refrigerant leaked out, so just because your compressor is running doesn't mean you didn't spring a big leak.

In reply to old_:

Awesome, I'll run through your list & report back. Thanks!!!

In reply to Vigo:

Most will run if low on refrigerant however there are are a few models (i know of a specific lennox) that do have a low pressure cutoff, just like a car. I believe that if the low pressure cut is tripped the condenser fan will not run either. Im not familiar with every model out there so I cant be 100% sure about that.

In reply to old_:

I finally had time to do a bit of troubleshooting tonight. The compressor definitely isn't running, nor are the lines getting cool. Here's a pic for reference, I'll explain the voltage readings below.

The relay is energizing when the unit turns on, and I read ~240v across both terminals on either the left, or right, sides(only the right side has power when the unit is off).

The black wire off the lower-left terminal on the relay looks a bit scorched, but the insulation isn't melted. The other end goes directly into a control box of some sorts on the side of the compressor, so I couldn't check continuity on it with the leads I have here.

The capacitor in the top of the pic had the following readings: From top-left terminal to bottom terminal ~500v. From top-right terminal to bottom terminal ~500v. From top-left terminal to top-right terminal 0v...that one puzzled me a bit, as it seems to me that the top-left terminal is the hot into the cap, while the top-right and bottom terminals feed the compressor and cooling fan, respectively.

The cap show no bulging on the other end or other physical signs of failure.

Does any of this point toward a bad cap, or is it time to call in a pro???

Try jumping the hp and lp switches, if there, and see if the compressor starts.

In reply to tooms351:

I'm not sure I know how to identify those? I didn't see any switches inside the outdoor unit.

The larger line going to the compressor is lp, exiting and smaller is hp. There is one wire in one out. Make sure the thermostat is calling for air, and jump or test for power in and out. Remember to kill power first if jumping.

No expert here but did you check voltage at the compressor itself?

TRoglodyte wrote: No expert here but did you check voltage at the compressor itself?

There are no bare terminals on the compressor its self, and my test leads I have with me aren't sharp enough to puncture the insulation.

tooms351 wrote: The larger line going to the compressor is lp, exiting and smaller is hp. There is one wire in one out. Make sure the thermostat is calling for air, and jump or test for power in and out. Remember to kill power first if jumping.

You're talking about pressure switches in each of the lines? I didn't see any, but I'll check again. I did make sure neither of the lines got cool, and I saw the valves, but it's possible the switches were obscured by something I suppose.

Didn't know if it had a junction box on compressor.When checking electricity I usually give wires a firm tug to make sure connections are tight.power off of course. Not much experience here though.

In reply to TRoglodyte:

There is "something" on the side of the compressor. I didn't see a screw holding it on, so I wasn't sure if it was just a cover or some type of control box - I also hadn't killed the power yet, so I didn't want to go poking/prying around where I wasn't familiar, or wasn't obvious.

Did you check your furnace filter recently? I've seen them so plugged up an ice berg forms in the furnace.

If no air is moving, no cooling is happening

The lines aren't getting cold even with 240V on the output of the contactor (relay) so the unit is calling for voltage. When you touch the case of the compressor is it warm/hot? I would take the cover off of where the wires go into the compressor. It sounds like you could have a burnt wire and may need a new pig tale from the compressor pins to the relay/cap/contactor.

You can also ohm out the 3 pins on the compressor itself (power off and wires off). That will tell you if the compressor has given up the ghost as well.

You still didn't say if this is a heat pump or straight AC only

http://hvac-talk.com/vbb/showthread.php?1359601-one-more-time-on-scroll-COPELAND-terminals-blow-out-fix

In reply to paranoid_android74:

Replaced it a couple weeks ago, although it wasn't bad.

In reply to eebasist: It's not a heat-pump, it's a gas furnace + central A/C. The compressor didn't get warm, or make any noise/vibrations at all. I'll try pulling the cover after killing power & see what I find inside.

Thanks!

I never updated this thread with a conclusion, but:

eebasist wrote: ...I would take the cover off of where the wires go into the compressor. It sounds like you could have a burnt wire...

eebasist nailed it with the "burnt wire under the cover" suggestion. I was able to crimp on a new terminal, clean the male-end of the connector, and everything worked fine...until last night. I went out this morning and discovered the other heavy-gauge wire going to the compressor had burnt all the way off at the terminal, so I trimmed it back to good wire, crimped on a new terminal, cleaned up the male-end, and we're back in business again.

Though now I'm thinking I may need to consider replacing parts - I'm just not sure what's what here:

This is the piece on the side of the compressor where the terminals have been bad. What is it, and is it replaceable separately from the compressor?

For reference, here's the compressor, and the tag on it.

The tool you really need to use is in your file cabinet!

You bought the house less than a year ago so you would/should/could have a home warranty.

Time to use that to fix this problem!

Besides the warranty, I don't think you can get inside that housing. It's sealed. So, it's use what you got or new compressor, I think.

This thread is a good reminder to get a back up capacitor for the house we just bought. In my last 2 rentals the item that always broke was the capacitor.

You can't replace the terminals sticking out of the compressor, they connect directly to the motor windings. You are lucky it's just burning the wires that feed the compressor. If those terminals on the compressor heat up enough it can melt that red insulation around them, if that happens all the refrigerant in the system will blow out of the hole. At that point you have to replace the compressor (or the whole condenser)

Either you just had bad connections at the compressor or you have other problems causing the compressor to draw too much current. Many things can cause that, dirty condenser, incorrect refrigerant charge, bad capacitor, etc.

Do you have a clamp on amp meter? You can measure the start and run current the compressor is drawing. The correct values will be printed on the data tag somewhere on the condenser.

It may just be time to have a hvac guy out to check over the system.