Think my A/C compresssor is gone...thoughts?

[Daedalus]

I have a 5 ton unit that's about 20 years old I think. It was running Wed, and I lay down for a nap. When I awoke 3 hours later I knew it wasn't working before I even got up. The attic blower was running, but the air from the vents was warm. Shut it off right away.

Both compressor and condenser fan are not working. Makes me think bad cap. BUT I also ohm'ed the start and run coils. Got 1.9 ohms on the start coil to common, and .1 ohms on the run coil to common. That seems way too low. Start to run, I do get 2.0 ohms.

Compressor is a Carlyle 5 ton, SRD600AC01.

Next step, I will confirm the contactor is working, and I will probably replace the cap, as it does look a bit poofed, but I am guessing both the cap and compressor are toast.

Can anyone confirm? Any other troubleshooting I can do on this? Wish I didn't let it run so long when it was out.

 

[walta]

The first question is the 240 volt power getting to the unit?
The second question is the contactor getting 24 volts and pulling in?

Walta

 

[PFSard]

I had the same problem a month ago. 19+ year old 4-ton(?) AC was working well. Started blowing warm air in the middle of the night. Shut it off.

In the a.m. : Dual capacitor bulging. Didn't even test. Replaced cap. Off and running. No issue since.

I'm sure someone will give the proper method for troubleshooting your issue. Good luck!!!

 

[Daedalus]

The first question is the 240 volt power getting to the unit?
The second question is the contactor getting 24 volts and pulling in?

Walta

The contactor is energizing and I get 120 on both sides of each leg when it does, so I know the compressor is getting 240. I do need to run 1 more check to see if the fan is also getting 240, as that goes through the board.

When the attic blower went out a few years ago I replaced the cap and all was good. But this time my spidey senses tell me I'm screwed, based on the ohm check, plus the cap doesn't look all that bad.

I have ordered the $20 cap, and it should be here tomorrow. If it doesn't fix the problem, then I'll be certain it's the compressor, and I'll be in it for a few grand at that point. I may decide to replace the whole system if that's the case. Getting used to making do with fans here, lol.

 

[MattT]

The contactor is energizing and I get 120 on both sides of each leg when it does, so I know the compressor is getting 240.

No you don't. If only one leg of the contactor is making it will feed thru' the motor and you'll read 120v on each leg. You need to check for 240v leg to leg on the motor side of the contactor.

 

[Daedalus]

Never thought of that.

 

[LS6 Tommy]

.1 Ohm between C & R is a shorted winding. It's toast. Sorry! I'm surprised it didn't trip a breaker or pop a fuse.

Tommy

 

[Daedalus]

Yeah, I know. I'm in denial. The $20 cap is a lottery ticket. I know I won't win, but I still have hope.

 

[gpiggaz]

Unfortunately- I had the same thing happen a couple of weeks ago at my house- 20 year old Lennox 5 ton heat pump, measured run .3ohms but didn't trip the breaker- however found the breaker to be no good too- blew the breaker's contacts. (un tripped breaker and read 100V line to line ) When I replace the breaker ( my lottery ticket) then the inline fuse blew- when I engaged the contractor manually- I knew I was screwed then. 17K later I have a new high SEER Lennox 5 ton with variable speed compressor and blower- Could have gotten by for 13K but more comfort in the house with the variable speed stuff. Got the "you need it now " price of course at least did it through Costco and will get some rebates to get it to ~14K. 20 years with no repairs was a good run for a 5 ton heat pump

 

[Daedalus]

Did you at least see a good drop in electricity usage? My current system has a 9 SEER. A 14 or even 19 SEER should save me some money, but if a new system is $14k I will probably be better off just fixing this. I'm guessing all AC shops worth a damn are super busy right now. I might **** it up until this heat passes and hope for some better pricing. Temps should be down into the high 70s starting this week.

 

[gpiggaz]

Yes- the usage was down considerably for similar conditions- we have a smart meter and I think we're using about 10KWH/Day less energy- which should save us about $75/month on our electric bill under the same conditions when it's super hot in Tucson. Old system was about 12 SEER vs 19 SEER for the new system

 

[u3b3rg33k]

Yeah, I know. I'm in denial. The $20 cap is a lottery ticket. I know I won't win, but I still have hope.

I was in my basement when I heard my compressor trip the high pressure relief valve. what a sound! went out side and saw the fan not spinning. bought a cap ($5 online, $25 at the distributor on a saturday morning), and back in service!

you could win - the capacitor is an integral component in the system. unless you have an oil filled cap (you don't, they're physically huge and rare now), i wouldn't be surprised if yours failed.

 

[yeldogt]

Confused -- did the condenser blow it's breaker?

would not the fan still be running if the compressor went off on it's internal overload ? If the breaker blew .. that would obviously indicate something else.

A few years back I warned my neighbor down the street that his AC condenser fan was failing and he should order one up .... he did not and it failed -- taking out the compressor.

 

[Daedalus]

Yeah, I'm a bit confused too. Breaker was not tripped.

Installed the new cap, and no joy. Neither fan nor compressor came on. I verified the contactor is working (resistance across terminals goes to 0 when 24v is applied. And my non-contact voltage sensor says each input leg is hot when the contactor is open.

Verified all wiring is correct after I put it all back.

I was thinking the same thing... the fan went out first, whether due to a bad cap or just old age, and the compressor died after that due to having to work too hard without a fan to cool the refrigerant? Or maybe the cap went out for both, and things overloaded without a start circuit to get things running? I know the basics, but I'm not totally up on the logic of all this stuff. If anyone has any other plausible suggestions, I'm all ears.

 

[yeldogt]

I think you need to figure out if indeed the contactor is switching both legs and you are seeing 240 .. seeing twin 120's is not the same.


or .............. you need to wire the fan direct to check it out.

 

[LS6 Tommy]

Forget the non contact tester. That 's a safety tool, not really a diagnostic tool. You should read 240VAC from L1 to L2 on the contactor with it open and 240VAC from T1 to T2 with it closed.

Tommy

 

[MattT]

And my non-contact voltage sensor says each input leg is hot when the contactor is open.

A high resistance feed will easily fool one of those pens when the circuit isn't loaded and fool it with the contactor pulled in for the reason I mentioned in earlier post. It'll even fool a decent DMM unloaded.

Forget the non contact tester. That 's a safety tool, not really a diagnostic tool. You should read 240VAC from L1 to L2 on the contactor with it open and 240VAC from T1 to T2 with it closed.

A good (Fluke) pen is an excellent diagnostic tool. Just the wrong one for this application

You're correct the OP needs to be measuring voltage

 

[Daedalus]

Hmm. With the contactor open, each hot wire reads 120v to neutral. But if I try to read across the 2 hots, my DMM says zero. Is it too cheap a meter, or am I doing something wrong? I bought a cheap meter many years ago when my tool collection was small and I needed to prioritize. It may be time to upgrade to a Fluke.

 

[MattT]

Sounds like you might have lost a leg somewhere between the panel and the contactor. First thing I'd do is pull cover off the disconnect and check for 240V, and fuses, there.

 

[yeldogt]

Hmm. With the contactor open, each hot wire reads 120v to neutral. But if I try to read across the 2 hots, my DMM says zero. Is it too cheap a meter, or am I doing something wrong? I bought a cheap meter many years ago when my tool collection was small and I needed to prioritize. It may be time to upgrade to a Fluke.

Could be a good sign. Start searching the power feed

FYI ... some of the outside disconnects have those long fuses in them. If so -- one could have blown.

 

[acmikee]

open your disconnect next to the unit and check your fuses.

L1-L2 =240 or each side of the fuse if you read 120 then that fuse is blown

 

[Daedalus]

The plot thickens. Double checked my DMM on my welding circuit, and it read 237V. So the DMM is good. Looks like the panel for the AC is coming down so I can really get to the bottom of this. I wonder if it's possible the main panel has a single tripped breaker before the sub? With a shorted compressor that might explain the 120v everywhere. The sub panel has dual 15A breakers for the compressor and fan, but they are not tripped.

 

[TRWham]

A 5 ton unit will need way more than 15A at 240V single phase (typically more like 50A MOPD). Are your sure you have identified the right circuit?

 

[Renfrick]

Do you have a circuit breaker outside for a disconnect?
You should have 4 wires out there.
2 are line
2 are load. Look closely.
Check across line side
You should have 240
Check across load side
You should have 240. If one has 0 that’s where your problem is. You have a bad disconnect.


Sent from my iPhone using Tapatalk

 

[Daedalus]

A 5 ton unit will need way more than 15A at 240V single phase (typically more like 50A MOPD). Are your sure you have identified the right circuit?

Good point. I will pull the panel down when I have time and investigate. I'm just going off the markings.

Subpanel has a dual 50A breaker labeled "A/C", and also separately side-by-side 15A breakers labeled "FAN" and "COMPRESSOR". The labels could very well be wrong.

There is a disconnect box between the sub and the fan unit. It is bladed, but no fuses.

Sorry this is such piece-meal info, I am furiously trying to finish another project and this is a lower priority right now. Thank you for all the help.

 

[u3b3rg33k]

if you really had separate breakers for the fan and compressor, both would be double wide breakers (fan is 220V, compressor is 220V). my guess is one is the furnace (typically they're 120V), and the other is probably a random mislabelled circuit.
or maybe someone wired up the outdoor unit to two single breakers - if they're next to each other it would work unless one of them tripped. it would be wrong do to this, but...

 

[LS6 Tommy]

if you really had separate breakers for the fan and compressor, both would be double wide breakers (fan is 220V, compressor is 220V). my guess is one is the furnace (typically they're 120V), and the other is probably a random mislabelled circuit.
or maybe someone wired up the outdoor unit to two single breakers - if they're next to each other it would work unless one of them tripped. it would be wrong do to this, but...

X2. you could technically use "side by side" breakers to feed the condensing unit, but they should have a mechanical tie so they both trip. IDK if it's legal by code to do that.

Tommy