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-   -   Help with older A/C unit (http://www.diychatroom.com/f17/help-older-c-unit-105329/)

pete0403 05-23-2011 01:35 PM

Help with older A/C unit
 
Hi All,

My AC stopped blowing cold air yesterday. I went out and saw that the fan wasn't on in the condenser and I couldn't hear the compressor.

Pulled out the dryer fuses and put them in the AC slot while the thermostat was calling for AC and they both blew instantly. (In hindsight this was probably dumb)

Called a tech, he says that the compressor windings have probably overheated. He determined this by showing me that the relay was on (there was a buzzing from the 24V and the little contacts were pulled in) and he said that terminals 2 and 3 were shorted to ground.

He did this by scratching off a little paint on the casing of the compressor and touching the terminals with his meter. I saw the meter read 16 (don't know if it was ohms or Mohms) and he said this was basically a short because it was too low.

I have a decent understanding of electricity and I thought an instant blow of the fuses would need a direct 0 resistance short.

I'm probably wrong but the reason I'm a little iffy is the guy instantly tried to pressure me on buying a new unit even though I told him I have a newborn and we wouldn't be doing that this year.

Rather than paying another $85 for a second opinion I was hoping I could describe what he did and get some thought from experts on here.

Thanks a lot!

Doc Holliday 05-23-2011 01:50 PM

Was the capacitor checked?

pete0403 05-23-2011 01:53 PM

no. Is this something I can do? I know caps can be nasty but What do I look for with my multimeter?

Doc Holliday 05-23-2011 01:57 PM

Quote:

Originally Posted by pete0403 (Post 653263)
no. Is this something I can do? I know caps can be nasty but What do I look for with my multimeter?


Yes, but you will need a multimeter that reads microfarads, shown as mfd on a meter.

You can also test continuity (ohms) of the compressor if you're up for it which would mean you'll have to remove the top of the unit and of course do so with the breaker turned off.

Doc Holliday 05-23-2011 02:00 PM

A cut 'n' paste.

First, check all 3 points against earth. Obviously you shouldnt get a reading on any.
Secondly, Take the 3 points of the compressor and measure between them. 2 of the readings should add up the 3rd so for example the 3 measurements you might get are 6 ohms, 5 ohms and 1 ohm, This is therefore OK as measurements 2 & 3 add up to the first.

pete0403 05-23-2011 02:01 PM

okay, so assuming the capacitor is okay, did he check the compressor properly? ie: 16ohms is indicative of a short inside?

Doc Holliday 05-23-2011 02:06 PM

There is another way to check it and that is simply to remove the leads from the compressor, safe them out, and turn the power back on. If the breaker does not trip, your compressor is bad.

To see if your compressor is the cause of the tripped breaker (is your breaker tripping?) you remove the 3 wires that go into the compressor and connect to the terminals. When you have the wires off it is important to make them safe by putting electrical tape on the ends that have connectors on them as when you reapply power these ends will be live. If those wires were to come in contact with one another or touch any metal, there would be a small explosion because they would short out.

pete0403 05-23-2011 02:12 PM

Thanks Doc,

The compressor in my unit has the leads going from the compressor to the terminals so I'll just take these off and test if it blows fuses. I'll still safe them out though as per your instructions. I'll have to go and buy some more 30A fuses and check it tomorrow.

Thanks again for your very quick responses.

Edited

Doc Holliday 05-23-2011 02:15 PM

Quote:

Originally Posted by pete0403 (Post 653272)
okay, so assuming the capacitor is okay, did he check the compressor properly? ie: 16ohms is indicative of a short inside?


Did he check to ground or to one another. I'm used to seeing OL for Open Line when the windings are, well, open meaning broken.

Since you have a multimeter, test for continuity yourself between each terminal on the compressor such as stated two posts above.

You will turn the power off to the unit first and foremost by way of breaker or disconnect, remove the lid of the condenser and then remove the three wires to the compressor. With your meter on OHMS, check from terminals s to c, s to r and r to c. They should be low numbers (not double digits) between each but will not be the same number between each and again as stated above, the two lower numbers will add up to the bigger one or be very close.

If you have that then your comrpessor is more than likely still very good and yes, he was trying to sell you a unit versus fixing it.

If you have an OL or no ohm reading between any of the two terminals than your compressor is shot, plain and simple.

Doc Holliday 05-23-2011 02:18 PM

Quote:

Originally Posted by pete0403 (Post 653279)
Thanks Doc,

The compressor in my unit has the leads going from the compressor to the terminals so I'll just take these off and test if it blows fuses. I'll still safe them out though as per your instructions. I'll have to go and buy some more 30A fuses and check it tomorrow.

Thanks again for your very quick responses.

Edited

Yeah, if it's blowing fuses than more than likely your compressor is grounded out meaning bad, sorry. Good luck and let us know what happens.

Doc Holliday 05-23-2011 02:30 PM

Quote:

Originally Posted by pete0403 (Post 653279)
Thanks Doc,

The compressor in my unit has the leads going from the compressor to the terminals so I'll just take these off and test if it blows fuses. I'll still safe them out though as per your instructions. I'll have to go and buy some more 30A fuses and check it tomorrow.

Thanks again for your very quick responses.

Edited


Or if you're mechanically enclined you can by pass the fuse box by turning of the breaker in the electrical service panel to the condenser, remove the line and load side of the fuse box wires and wirenut each respectively to itself (tying through) , remove the terminal wires from the compressor, make them safe and then turn the breaker back on. If your breaker does not trip then your compressor is bad.

A fuse just protects the breaker from tripping but they both act the same so in actuality you don't really need those fuses, you have a breaker. Nowdays we remove those fuses and add disconnects so only the breaker would trip.

Good luck!

kb3ca 05-23-2011 02:59 PM

Some units have an external crankcase heater located in a sleeve just below where the wires connect to the compressor. They sometimes short to the compressor case due to corrosion, kicking the breaker. They ere generally wired to the contactor, so, if you have a schematic you can see which wires to disconnect from the contactor. Disconnect the CCH and then reset the breaker to see if the unit will run. Make sure the power is off before messing with the internal wiring.

pete0403 05-23-2011 03:24 PM

Thanks...

Doc, I only have fuses. No Breaker at all, I must have used the wrong word somewhere in a previous post :001_unsure:

So I won't be tying through the fuse since that will probably burn my house down :laughing:

Doc Holliday 05-23-2011 03:34 PM

Quote:

Originally Posted by pete0403 (Post 653347)
Thanks...

Doc, I only have fuses. No Breaker at all, I must have used the wrong word somewhere in a previous post :001_unsure:

So I won't be tying through the fuse since that will probably burn my house down :laughing:


You sure about that? You don't have an electrical service panel?

http://www.renovation-headquarters.c...rvice%20cr.jpg

http://3delectrical.com/wp-content/u...3498XSmall.jpg

Doc Holliday 05-23-2011 03:37 PM

http://buildingspecs.us/images/inspection/7.gif


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