I have a central AC unit that is old but still cools very well. I would guess it dates from the mid to late 70's. I want to keep this one in service a bit longer for reasons on which I will not elaborate. I am not sure of the manufacturer but I can provide information from the service plate if need be.

Late last summer it became hesitant to start. I could hear, outside, the contactor closing and opening several times and finally the compressor and fan would start, run for less than a minute and stop. After a short interval it would repeat this. After three or four attempts it would start, run and cool.

I shut off the power and had a look at the unit. I took some pix, drew my own wiring diagram because the old one had been used as a mouse nest and disconnected the few devices that were easily removable. They were two run capacitors and the contactor.

I replaced these items with new from a local HVAC supply. But now the AC would not even try to start when I turned it on at the thermostat/control.

Back outside I smelled something burning. Power off. I realized the 240v-24v transformer was overheating. Back at the supply house I got another multi purpose transformer.

Like a fool I wired the new transformer incorrectly. I bought another transformer. It is a White-Rodgers 90-T60C3. I got this because it has an integral circuit breaker. This time I made sure to use the 240v and common wires.

Still, no start. The integral circuit breaker tripped.

With the control circuit off and power to the unit on I am able to start the compressor and fan by manually closing the contactor. So I know they work.

Inside the house, I can also close the relay for the furnace fan manually and it runs. So I know it works.

That leaves the control circuit. The control unit itself is also quite old. It's analogue, has the Kenmore brand name and it is only used for the AC as I have hot water heating with separate controls.

I also happen to have a newer Honeywell control which I believe will work for this purpose. But I first need to determine what is the problem with the control circuit.

Is it the control wiring? The control itself? The relay coil at the furnace? How do I check these things?

I have checked the outside unit with the multi meter and it's reading in the 220v-240v range. Likewise, the transformer is providing 26.3v which is within the operating range of the old control unit.

That's all I know at the moment. Any advice is greatly appreciated.

 

Your trying to trouble shoot a 45 year old unit. Hmm.
Time to call in a Pro.
First thing there going to tell you is it should have been replaced at least 20 years ago.

 

Welcome to the forums.

Your original problem with the contactor chattering several times and then latching is more than likely caused by low refrigerant. That would have been a basic recharging.

Now you're in the middle of wiring that we can't see.
Your transformer is outside..... is this a packaged unit ?
You're going to have to draw a diagram or take pictures of what you have as it sounds like I won't be able to find any service info on it.

http://www.doityourself.com/forum/el...rt-images.html

The 24v goes to the thermostat and the stat sends back 24v to the blower control relay and the contactor outside.

 

I don't know what a packaged unit is so I am unable to answer that. But, yeah, a couple of other people (who have not been much help) seem surprised that the relay is 240v to 24v and is outside. Since it is not 120v and not at the furnace they are reluctant to help.

I can try to describe what's there and I'll see what I can do about providing a diagram. Would the info on the service plate help you ID this unit?

About low refrigerant...Pete, I can rig the contactor so that it is closed and likewise the relay at the furnace and it will run and cool. I have to turn it on and off at the breaker box. But it gets downright cold. Can low refrigerant cause the chattering and still cool the house?

 

The 24v control circuit is run thru a low pressure switch inside the condenser. If the refrigerant is low the contactor won't close. If the pressure is on the edge of the switch specs.... you can get that chatter.

 

So, in theory, low refrigerant might be the cause of the problem.

But if the refrigerant is low or on the edge, would it cool? Because, as I said earlier, when I force it to run it cools pretty well.

And would this also account for the integral breaker in the transformer tripping?

 

If the unit is low on refrigerant, the evaporator coil can freeze up, so yes the unit will still cool even though it's low on refrigerant. You might be just above the point where the evaporator freezes up. Pete's idea of the pressure being low is a good one. That might explain the hesitant operation last summer. It's conceivable that the low pressure switch was making the contactor "oscillate". After a little bit of trying to start, it built up enough pressure to activate the low pressure switch and let it run normally.

If the unit is low on refrigerant, that should not cause the 24V transformer breaker to trip. You likely have a short in the wiring somewhere. If you know where the cable with the wires bringing the control voltage (24V) to the outside unit connects to the furnace, make a note of the wiring and temporarily disconnect the 2 wires going to the outside unit (at the furnace end). Set the thermostat for cooling and see if the transformer breaker trips. If it doesn't, you have a short somewhere in the wiring going outside. If it still trips, then the problem is with the furnace (possibly the control board) or the inside wiring.

 

Bob

Thanks for responding.

After Pete's reply I wondered if I might have two different problems.

I believe I can determine which wires run between the outside unit and the furnace. I was just looking at a picture I took of the fan center but I need to go downstairs and see what goes to where.

So I will try your suggestion and report back on that.

According to the service plate the refrigerant is F-22, which I probably didn't have to tell you. I gather it was the standard for decades.

Is this still used? I'm just wondering if service people will be reluctant to charge it.

 

The refrigerant is actually R22. Manufacture of R22 has been banned for a number of years now because it depletes the ozone layer when released into the air. Needless to say, the price of R22 has skyrocketed because of this. It's still available, however there is a limited supply and it's not getting any larger. In addition, you need to be EPA certified in order to purchase it. If it is low on refrigerant, be prepared to pay $$ to replenish it. Of course, how much it requires depends upon how low it is. If it is low on refrigerant, it means that you have a leak. Ideally, the leak should be found and repaired before recharging, otherwise the new refrigerant will leak out (eventually). However, depending where the leak is and how small it is, it may be practically impossible to find.

 

Well, that pretty much settles it. No sense in beating a dead horse.

I will still have a look at that control wiring. I assume it could still be used with a modern unit. Or am I wasting my time there also?

 

If you decide to replace the A/C unit (compressor/condenser unit & evaporator coil), you can do that independent of the air handler (furnace). However, if the air handler (furnace) is more than 20 years old, you may want to consider replacing it as well. The upper portion of the furnace (where the evaporator coil likely is located) will have to be removed to install the new evaporator coil, so there is some "teardown" of the furnace involved. It's an ideal time to replace the furnace, should you decide to do so.

 

I would definitely replace the furnace. It is ancient but it has not been used for heat for more than twenty years. Just as an air handler for the AC. I have always thought that when I finally get new AC I'd switch back to forced air heating. Currently it is hot water. That system is twenty plus years old and has been well maintained. But a modern furnace would be more efficient and quiet.

Thanks for your input.

 

After reading your posts I believe that you could have miswired the new contactor or you simply bought the incorrect replacement contactor simple as that. I would start there, these things are super simple but also can easily be miswired or a wrong part bought if you are not experienced. Retrace your steps back and lets check the basics. The new contactor will have a tag with the ratings. Contactors come in several different control voltages and ratings.
My guess on the original problem of the chattering contactor is that the points were simply burned up, happens all the time not unusual at all.

 

Points were definitely burned in the old contactor. I could give the new contactor info but since I have decided to replace the unit it's a moot point.