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Old 08-29-2010, 07:09 PM   #1
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contactor/relay issue???


Greetings!

I have a split A/C unit that has me perplexed at the moment. When I turn the thermostat down the 24V contactor-relay (mounted on the outside unit) closes and the compressor and condensor both turn on. After about five minutes of run time, the relay starts to randomly open and close. Not "chattering" just random. Maybe twice a second, maybe once every 20 seconds. Totally random. This on-off-on-off cycling eventually causes the compressor to overheat and it eventually thermally trips and shuts off. I confirmed that the relay is losing it's 24V supply voltage from the circuit board mounted in the blower unit in the attic. Strangely, this only happens after the compressor has been running for 5+ minutes.

Now, I have discovered that if I pull the 240V quick disconnect first (isolating power from the compressor & condenser fan) and then turn the thermostat down to command air conditioning, the 24V relay will close and stay closed indefinitely until the thermostat commands it to once again open (temperature satisfied). The exception to this is if I push back in the 240V quick disconnect the compressor runs for another 5 minutes or so and then the 24V relay starts losing power again.

So, my hunch is that the circuit control board mounted in blower unit in the attic is going bad. I checked the circuit board and the transformer and they both have a steady supply of 24V AC accept for the wires that go down to the condensor/compressor unit.

If I manually hold the relay contacts closed the system works great. I just can't figure out why the supply voltage to the relay keeps cutting out?

Anyone have a better diagnosis?

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Last edited by cactuskid; 08-29-2010 at 07:11 PM. Reason: accidentally clicked on polling
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Old 08-29-2010, 07:22 PM   #2
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contactor/relay issue???


Any float switches between the air handler and the AC?

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Old 08-29-2010, 08:11 PM   #3
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contactor/relay issue???


I don't believe so. Just ordinary telephone type wire run from the Condenser unit to the evaporator unit. Looks like a direct run.

I forgot to mention that I have a two story house and the upstairs unit has been working fine albeit a little harder trying to compensate for the broken downstairs unit. So I got to thinking and wired the 24V AC supply voltage line from the good unit to both unit's relay/contactors in parallel (I disconnected the 24V supply voltage to the bad unit) and now am basically commanding both condenser/compressor units to run from the upstairs thermostat. So far so good. I'm sure the lights will dim a little when both units come on at the same time tonight. BTW I turned on the blower fan on the bad unit (downstairs) to run continuously so I don't accidentally freeze the evaporator since the downstairs thermostat isn't in control anymore. At least both floors are cooling now.
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Old 08-29-2010, 11:40 PM   #4
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contactor/relay issue???


OK. So my jump wire idea seemed to be working and I left both units running. After an hour or so, I checked the compressors and found ice/frost on the bad unit (it was still running though). Inside the house, I noticed that the problem unit's thermostat was blank and the blower fan had quit running. A quick trip to the attic confirmed that the 110V/24V transformer failed. The 110V side has no resistance at all. The 24V side has less than 1 ohm but it does have continuity between those leads. No continuity between the two 110V leads. So, originally being an intermittent problem, I am now thinking that maybe it was the transformer all the time and not the circuit board. It's quite possible that when I tested the transformer earlier that it was working OK. Not sure but I think I will replace the transformer first and go from there.

It is my understanding that if a device on the 24V side shorted it would have cooked the 24V side of the transformer and not the 110V. Is that correct or just a myth? I've read this more than once.
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Old 08-30-2010, 05:48 AM   #5
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contactor/relay issue???


Depends on what fried the transformer.
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Old 08-30-2010, 07:12 AM   #6
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contactor/relay issue???


Quote:
Originally Posted by cactuskid View Post

It is my understanding that if a device on the 24V side shorted it would have cooked the 24V side of the transformer and not the 110V. Is that correct or just a myth? I've read this more than once.
It's the opposite. When the 24 volt side shorts it burns out the high voltage side.
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Old 08-31-2010, 12:07 AM   #7
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contactor/relay issue???


OK. I replaced the transformer and wired a 3A fuse in the 24V side. I also replaced the contactor relay. The unit ran for about 10 minutes and then the 24V supply voltage at the contact relay started cutting in and out again. Within a minute of acting up, the 3A fuse blew. So I replaced the fuse and clipped onto the common and hot 24V terminals on the circuit board with my meter. Again, the unit ran for a few minutes and then I heard "click, click, click" and watched the voltage come and go a few times before it blew another fuse. So my guess now is back to the circuit board. I'm thinking there is a bad relay or other component on the board itself.
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Old 08-31-2010, 06:56 PM   #8
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contactor/relay issue???


Or a bad thermostat wire.
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Old 08-31-2010, 10:33 PM   #9
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contactor/relay issue???


OK so I bought a 4 foot piece of thermostat wire and wired the thermostat to the control board right there in the attic. Fuse on the 24V side of the transformer always blows after a few minutes. I did find out that flexing the low voltage wires harness near the control board causes the 24V power to cut out. Lots of clicking from the relays on the board and some other strange stuff happening like the glow coil turning on intermittently as well as the combustion fan. I triple checked the wiring and compared it to the other unit (and I wrote it all down before I fiddled with them) so tomorrow I'm going after the control board. If that's not it, I'll at least have spare one sitting around.

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