Relatively New LG Room A/C Compressor Stuck On When Turned Off

[Mark Weiss]

My new LG air conditioner, 24,000 BTUs, is just out of warranty. It replaced a trusty old Emerson Quiet Kool unit that's served me since 1984. During a recent remodel, I replaced that noisy old energy hog with the LG from Home Depot. It's in the ceiling of the basement, in a corner between two walls. Not really accessible. I used to turn the old unit on off via circuit breaker, which can be reached without a ladder. The LG operates via a handy remote control.

This unit worked fine until today. When I shut off the unit, the display went dark as usual and the fan stopped, but I could still hear the compressor humming.

I shut off the breaker to this a/c for a while and later turned it back on. The compressor came on, despite the unit being in the 'off' state. If I turn the unit on, the fan comes on and it cools the room.

I'm thinking there is a relay that controls the compressor and perhaps the contacts got welded shut so it won't disengage. If the controlling electronics is immediately behind the plastic front panel, I may be able to get to it on a ladder. It's somewhat recessed into the wall, which is about 22" thick. I'm thinking either a stuck contactor/relay or something wrong with the controller PCB.

Ideas or suggestions welcome.

ADDITIONAL INFO:

I pulled the plug and measured hot to hot 2.5 ohms. This suggests it's not the controller, but a contactor that welded shut.. the compressor is connected to LINE even when off.

 

[Dana]

Sounds like the control board is fried- not sure if it's repairable at the component level- most modern AC units have all the controls and relays on a single board. Did you search out an online technical manual for that exact model number or series?

At 2 tons the AC sounds crazy-oversized for the cooling loads of a basement, unless it's a walk out basement with a full wall of west-facing sliding glass doors or something. That's more than the cooling load of my entire 2400' house + 1600' of insulated basement.

I'm curious what the wall stackup is that makes it 22" deep? That is deep enough to become an air flow issue for many window and through-wall AC units that have air intakes on the sides.

 

[Sponsor]

 

[Mark Weiss]

Sounds like the control board is fried- not sure if it's repairable at the component level- most modern AC units have all the controls and relays on a single board. Did you search out an online technical manual for that exact model number or series?

At 2 tons the AC sounds crazy-oversized for the cooling loads of a basement, unless it's a walk out basement with a full wall of west-facing sliding glass doors or something. That's more than the cooling load of my entire 2400' house + 1600' of insulated basement.

I'm curious what the wall stackup is that makes it 22" deep? That is deep enough to become an air flow issue for many window and through-wall AC units that have air intakes on the sides.

I'm trying to determine the exact model, but there's no model number on it, and I've misplaced the manual. It's similar to the LW4001815, which we have cooling our upstairs.

The lower level is a Cold War Era bunker. The concrete is 3' thick is some parts. Complicating things is that the a/c is caulked thoroughly around every edge, so effectively, it's cemented in good. The a/c is recessed in the wall for the reason that it's the only way to get the outside air vents not to be inside the wall. Being recessed makes accessing the front very difficult. The unit was installed from the outside, if I recall correctly. The large capacity is necessary to cool our small version of an IMAX theater and recording studio and 20,000 watts of amplification and projector. The place needs no heat in the winter--the computers, equipment alone will keep it 67F when it's 10F outside. The sound system and projector will heat the room to unbearable on the coldest winter nights.

There are controller boards for sale, but they don't have relays on them. This one matches the switch and LED locations for my control panel: https://www.repairclinic.com/PartDetail/Display-Board/6871A20604A/1359522
No contactor. Just a multipin connector for logic signals. No idea where that goes.

I've no idea where the relay itself is located. I suspect it's called a PTC starter compressor relay. I take the name to suggest it's both the overload protection and the on/off control. In which case, that means there is NO protection whatsoever now.

I need to figure out exactly where that relay is located and if it's accessible from the inside front. I suspect that multipin connector on the edge of the PCB connects to the PTC and controls the flow of mains power to the compressor. That is apparently welded shut, as the compressor winding resistance is reading across the power plug hot to hot terminals. Normally I should see several hundred ohms there when the PTC is in the OFF state.

The only sticker I found under the front panel has a barcode and the following numbers:
93EX6212 and 903TASW09934

I was unable to find any search results for these numbers.

 

[Mark Weiss]

I just got home and turned on the breaker for the a/c unit. The compressor came on right away and a moment later, the fan did too. As expected given the circumstances.
After about 7 minutes, the compressor shut off normally! It looks like the relay unstuck itself. But it will likely get stuck again at some point.
Many of these relays are underrated and if switching at a peak voltage (90 or 270 degrees), the surge current can be at its highest, causing the contacts to have a brief arc, which can weld the contacts together.
As I write this, it cycled on for two minutes and off again. It's working normally at this time. But I don't trust it. I can't leave this unit on while I'm out of the house.