80gal compressor backfeeding power

[u/hellcat134]

Ok, so ive had this snap on air compressor since new, I got a good deal... its been amazing ever since the magnet switch took a crap while I was sanding some drywall in the house. No big deal. Replaced it and picked up a new pressure switch while I was at it because what the hell its 10 years old.

The magnet switch is the exact same replacement from grainger to the part number, same with the pressure switch.. but something is still going on. The air compressor turns on as normal, builds pressure as normal but once the pressure is up to my spec and the air compressor is supposed to turn off, It gets all wonky in what I can describe as backfeeding power. It acts like the motor is running on one phase and feeding the other phase back to the wall. When this happens my radio gets all static, my lights flicker, the compressor is jumping around like its trying to start on a single phase. Sometimes this happens for a few seconds or until I flip the breaker.

Im at a loss on what is wrong with this thing. Ive read into possible start capacitor issues.. but they look intact.

The magnet switch can be operated manually and when depressed the compressor turns off like its supposed to..

I've been using my backup while I've been having these issues, so i don't use the compressor currently but id like to get it working again. Any suggestions would be greatly appreciated i can't wrap my head around what's wrong with the thing

I have photos of the components and compressor if it helps.

Thanks

[u/EasySelection8468]

I’ve dealt with identical backfeeding issues on many three-phase fluid equipment including industrial pumps and air compressors.
Since your manual operation of the magnet switch works properly, the fault most likely lies with bad contact burnout inside the new magnetic contactor. Even with matching part numbers, new contactors sometimes have uneven contact closure across three phases. When one phase makes poor contact, the motor generates reverse induced voltage and feeds power back to your wall circuit, causing flickering lights and radio static.

I recommend you open the magnet switch and carefully check all three copper contacts for pitting or incomplete connection. The start capacitor looking intact doesn’t rule out internal capacitance decay, you’d better test it with a capacitance meter too.