I've been a member of this site for about a year and a half, but this is my first question. Some background: I completed an HVAC/R program at a community college a few years ago, and I've been working on med/low temp equipment mostly in ice cream shops and other restaurants, as well as some small commercial AC.
So I have a customer with an upright Kelvinator freezer
model ULG30BSP-5 serial D2083725
404a 18 oz, txv Sporlan Y1232-FBS 1/4
copeland comp KAJB-007E-IAA (3/4 hp, 115 V, semi-hermetic)
Customer complains unit not cooling and says another tech told him he needs a new compressor. Customer wanted second opinion as he didn't fully trust the other tech.
When I arrive unit not running. Before trying to start, I check windings (1.4ohms, .6ohms, no short, not grounded). Check start cap, ok. Check potential relay normally closed contacts show very high resistance so I replace. Turn on unit. Compressor runs fine. I figure other tech condemned because it's not pumping right so I hook up guages and do comp efficiency test. Pulls vacuum, but doesn't hold. Customer doesn't want to pay for new comp because unit is not that old. I explain to him that I can replace the valve plate (I know, many folks here will say just replace the comp, but businesses are struggling right now and money is tight, and I've done a few they're pretty easy on the copeland k body comps).
I decide to pull the existing charge and weigh it instead of valving off system with the service valves. Turns out there is only about 1 (one) oz of refrigerant in the system! I leak check with 200psi nitrogen, sniffer, and big blue but unable to find a leak. I ask owner for service records because I want to know what is going on here! Still waiting on them...
So I replace valve plate, leak check again, evac to 500 microns, and weigh in 18 oz 404a. Start up and pull down. Check run amps at comp R terminal: 9.9A (normal). As box gets to 0*, I start checking subcooling and superheat, even though it's not quite down to temp. Pressures 203/8, SH 16* at txv bulb, SC 5*. But (this is where it gets weird) after a few more minutes, the compressor/freezer starts sputtering, knocking, and jerking, and the inside lights flicker a bit. One of the employess then tells me that sometimes the freezer "dances" like that and it drives them crazy. It turns out this is/has been an intermittent problem for a little while.
I turn off the unit and pull the plug and notice that they have it plugged into an extension cord. The extension cord prongs are burnt and black, and the original power cord prongs going into the extension cord are also very burnt. I figure that this is causing voltage issues, so I get them a new 20A/600V cord to specs, and replace it but this one is a couple feet longer than the original so we don't need a stinkin' extension cord. But this still doesn't solve the problem of the compressor "sputtering and shaking" (I know, not exactly technical terminolgy). Its not because of the mounts, and I double checked the electrical connections, terminals, wiring.. it all looks clean and tight. Discharge line doesn't have any obvious kinks. I keep thinking it is electrical because the lights flicker just a bit when this happens and it sounds like the compressor is "surging", if that makes any sense.
Anyone have any ideas? Could the compressor have intermittent mechanical or electrical problems that cause this? What am I missing? thanks in advance...