Rocking Compressor

[tgoods]

I've got a weird one. Frigidaire built 253.618879401, LA43406427, manufactured August 2004. Problem is a knocking sound. Had a couple of service calls but never made the noise for me.  The tenant complained enough so now it's in my basement. They want it back for a spare if I can fix it.  The problem is when it cycles off the compressor starts rocking.  It will rock enough to start banging against the inside of the casing. The banging will sometimes go on for a minute or more. It's NOT the common rumbling sound that all of us techs have seen and heard before.  Here's a video captured at the end of one it's knocking episodes.  Keep in mind that the compressor is off and not running while this is happening.



What do you think?

[krazytech]

broken spring when the compressor cycle off back pressure rocks the compressor...

[appdoc]

Thats what it sounds like to me also. Used to change those things daily for that problem.

[tgoods]

I don't think it's a broken spring because it's not acting like a typical broken spring.  This video was taken at least two minutes after the compressor had shut down and it had been banging for at least a minute before I could turn on the camera.  Every broken spring I've seen, it will bang for maybe a second after the compressor shuts down.
The owner is aware that it is probably a sealed system issue and may not get his fridge back.  I have a couple of things I'm going to try.  I'll let you know what I find.

[tgoods]

I found a service bulletin for evaporator feed noise.  This isn't evaporator noise but I figured I'd give it a shot.  The bulletin says to add some dry nitrogen to the system to increase the head pressure.  It seemed to work at first, hadn't heard the knocking all day.  Then it knocked a couple of times this evening.  It seemed to help but if it still knocks the owner doesn't want it back.  So off to the scrap yard it goes.  With scrap at 10¢ a pound I should get at least $20 for it.