Had a call to a McDonalds today. They have a 3 month old MAC 7/8 system with the two digit display and only two buttons on the controller. The freezer evap was VERY frozen up. I put the machine into manual defrost. The compressor cycles on and off on the low pressure switch maybe 10 seconds on, 10 seconds off. The LPC was set to cut-in at 30psig and out at 10psig. Being that it was cold outside the suction pressure in defrost was causing it to cut out. I manually cleared all the ice off the coil with a hose then restarted the unit and let the box get to temp. While this was happening I checked all the settings in the Master Controller and found nothing out of sorts. Once the box was nearing temp (and it did so fairly quickly), I put it back into defrost. In the past I have had Master-Bilt tell me to set the cut in/out on a reverse cycle to about 20-25 and 2-3 for 404A, so I reset the LPC to about these points and the compressor did not cut out. The coil cleared and defrost terminated, though it took nearly 20 minutes.
I am wondering if the charge is suspect? The unit has a sightglass, but I haven't found them terribly useful for these systems since they normally bubble quite a bit. Superheat is maintained during freeze, which makes me think it isn't undercharged. The sightglass bubbles during freeze, so this makes me think its not overcharged. But, defrost takes a long time and the suction pressure at the compressor is riding the edge of cutting out on the LPC during defrost even with my resetting. The only way I know to properly charge these things is weigh in a calculated amount, or a little less, from the installation manual, then add small amounts while watching the superheat on the controller. I didn't install this unit, so the only way to verify charge is to pull it out.