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Sub Zero 690 Freezer Compressor Always On

Liam J.

Premium Member
Aug 25, 2020
Model Number
More than 10 years
I have a 20-year old Sub Zero 690/S which has been working fine, except I did notice that condenser fan rarely stops. In recent months, it started to give me "vacuum condenser" warning message. Not long after I vacuum the condenser, the warning message would come back. So I got the factory authorized service technician came out to take a look. He pointed out the compressor was running very hot so he suspected it's the compressor about to fail. Since it's 20-year old, he asked me if I want to buy a new unit replacing it or spend the money to replace compressor. Since replacing compressor is only about 1/10 the cost of buying a new one, and the unit seems to be working well other than this, I bit the bullet and had the freezer compressor replaced.

You probably have guessed, the reason I'm posting here is that the new compressor did NOT solve the problem. The freezer behaved the same, it can hold the target temperature but it keeps running all day, gave me the "vacuum condenser" warning message and got very hot from time to time (the grille is hot to the touch). Of course, I was not happy with the expensive repair. The technician came back, he couldn't figure out why it did not fix the problem. The pressure on refrigerant indicated there was no leak in sealed system. After scratching his head, he decided to replace the compressor with another new unit making sure the new one he just put on was not faulty.

Well, 2 new compressors in, the problem remained the same. By that point, service manager was involved. He suspected the problem was in thermistors or electrical circuit system. So he came out himself and replaced both thermistors inside freezer and main circuit board. I was really hoping it would fix the problem. Unfortunately the problem remained the same!

I contacted them again. The technician came out and installed a new condenser fan/motor as the last resort effort even though he did not believe it was the problem because the fan was working well as far as he can tell. I was told they have run out of options other than doing a complete freezer sealed system rebuilt. The only components they had not touched were evaporator (which is not leaking) and tubes. The technician said he would not recommend doing a rebuild because he has lost confidence on fixing this 20-year unit. He gave me 50/50 chance of fixing the problem with a complete freezer rebuild.

So after 2 new compressors, new set of thermistors and new circuit board, new condenser fan/motor, the problem is still the same. Even though I'm not happy about the expensive repair that did not fix the problem, I know the technicians have tried very hard. I'd like to get a second opinion and expertise in this forum to see what could have been missed in the whole repair process.

What I notice:
1) For some reason the freezer compressor just keeps running, even though the temperature from thermistors seem to indicate it had reached or exceeded the set target. For example, I set the freezer temperature to zero (recommended by factory). By checking thermistors inside the freezer (from touch panel) this morning, the front one was -2, the back one was -7. But compressor was still running? Why does it not stop?
2) It seem to be sensitive to room temperature. During morning/night when temperature is closer to 70 degrees, the compressor was not that hot. But during afternoon when temperature is 75 degrees or higher, it starts to get quite hot.

The technician suggested me trying to install a small fan blowing air to condenser to see if it would improve. I'm fine trying it as a small fan is very inexpensive. But before I do it, I would like to get some feedback if this is worthwhile to try?

From reading this forum, I noticed the recommendation of replacing filter drier together with compressor. I'm pretty sure the technician did NOT replace the filter drier both times when he replaced compressor. What is the chance the filter drier is not working as supposed?

Is there any recommendation/suggestion for me on what to do or how to further diagnose the problem? I still believe this unit is in relatively good shape despite of its age of 20 year. I'd really want to keep it running as long as possible if I can fix or mitigate this problem.

Thanks a lot in advance for your help! Sorry for the long posting!
One correction: I asked the technician about filter drier and he said he did replace with new one when he installed new compressor, perhaps I didn't pay attention or know what to pay attention. Therefore, I should rule out the filter drier.

After closely monitoring the freezer behavior, I found the main problem seems to be it can't get the freezer to be cold enough to cross over the compressor shut off low offset. The temperature at freezer evaporator won't go below -9 degrees and basically it saturates there. Therefore the freezer compartment temperature have a hard time go below -1 or -2 degrees which is still higher than low offset (when target is set to 0).

I also found something very suspicious -- I think it might be the partial restriction on refrigerant flow. There was a kink in the suction line close to compressor. The technician noticed the first time replacing compressor but said it had been there since new so it should be ok to live with it. Right now, I notice frost coming up centering around that kink when the compressor is working hard (as shown in photo). My speculation is that over the years, there might be some deposit formed around the kink in the line, therefore it is more becoming partially clogged? Do you think it is the root cause that evaporator temperature does not go beyond -9? Thanks


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You have the right instinct about refrigerant flow restriction. There should be no frost there. Another causes for frost on the suction line is refrigerant overcharge. Except for some of the last Frigidaire models made by GM that had a side-stream compressor cooling cap tube directly off the condenser, your compressor relies on refrigerant flow to cool itself. Without adequate refrigerant flow, good bye compressor. I leave out obvious causes, like dirty condenser, failed defrost element or defrost timer, etc.

Given the age of your unit, it's on the border of the changeover from R12 to R134a refrigerant. R12 compressors were often lubricated with mineral oil, which no matter how carefully refined, always manages to retain a bit of dissolved wax. This wax can congeal on the inside of the capillary tube over time, slowly throttling back the refrigerant flow. Hot-running compressors eventually break down their oil, and this break-down sludge restricts the filter-dryer and the cap tube.

Plan-A - The cap tube can be flushed clean with an Imperial "Cap-Check Model 1003 tool" and a 3/8" slug of lead wire that is a few thousandths smaller than the cap tube ID, plus a new filter-dryer.
Plan-B would be replace the cap tube with an identical length and bore diameter, plus a new filter-dryer.
Plan-C would be removing the compressor and dumping the old oil, flushing the entire system from the cap tube end with New-Calgon's RX-11 flushing agent, refilling the compressor with new appropriate weight mineral oil or alkyl-benzene oil per advise for Sub Zero tech support.
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