Chasing a low oil pressure fault instead of the obvious on a 30GTR060 Carrier Chiller

Note: Watch the video FIRST before reading the rest of this information as to not give away the diagnostic before watching the video. :-)

The nitty gritty:
We just got this property (our parent company bought it) and inherited this Chiller system too.
We found out that the other maintenance guys had to come over and reset the thing all the time. I probably can't blame them much for the system because I think they were coming over as a favor to fix the little things. An hvac co. was called out for other repairs.

Anyway just as we were inspecting and learning the building we found out that it had to be reset 3 times on Sunday alone and then the maintenance co. washed their hands of it since they were no longer responsible for it.
The auto dialer was calling them with the automated messages when the chiller loop got warm.

Anyway on Monday it did it again and I found that it had an alarm for low oil pressure. The supervisor told me to just call out the hvac co who's familiar with it since it was 4PM and a medical building, so no messing around.

The tech comes but after I reset it, the dang thing ran ok. He suspected maybe the oil pressure (differential) switch.

Then later that night I came out and had to reset it again.
It seemed intermittant.
Eventually by 10PM last night I was able to let it fault and the other smaller compressor was enough to maintain the coop temp by then.

I went back out this AM and dug into the unit.
I also searched and downloaded some pdf files of the system.

As I am checking it out I realize that it is faulting after a normal pump down and stop of the compressor whenever that compressor staged down and was no longer required to run.
On the next start it would not make oil pressure to close the diff. swith before the 2 minute proving time elapsed.
When that happens the compressor just shuts off without a pump down.
Then I would reset the alarm and the compressor would restart then the oil pressure would go to normal.

Hmm. It did this every time. Always failing after a normal compressor pump down and shut down. Then run after the alarm is reset.

I noticed that it went into a good vacuum during pump down which bugged me.
But no low press switch to adjust on this beast.
Then I looked for an adjustment in the comforlink processor.
Nope. Fixed pump down time.

I had ignored the bubbles in the sight glass so far because I was busy trying to figure out the low oil press fault.
It finally occured to me that the unit being under charged may be causing the excessive run in a vacuum during the pump down.

So I then corrected the charge and presto, it seems to be ok no!
Wow. Silly!

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