Ah, the old mud problem
That might make a little sense to me given the age of the chiller. I do drain the chiller every year and I haven't been getting gruesome stuff out of it, but maybe unless I agitated or blew some water through it while draining I wouldn't really get any indication of 'mud'.
I don't believe the Trane manual gives a target delta T range, but I am going to check it carefully once I make up my 2" temperature clamps this week.
Thanks for your ideas and your response on the burnout circuit cleaning thread also.