So my AC problem is fixed, see the rest of this topic for symptoms. Here is what I did, hopefully it will help someone out there.
I can tell you now you cant diagnose AC without a gauge set (low and high gauges). It was only $40 on amazon/ebay, well worth the price. A shop will charge you double that just to hook theirs up and renting isn’t much cheaper than buying. Read up on how AC works and what common problems are and general rules on diagnosing with gauges. Remember to keep all valves closed so you don’t drain your system or cause injuries. Glasses are a must, gloves recommended – this stuff will freeze-burn you if a hose ruptures, etc.
Other things needed will be: soldering iron, core flux solder, infrared thermometer ($15 ebay/amazon) water spray bottle and computer duster can (as used in PC shops to blow dust out of PC cases).
First thing I did was to remove ATC module and reflow/re-solder the AC clutch relay (under the temp set knob). Unfortunately that did not fix my problem, as it did for others.
Next, I hooked up the gauges (close all valves) and checked static pressure – normal as for 134s chart, so no apparent leaks. Then started the van and let it run with AC on for a few minutes (doors open, fan on high) until pressure stabilized. The pressure High readings were near normal for ambient temp (see chart) so that eliminated the compressor and its valves. The AUX fan (front, behind grill) would come ON when the High pressure reached about 300psi. So, fan is good. Low pressure would occasionally dip to about 25psi, than back to 40psi (proper for ambient temp at the time). Had to restart van when I got the “fluctuation, then clutch off” symptom.
Next, removed the grill, got the thermometer and scanned the condenser for “cold spots” that would indicate obstructions inside, none found - condenser good.
Then to evap and TXV. Thankfully in my 2005 Sprinter the TXV is just behind windshield wiper motor, easy access. Checked temp delta between in and out of its evap side, was about 20F, evap normal.
Did TXV test: took the duster can, turned it upside down (freeze mode) and blasted TXV for about 5 seconds to cool it while watching the High pressure gauge: moved UP about 15psi, good. Next used spray bottle filled with hot water and sprayed the TXV for about 10s to heat it while watching High gauge, it moved DOWN about 15psi. So, TXV valve is good. No movement or very slight – replace.
That left the last component: the ATC module. I recall Midwestdrifter mentioning that the codes I’m getting were not published, so most likely internal module error codes. Bought used ATC module off ebay thinking I can resell it if that’s not it. After just plugging it in and starting the van I noticed a change, no more ATC errors. Let the van run for whole 30 minutes, no weird “fluctuation” the compressor clutch no longer stayed OFF (did cycle tho, as is should) and the temp out of outlet stayed at around 41F. After assembly of the console I took the van out for an hour long run and the AC hasn’t failed. Will keep the old ATC for spare parts, refurb on ebay was about $100. Total cost to fix: $150 plus many hours of learning and working on it. If I were to consider that most auto repair shops will replace every single component in AC system until they arrive at the problem – saved myself thousands. Hope this will help you do the same.
Thanks again to Midwestdrifter and others on this forum for their knowledge and time taken to share with us.
PS. The High pressure never went over 300psi, and this was on startup cold before the fan kicked in and would not go pass 260 with 90F ambient temp. So my old ATC MBII reading of 29bar (420psi) was also misreported. Should have gotten the gauges right away...