<When running in HOT weather, in bumper-to-bumper traffic, occasionally my engine temp climbs above the A/C compressor turn-off temp - turning off the compressor. When the engine temp cools back down, the compressor turns back on. He and I agree that it's most likely my ETR.

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Well, not to disagree, but if the temps in traffic are getting to a/c cut-off spec of temp sensor, then the problem is an engine coolant temp problem..the comp cut-off is working as designed..and if one has a hot engine ,the ac system is less efficient than normal temps, so how do you justify the ETR is at fault,,that just senses evap temps..which would not be low at an 'In traffic/high engine temp condition"

You want to be looking at coolant system efficiency and a/c condenser airflow for problems,,, [ my take on it].
..and be sure your comp cut-out is not the product of high side pressure cut-out at drier ; tho you said your guy has verified that one, but did he do it under condition?
The trick to this diagnosis is to be absolutely positive as to which sensor is actually cutting out the comp..I doubt it is the ETR

The reason being that I EXPECT the AC compressor to turn off at high engine temps. Seeing it turn off and then back on is the expected mode of operation.

The failure I'm trying to trace is when the engine is NOT hot, and the weather is not HOT, but the AC compressor shuts off after 20 minutes and does NOT turn back on. I am seeing a failure of the system corralated to vent temps:

At HIGH outside temps, the vent temps never get below 50, the AC never fails.
At moderate outside temps, the vent temps eventually get below 50, and the AC fails sometime after the vents reach about 47
At low outside temps the vent temps quickly get below 50 and often below 45 very quickly, and the AC fails at that point.

By failure, I mean the compressor shuts off, and does not come back on until the car has thermally reset (Turned the engine off, and let it return to close to a startup condition). If I leave the AC and blower off and drive for a while, it has no effect on the failure mode (i.e. I can drive for an hour without any vent blowing and everything will still start properly).

The thinking was that it was either the Compressor Clutch failing or the ETR in failure mode (First suggested by David S. Poole on this thread). I want to attempt to bypass the ETR to either ID it as the failure point, or rule it out as the failure point.

Very Good..
I mis-interpreted you complaint to question the hot cut-out.
I aggree..

So, is the spot I circled about right for the connector for the ETR, or am I out in left field (or do I need to take more pictures)?

The part that is circled is a hard-pastic connector between two cables. It is basically loose and accessible, and there are part numbers on it (I can go read them off it that would help - but I didn't before I came in for lunch).

I have a DMM that I will use to read the status of the ETR - once I find it - and determine if it has indeed failed; and I plan to bypass it during testing to verify that the AC works without failure when it is bypassed. I just need to know that THIS connector (among the dozen or so cables, all of which are black) is the right one.

There are 3 sensors there [heat/cabin/etr]
B10/6 is ETR ...it is a 2 wire thermistor and the wires feeding it are gy/rd on one side and br/yl on the other side. [ 2br/yl]
It is to the right of the steering column.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0MN3YU24S0UPKUQ&year=1988&make=MB&model=300-TE-001&category=R&part=A%2FC+E.T.R.+Switch

That's the one. Which side goes to the ETR?

Did you monitor the voltage at S31 to see if the ETR is at fault yet??

..if you think the ETR is cutting out the comp b/c of low temp, then just pull it off the evap and leave it hanging in the ambient cabin air..leave it plugged in, b/c it can not cycle the comp if it is not there to sense evap low temp.

The one with the 2 single wires [ that is sensor side]..the other side of the connector has one wire on one side and two on the other. [ that would be feed side]

That's it. Take off the plug and slide it out.

Ok. Spent this week monitoring the voltage drop over and resistance of the ETR before, during, and after operation and failure.

One day's typical results:
Initial ETR resistance: 7.1kOhm
initial circuit resistance (once ETR was connected): 4.36kOhm
initial voltage drop over ETR: 2.085v

Max Voltage Drop: 3.5v (this is also the point at which the system fails, roughly corresponding with a vent temp in the mid 40s)
Circuit Resistance shortly after failure: 5.6kOhm
ETR Resistance shortly after failure: 11.7kOhm

The highest voltage drop I've seen is 3.554v, and the system fails at that point.
The highest circuit resistance I've seen is 7.8kOhm
the highest ETR resistance I've seen is 12.7kOhm

Today I disconnected the ETR from the core, and let it hang while plugged in.
Vent temps very quickly dropped into the low 40s, and even into the mid 30s before I shut the system off to prevent actual damage (switched to Economy setting). Once the vent temps returned to the 50s I switched back to AC and the vent temps again returned into the 30s.

At no time did the disconnected ETR voltage change significantly.

I brought the ETR inside, and placed it in my freezer. It went to Open Circuit shortly after 40.25kOhm. The chart indicates nothing higher than 20.5kOhm (corresponding to +10C, or roughly +50F). If I extrapolate out, 40kOhm comes to about -400C (not a realistic reading, obviously).

Sitting in my Air Conditioned living room at 75F, it is reading 10.5kOhms, or roughly 72F.

How I read these results: the ETR itself seems to work, but I think the Control Unit is forcing the system to failure based on a bad reading of the temperature. Certainly the system is able to blow far colder with the ETR removed than it was with the ETR in place - but at the risk of freezing the evaporator core, and forcing the system to failure by that method.

My question: Is it possible my ETR has gone out of spec, and is leading the Control Unit to think it is colder than it actually is? Or has my Control Unit failed, and I need to replace that instead?

Do a Temp/Ohms camparison test on sensor.

How do you mean? Do you mean for me to measure the temp, and the resistance from room temp down to failure (or at least down to freezing)? Or is there some other specific test you mean?

There is a test chart that you use to verify accuracy of sensor [ Thermistor Chart}
It is in the CD , or it is here in the archieves .I posted it several times , but years ago.
If you can't find it , I will try to post later , but I am back-logged up right now..

I did that. The ETR seems to read a degree or two lower than actual. Is that a significant enough difference to cause a problem?