That's very helpful...thanks. I hate trying to decipher those diagrams in the FSM.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 72k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

The data is all from the W126...........previously mine.

K......my bad.

If the vent temp gets down below 37°F or so, you can be certain that the evap is at or below 32°F. It will begin to accumulate ice at a rate dependent on the temp and humidity at the time.

The 45 minute stated time is completely variable and depends on the temperature and humidity at the evap.

If the ambient is only 80°F. and the vent temp shows 28°F. and the humidity is quite high (80%), I'll hazard a guess that the evap would freeze almost solid in less than 45 minutes and the resulting vent temp climb will be permanent for quite awhile until the system is shutdown and all that ice can melt.

No worries! You better than most know how things can be misinterpreted on forums and the internet in general.

What are the lowest vent temperatures that are safe at 70-90°F ambient? R-12 assuming everything else is in good working order? See answer above, Brian can type faster than me.

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1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

Brian, in your experience with the SDL, how did the failed ETS afffect ability to defrost in the winter, if at all? My son is driving the 300D. From a cooling-in-the-summer perspective, I'm not too motivated to fix it. But, there is the issue of visibility in the winter.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 72k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

With the low humidity generally present in the colder months, I never had the need to run the a/c.

The warmed air from the heater has humidity levels that are so low that it's a non-issue.

Then maybe the only situation to worry about would be some muggy, rainy day.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 72k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

What would happen if I simply remove the sensor for a while? Would AC still operate as it does with a faulty sensor? Testing it, getting the new part, replacing it could take a little while and I want my son to be able to drive the car (and have AC) in the interim.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 72k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I don't see the need to press the limits. If the ETS is switching at somewhere between 38 and 42 degrees, all will be good.

Remember, at about 90°F ambient, there is no way to freeze the evaporator no matter what the setpoint of the ETS. At 80F, the cooling capability is excellent and there is no real need to get 35°F. evaporator temperature.

With the sensor "simply" removed, the A/C will not work at all; the compressor clutch circuit will be open.

I shot into the center vent at the evaporator with my IR temp gun today. Outside temp was 90(°F). IR gun read 20 (°F). My road speed was 40-55 mph.

The car was recharged last summer when I replaced the A/C pressure switch. I also replaced the resistor so I have low speed electric fan when the A/C calls for it.

So do I need a new ETR switch? Or is there something else that will cause a temperature that low?

__________________
1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

I'd question the 20°F reading. That's almost an impossible air temp to achieve from a W124 AC system. maybe that was reading the evaporator (or rather, heater core) surface? The shiny aluminum fins will cause an inaccurate reading, so if you're trying to use IR, aim at the flap or the vent fins.

An IR thermometer is not really the proper tool to use, you should really be using a probe type thermometer stuck in the vent outlets, like the one below (available from Amazon here).

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I agree with Dave. Before you go and condemn the ETS, get one of those dial thermometers with a probe that can be placed into the vent.

20F. isn't possible under any circumstances other than ambients below about 50F. The evaporator would freeze solid in about 15 minutes.

I had a meat thermometer that was 5" long so I tried that. It didn't quite reach the heater core/evaporator whatever the first thing I see when I look into the center vent is. I wasn't on the highway, but around town it did get down to 44°F.

Attached Thumbnails

 

__________________
1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

That thermometer is similar to the proper one used for a/c work. There is no intention for the thermometer to reach the evaporator. It's only designed to measure the temperature of the discharge air. The evaporator is a bit colder.

44° around town is excellent.

It was 100°F. yesterday and the SD wouldn't go below 65°F. around town. Took 30 minutes at speeds above 2000 rpm to get down to 59°F. That's the best it could do.

I definitely see the limitations of an R4 with R-134 for the folks who routinely see these temperatures. It's just not adequate.