Accessing, Replacing Evaporator Temperature Sensor 92 300D 86 300SDL
 

Could it be that I'm the only one on this forum with not one but TWO bad evaporator temperature sensors? :D

According to PO, my 86 300SDL has a bad one (though I haven't experienced the symptom).

On my 92 300D I think I may have a bad one...though I'm still thinking through possibilities.

How do I access it? What's involved in replacing it?

Thanks.

That is one of the only A/C related switches that I have not replaced. From the description it sounds like it is in front of the evaporator, which is not very accessible. Maybe it is accessible from the glove box if you have one on the '92. I can't recall when the passenger air bag became standard.

This is the E.T.R. sensor which senses the cabin air temperature at the evaporator. 1 per car. Limited availability.

I believe it's accessible from the driver's side footwell.

I don't have the air bag so maybe I could get to it.

Maybe this is where it is on the 92 (assuming all 124s are the same):

http://w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Climate/83-646.pdf

Well that seems like it should be pretty easy. I just replaced my evaporator drains, had I known that was where it was I would have taken a picture.

How much is a new one?

BTW, what are you symptoms that make you think you need new ones? Other than BC telling you that you do.

$125 for a new one. On the SDL, it's BC telling me (I haven't yet experienced any symptoms). On the 92 300D, I've measured vent temps as low as 35F and, after driving for two hours or so, it doesn't cool nearly as well. So it sounds like the ETS is the culprit...but I'm open to other suggestions.

The symptom is 28°F. vent temperature.

This would be "other than BC telling you that you do".:rolleyes:

Don't know about the W124 but for the W126 you can access it from the driver's side foot well. I can't recall the exact location but it is effectively right near the expansion valve...maybe a bit lower and towards the center of the car. I have experience with an 84 so the appearance might have changed by 86.

On mine it appears as a 1X1X2 inch fuzzy looking insulated thing. Kind of odd looking and soft to the touch. You can't really miss it. In a sea of plastic it sticks out. It is screwed into the HVAC box with two Phillips head screws. When you remove them pull on it and the unit will come out. It has a long wire probe that extends into the evaporator which you will feel as you remove. Don't know the symptoms when these go bad but it is not usually an issue so I would think long and hard about spending $125 bucks. Maybe try a used unit from a junkyard. If the evaporator is freezing, it definitely sounds like it could be the source of your problems.

Also, be careful when putting it back in. The two screws go directly into the HVAC plastic so thread the screws carefully.

If you are really getting 28 degree vent temps maybe you should just leave it all alone!!! That is pretty chilly

Except
 

28 Degree "F" vent temps positively assure Far Below FREEZING TEMPS @ the
Evaporator.
(Hence,the lower transfer performance after protracted operation.)

Southeasterngeorgiarednecktranslation:
Y'all won't get 28 "F" vent temps for long before the evap freezes.

I meant I haven't measured the SDL....just taking your word for it. :)

On the SDL, I haven't really had a problem. I can regulate the cabin temp with fan speed and vent positions. Haven't taken it on a long trip yet...first opportunity will be in August.

In fact, if it weren't for long trips, I think I really prefer the cold vent temp.

On page 233 figure 6 there is a picture of the ETR location. It is a very large file.

Those vent temps are from the 124 not the 126.. Did the 126 experience the same symptoms?

I just trying to learn what the symptoms of a bad E.T.R. are, no reason to get your panties in a bunch.

Brian could speak to the issue more precisely, but my recollection from talking to him is that, after some period of time (45 minutes?...Brian could tell us) vent temp would begin to rise to 50F. So apparently it would still continue to cool, just not as well. And it would only have this problem if ambient was below a certain temp. (80?) My 124 won't get as low as 28 and has the problem with a relatively high ambient.

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

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.

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.

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.

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.

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?

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).

:confused:

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.

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.

OK, so it seems all is OK. Who can I talk to for help with my Mercedes hypochondria? I can't seem to locate that in the DIY links and resources. :D

44° is correct for a good/normal system in a 124 chassis.

59° after 30 minutes (even in a 126) indicates that something is wrong, IMO... overcharge, undercharge, lack of airflow at condenser, something. That's worse than expected even from an R-12 to R-134a conversion. One of my old W123's (which should have had basically the same AC system) was converted to 134a and that got into the mid-40's at the ducts with ambients near 100F. I'd ditch the 134 and try a blend like ES-12a (or similar), which should perform much better, and at a reasonable cost...

:blink:

Remember the ambient............100°F.

A 40°F. differential is the generally accepted limit for this system (with a 10 year old compressor, but close to full charge of R-134).

If we expected 100°F. on a daily basis, I'd switch it back to R-12 or use propane.

Yes, I saw that, but with recirculation, you can maintain ~45F vent temps (with vehicle speeds above 30mph) even in 100F ambients. My cars do that all the time. In the 124, the recirc is triggered automatically when the control unit determines it's needed, or it can be triggered manually via a dash switch. I'm not sure how the 126 controller works.

The temp differential you mention assumes that the inlet air is equal to the ambient, which is rarely the case, unless your main air flaps are not working.

:boat:

I suspect that I have main flap problems on the SD. The system is not functioning as designed and I haven't had the time to properly troubleshoot it.

I need to understand the function of the recirc on the W126 and confirm if it is doing the job.

I'd love to see 50° vent temps at 100° ambients............!!

That's interesting....I didn't know that. Any idea what the conditions are for automatic triggering of recirc?

I'm not sure, but I think I read this somewhere in the FSM (in the climate control manual)...

:stuart:

Right you are....it's on p. A-18 here: http://www.w124performance.com/service/w124CD1/Program/Climate/1986.pdf

Well, you prompted me to do some research on the W126. Apparently, the first generation W126 requires the operator to move the temperature dial all the way to the click stop to engage recirculation. In this mode, the main air door is closed 80% and the scoop is open.

The second generation has a switch on the panel to select recirculation.

I need to test this to determine if it's effective.