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  #1  
Old 05-07-2004, 10:01 AM
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Question A/C Temperature Readings

Now that the summer heat has arrived throughout the USA, I thought I'd ask you folks about the temperature readings you've been obtaining (if you've been measuring ) from the A/C vents in your Mercedes-Benz diesels.

I noticed that whenever I get to a stop light the temperature of the air from the vents started going up. It's as if the compressor stops running or something.

The best temperature readings I got were as follows:

Outside Temperature : 93F
Temperature of air from vents : 51.9F
Humidity outside : 70%
Engine rpm : 2500

All this from my 1984 Mercedes-Benz 300SD. After I got the car I had it converted to R134a as the A/C wasn't blowing out cold air. Little did I know that I had a major leak. Eventually my A/C expert found out that the compressor was just leaking like no man's business. So I just replaced the compressor and clutch and accumulator drier with brand new ones. Wanted to replace the expansion valve but looks like the engineers at Daimler-Benz stupidly put it up in the dash, so we didn't bother to replace it.

So what have you all been getting? Please specify R12 or R134a.

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  #2  
Old 05-07-2004, 10:31 AM
Fritz Yoda
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Several things ... you don't say that the air gets warm, just that it isn't as cold as it is when cruising. within reason, consider this ok. The a/c compressor spins pretty slow at idle on this car.

A 40 plus degree drop from ambient is good. With fine tuning you may be able to find another 10 degrees. (consider the expansion valve, a very important component)

R134 requires less to fill the system. I assume your tech knows this.

Does the condenser fan (in front of car) have good RPM?

My R12 85 300D has a 46 degree duct temp at cruise (95 + ambient temp) and it DOES go up some when sitting at a light idling.
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Old 05-07-2004, 10:31 AM
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Location: Olive Branch, MS
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Kinda off topic, but the expansion valve needs to be as close to the evap as possible so that it can sense the evap's temp to regulate flow of the refrigerant.

I can't give you any precise numbers, but my vent temperatures go up at least 10 degrees when I'm idling. On a day in the mid 80's, 90-100% humidity the vent temps while idling were about 70-80. Going down the road the a/c does just fine. As most people on this forum will tell you "the limiting factor of the a/c is airflow over the condensor" and when you're idling away, not much air is flowing.

FYI the car has R134a in it.

John
'79 300SD
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Old 05-07-2004, 10:43 AM
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Does your car have the 9 bladed fan installed ? If it came with the 6 blade you can upgrade pretty easily...
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Old 05-07-2004, 12:11 PM
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Quote:
Originally posted by Fritz Yoda
Several things ... you don't say that the air gets warm, just that it isn't as cold as it is when cruising. within reason, consider this ok. The a/c compressor spins pretty slow at idle on this car.

A 40 plus degree drop from ambient is good. With fine tuning you may be able to find another 10 degrees. (consider the expansion valve, a very important component)

R134 requires less to fill the system. I assume your tech knows this.

Does the condenser fan (in front of car) have good RPM?

My R12 85 300D has a 46 degree duct temp at cruise (95 + ambient temp) and it DOES go up some when sitting at a light idling.
Indeed, the temperature doesn't become warm, but goes up from about 53 degrees to about 60.1 degrees. The exact amount by which the temperature goes up depends on how long the car idles.

My condenser fan has excellent rpm actually, and mine has the 10 blade fan in it. I bought a brand new expansion valve and I intended to have that changed as well, but due to its location and the cost of the enormous labor hours required to get to it, I'll just have to pass on it for now and change it later. When I do so, I'll change the evaporator as well since they are located close to each other.

Since you get 46 degrees from your R12 system, then your system isn't that much colder than mine. I thought R12 was supposed to cool much more than R134a. Perhaps the age of your system has something to do with it.
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  #6  
Old 05-07-2004, 12:17 PM
LarryBible
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The most critical factor in most a/c systems is the amount of heat exchange that can take place at the condensor. When you idle at a stop light you are getting the least amount of heat exchange because of the minimal airflow across the condensor, so this will naturally be your worst situation and your vent temp will go up under such circumstance.

All other things being equal, you lose about 15% of system capacity when converting to 134. To get a decent vent temp at idle you will have to reverse convert to 12 or modify the system. A large parallel flow condensor MIGHT regain part of the lost capacity. If you are lucky and get a big enough condensor shoe horned in there, you could get it all back.

Best of luck,
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Old 05-07-2004, 03:21 PM
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There's one other critical factor outside the air conditioning system itself. It's the ability of the car to switch from fresh air to recirculated air. When dealing with 20 year old cars it's far from certain that the various vacuum elements and doors in the climate control system are functioning correctly. So comparing duct temperatures from one car to another is something of an apples and oranges comparison. You get a lot cooler air coming out when passing dehydrated 80 degree interior air over the evap than when suckin in humid 95 degree air.

As an example, the 123 chassis has two modes - fresh air and 80% recirc. A standard "Texas Hack" was to convert the car to run 100% recirculated air. I did this on my '84 one summer when I got tired of sweating. On a 105 degree day it was good for a 5 degree drop in duct temperatures. As I recall it went from 48 to 43 degrees - a nice little difference.

Don't know what the 126 has for recirc options. (The 124 has a three position system - fresh, 80%, 100% - and the 100% fails a lot.) Without knowing whether the vacuum elements and doors are operating correctly, you cannot determine if the a/c system itself has a problem, or if it's just being fed hotter, more humid air.

- JimY
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  #8  
Old 05-07-2004, 03:45 PM
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There is a bit of discussion about the expansion valve, do these go bad? What is the reason for changing them. I did it on my 300DT 16 months ago because the compressor failed and I changed the compressor, accum. and the expansion valve, a little of a PITA but not too bad to change, I think I needed 19mm and 21mm wrenches.
Once I had it out I couldn't "see" anything wrong with it.

I like the GM approach with orifice tubes and little filter screens to catch crap in the system.....

PS

My $52 NEW R-4 Compressor came today, an ebay special... I'll never let myself get bent over the dealer's counter again!!!
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A/C Temperature Readings-mvc-042f.jpg  
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  #9  
Old 05-07-2004, 04:29 PM
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Benzcrusher, Usually the reason they need to be replaced is exactly what happened to you...on the chance that crap from an exploding compressor or dessicant bag from the R/D gave up the ghost... and went to all the important small holes... the expansion valve being THE most important...
However, Larry and I have long suggested an inline filter be installed to help mitigate explosion scattering events...

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