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  #1  
Old 06-21-2005, 05:53 PM
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Join Date: Jan 2004
Location: Titusville, FL
Posts: 244
w124 300D A/C Troubleshooting

I am trying to resurect the A/C system that has not operated for a couple of years. PO reported that it had a slow leak, when he got it last recharged the shop said that a hose needed replaced. I have had it for a year or so.

Here is what is happening:
It is still R-12. Did not install new dryer yet (wanted to ID leak first). Pulled vacuum on system to 29 in Hg for about 30 minutes. It would drop about 1-2 in over 30 minutes of hold. Added R-12 to check for leaks with dye. Could not see any on visible equipment. Did find the Hi pressure schrader valve leaking. Did not have time to dig under dash yet.

System pressures at 2000 - 2400 RPM are:

Hi 250 - 260 psi
Low 18 - 20 psi
Ambient temp - 85 deg. F
Center reg. temp - 62 deg. F
Humidity in the 90 - 95 % range (FL summer afternoon).

Do these look ok pressure wise? Would the poor temp difference be due to the state of the old dryer? Also in looking at expansion valve, there is no insulating tape on it (like I usually have incured on other auto a/c systems).
Should there be?

Thanks for the help.
Lee

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  #2  
Old 06-21-2005, 06:07 PM
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Join Date: Jul 2002
Posts: 1,141
Lee,

Doesn't seem like the expansion valve is throtaling the freon correctly. Do a search on the Larrybible. He's one of the better A/C guys on the list.

How much freon have you added. It was completely empty before- I believe that system is right about 1 kg or 2.2 lbs.

If you are looking for a leak in the evaporator, I think many ppl add a florecent dye and check the condenstation water with a black light(that's my plan).

If the shrader valve is your leak point, they can be replaced with out discharging the system with a special tool. I purchased mine in 2000, and it was about $30-35. A appliance store ordered one for an R-12 system. Low side is always easy-high side valve I'm not sure if there is enough room to get at it. Was a bit tight- but I managed to do it on my W123. My W124 has both of them leaking.
Previous shop said they couldn't find a leakage... I'm going to assume his gauges were still connected. He was a very well respected Mercedes tech and checked the evaporator.
If the system was not completely empty- I wouldn't worry about a dryer.

Michael
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Michael McGuire
83 300d
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  #3  
Old 06-21-2005, 10:41 PM
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Location: Titusville, FL
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Michael,
Thanks for the info. The system was completely empty when I started working on it this weekend. I know the system holds 1.1 Kg of freon (38 oz) but am not too sure how much I added (have old tank of freon but no scale). So that was another question as to any way, beside having a scale or small cans of freon, to determine if have sufficient freon in system. I had heard in the past that you could look to the sight glass to see when bubbles were removed and I was trying to target that. The dye in the evaporator condensate is a good idea. I will have to look at the puddle.
Thanks,
Lee
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  #4  
Old 06-22-2005, 09:40 AM
LarryBible
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From the information you gave, my gut says you have two problems. The first is obviously that there is a leak somewhere and the other is likely a leaky vacuum pod on the fresh air door.

I assume that you are looking with a UV light and yellow glasses. Sometimes the dye will be only a trace and while it is cooling look in the condensation drain water underneath the car with your UV light and glasses to see if there is dye in the drain water. If not, then the evap is probably okay.

Also when looking for a leak, look for oily places. See if there is any oil around the expansion valve. BTW it does not need presstite tape.

The other problem that I think you are dealing with is a bad vacuum pod on the fresh air door. Remove the glove box and you will see a vacuum manifold called a switchover valve. Remove the hoses one at a time and draw a vacuum on them to see if you have a leak. I would not be surprised if the one or both of the top two lines on that manifold have a leak. Check them all while you're there.

The best way to check for an a/c system leak is with the system evacuated, add two ounces of R22 and then pressurize with nitrogen. This gives you the very best and most detectable way to use an electronic leak detector. An electronic leak detector is VERY sensitive to R22 and by having the entire system pressurized, you are forcing it through the leak better. You should recover your R12 if you decide to do this. After finding the leak, it is legal for this procedure to vent the R22.

Don't assume that the problem is the evaporator just because they are known to leak. There are many other sources for a leak including the expansion valve and compressor seal being the most common. To check for dye at the compressor seal remove the front plate of the clutch so you can look at the shaft itself with your UV light and yellow glasses.

Keep checking things and posting the results here as you get them. We will help you through it.

Good luck,
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  #5  
Old 06-22-2005, 09:58 PM
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Join Date: Jan 2004
Location: Titusville, FL
Posts: 244
Larry,

Thanks for the guidance. Can the compressor front plate be removed without taking off the belt? Also, how does air reculation switch factor into the equation? Did not get to work on it today due to work and rain.

last thing, I found the temp/pressure/humidity diagram for my car and all I can say is wow for the max high pressure of 30 bar (435 psi) value given for a 95 deg F (35 deg C) with 90% RH. Can the system really handle that much pressure?

Lee
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  #6  
Old 06-23-2005, 09:50 AM
LarryBible
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Air recirculation is a HUGE factor in vent temp. Think about it. If you are recirculating cabin air, you are drawing the cooled air across the evap. It will get colder and colder. If you are drawing in fresh, outside air, you are always drawing hot air across the evap. It typically can only drop the temp about 35 degrees, thus if the outside temp is 90, you will typically have 55 degree vent temp rather than the 38 degrees that you will typically have with recirculated air.

Good luck,
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  #7  
Old 06-23-2005, 11:27 AM
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Join Date: Sep 2001
Location: central Texas
Posts: 17,281
ALSO, when measureing vent temperatures ... you get lower temps with the fan on low... due to the air spending more time in contact with the evaporator fins...

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