87 300dt
I've done a good bit of troubleshooting on my A.C problem. I know the problem is not the Klima relay (thanks to this board). I thought the problem was the Klima at first and had replaced it. The compressor comes on when the appropriate Klima sockets are shorted.
I've discovered that the compressor will not come on if the system presure is too low. Thankfully, at the last recharge, they added some flourescent dye to the system.
So 11pm, dark garage, and with my Blacklight in hand, I went searching for a leak. I saw nothing in or around the center nor side vents, and nothing on the floor. Then, I opened the bonnet, and found the only glow coming from the indicator on the dryer.
Next, I decided to lift the car and inspect beneath. Bingo! I was sure that this pipe was the culprit!(see pic) So I take the car back to a shop where mechanic say's "this is NOT an A.C. part"! It was glowing brightly and sprayed the surrounding engine parts (especially the bottom of the alternator), but has nothing to do with the A.C. He mentioned that this is a tranny cooler line and the glow is comming from a water pump leak. I use the Mercedes clear coolant and mix it with 50% h2o.
I scratch my head and take his word that the problem is the evaporator, even though his searching with a black light showed no dye-leaks anywhere. Next evening I fill a cup with some of the Mercedes clear coolant and hold my Blacklight nearby, NO GLOW! hummmm!
So, before I rip the dash out, can anyone give some other suggestions or shed some light on this condition?
Thanks,
Freybird

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1987 Blue-grey turbo 300d "The Panzer Vagon"

I've heard $3000-$5000 repair cost for the evap replacement, can someone confirm?

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1987 Blue-grey turbo 300d "The Panzer Vagon"

A couple of thoughts.

Dye will only be visible as it exits with some kind of medium. Water, oil or freon. As you have done, look for traces of dye. In the engine compartment, be aware of the fill point for the system. Trace the hoses from the compressor as they go through the engine compartment. Along the hoses you will see two fittings, one on each hose. This is where the dye has been ADDED so some may have "spilled" when being added. If it is around here, be suspicious of this being a leak.
The evaporator in the dash. Using dye, the only way you can determine a leak is to look below the car ( after it has been running for a while ) in the center of the car under the dash area. You will see the dye in the water that drains from the AC system. Looking in the vent does no good.

I would suggest spending or renting a AC freon "sniffer". The tool beeps when it senses freon. It's available for under $100. Once in your hands, trace the entire system in the engine compartment , slowly, for a "sniff" of freon. If nothing then make sure you get a confirming "beep" from the sniffer while sticking it in the vent ( center and side vent. stick it in as far is you can). If the vents give you a beep then its an evaporator problem and the dash has to come out. Think $$$$$.

When was the last time the system was recharged. A small, very small leak is sort of normal in a system. So if it's been 10-15 years since is has been serviced then the system could be low on freon. But it is an old car and a leak is probably the norm.

Spend the $$ to get the sniffer and test the system. Get a good confirming beep from the vents before removing or having the dash removed.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

I jsut had my evap drains cleared and if the evap leaks it coems down the drains, which are jsut above the tranny. So there's your glo.

Evap is a HUGE PITA since you have to pull the dash apart. To pay someone shoptime for that kind of R&R is wickedly costly.

Also found UV-dye in the A/C drains pipes, so it confirmed that the evap was leaking. Same as WINGAS found.

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Regards . . . . JimF
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'94 S500 Cpe

Visit my Mercedes Web Page

Check the expansion valve to under the dash by the evap. I could not find my leak for the life of me and one day I was trying to chase a water leak problem on my carpet. I was pretty sure it was the evap drains, so I pulled all the carpet and my parts tray and found oil and dye all over the expansion valve. glad it wasnt the evaporator.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual