Need a bit of AC testing advice - PeachParts Mercedes-Benz Forum

JHZR2 · #1 07-13-2007, 09:38 PM

Hi,

My 85 300D doesnt have any freon charge right now. I am not entirely sure why, though I have an 83 engine in an 85 car, so I guess the syste was opened up at some point.

Though some are against, Ill likely do a 134a retrofit, as I had it professionally done on my 83, and it cooled well and ran great. That system was tight, however, and I knew it for a fact. This car Im not so sure of.

So, two questions: First, given that my car is an 85, but the engine is an 83, I have an issue with the Klima relay. Provided that I jumper pin 7 to 10, is there anything else that I need to do to bypass so that the ACC and the AC system work together?

Now, regarding the AC system - the compressor will work, I verified this by jumpering the pressure switch on the reciever dryer (which I also assume answers my question about the Klima above, but just making sure...). If it leaks or anything else is another question, but the clutch engages.

Pressing the schrader valve on the low pressure tap indicates that there is no pressure in the system whatsoever. OK, fine... question is, how tight is the system? THough 134a is cheap, I dont like suprises, and would hate to find out that after a suckdown of the system prior to refill, the vacuum ump kept pulling air... or that after two weeks I have no pressure because of a leak.

I was thinking that given that I will have the system vacuumed down overnight, solvent flushed, and likely will just get a new compressor put in (to be safe and definite), could I simply use a bicycle pump to pressurize the AC system at the low pressure tap to say, 90 psi, and then just use a tire gauge to measure charge loss??? Given that there is no freon in there to recover, the worst thing going on is that a bit of moisture would enter... I cant see it to be a big deal if Im going to have the system opened up and completely worked on in a week or two.

I just want to eliminate suprises.

I was considering buying freeze 12 or something similar tht I could dirrectly fill in, but my nderstanding is that these options are not good...

So, any reccomendations of a good way to test this? I have an air compressor and a bicycle pump. I do not have a vacuum pump, and unless I can get one cheap, I dont particularly care to purchase one. AC systems are not smething that I work on a lot, and for the work that I will have done, a pro will be performing it.

Any insight or advice would be appreciated. Sorry for the long post.

Best,

JMH

Matt L · #2 07-13-2007, 10:14 PM

One standard leak-testing charge is 4oz R22 and nitrogen to charge the system to 175psi. This will expel enough refrigerant to find any leaks. You can then discharge the R22 and nitrogen mix into the atmosphere (according to the EPA).

Buying the equipment to do this once or twice would not be worth it, but perhaps you can find a local shop that will do it for you. It will cost money, but perhaps save you a lot of aggravation and money over the long run.

Once you find what's leaking, you have to decide whether to tear the system apart to flush it. The system would probably work better if you do, and if you replace any leaking parts and use new O-rings, you can probably then charge with R12 and oil, and check for leaks again.