how to test a/c system? - PeachParts Mercedes-Benz Forum

ambush276 · #1 08-04-2015, 12:25 AM

So i am trying to figure out what is wrong with the A/C. I have never worked on an A/C system before, but i do have some of the components needed to work on it. My friend has a vacuum system i can use and i have some R12 as well. I guess i am trying to figure out what i should test before putting new R12 into the system to make sure all the components are working (before i waste the r12)?

I know i should also add some oil to the compressor, but was wondering if there is a good way to test the compressor, condenser, evaporator etc...?

Thanks!

vstech · #2 08-04-2015, 07:38 AM

Only reason to add oil is if components are replaced.

First step is to get the system pressurized with a dry gas and verify no leaks.
Next, of course, fix any leaks found, and reverify.

If you are replacing the compressor, it will need 2oz of oil... If you are replacing it, you will also need to replace the receiver dryer and it needs 2 oz oil. Any hoses needing replacement, need a tiny amount of oil. Condenser and or evaporator coils also would need some oil if they are being replaced, or if you are flushing them out with cleaners.

vstech · #3 08-04-2015, 07:43 AM

Testing the compressor first grab the center section of the clutch and see if you can turn it by hand. If not, the engine will need to be started and the clutch powered.
Jumping the pressure switch should power it, when the ac is requested.

It should engage, and pull a vacuum on gauges if no pressure is in the system. DO NOT RUN IT LONG WITHOUT FREON.

leathermang · #4 08-04-2015, 08:05 AM

John did not mention that a LEAK can also take oil out of the system... in addition to replacement of items...
check the AC thread in my signature.. years of good info there...
to do it RIGHT.... meaning have a chance of lasting a long time.... 6 to 8 years before needing topping off with luck is tedious.... but worth it....
Too much oil or not enough are both AC killers....and the only way to be sure someone in the past has not added too much.... take all the parts and flush them... and then measure the specified amount of oil spread around the systems parts and start over. If you read the AC thread you will find all sorts of important warnings ...

Maxbumpo · #5 08-04-2015, 08:53 AM

I agree with a basic compressor test (will it spin by hand or engine power) and a dry gas pressure test (nitrogen is most commonly used) to make sure you don't have any leaks. If you can round up the equipment and information to get those two things done, then think about a complete system flush. If you find leaks, once you start taking the system apart you had might as well flush "while you are there".

Section 83 in the factory service manual has a ton of critical information. There are about 20-odd steps to troubleshoot the fully automatic system, and you will need a good wiring diagram and good digital multi-meter skills to perform those if you don't have the special equipment the manual calls for.

rrgrassi · #6 08-04-2015, 10:29 AM

Working on an A/C system is not bad, but can be tedious and it is labor intensive. If you understand how and why the system works, then repairs and what happens will make sense.

ambush276 · #7 08-04-2015, 03:06 PM

yea its a 1982 240d. No complicated climate control issues. I will start with the compressor hand test and engine test and report back. I will have access to the vacuum pump this weekend and will do the pressure test then.

BillGrissom · #8 08-04-2015, 03:14 PM

Does it currently have refrigerant pressure? You must have a pressure gage. Best if both high and low-side, though you can get by w/ just low side. You should find old R-12 gages cheap on ebay. Over-charging refrigerant or oil can be worse than not touching it, and how many people ruin a perfectly good AC system.

Is your AC compressor a rectangular aluminum box? If so, it is a York, which were common on Fords. I have one on my 1965 Chrysler and still works flawlessy. If the electric clutch proves to be the problem, that can be easily changed without disturbing the refrigerant.

rrgrassi · #9 08-05-2015, 10:38 AM

The 82 should have the GM R4. They can be difficult to turn by hand. You can see if the compressor turns by giving the clutch 12 volts and have some one turn the key to engage the starter, but do not let the engine run by keeping the engine stop pressed down.

ambush276 · #10 08-17-2015, 02:24 PM

so i can turn the compressor by hand... there is very little pressure on the low side. I have some R4 but i am going to a buddy who is certified to install it to recharge it.

There does not appear to be any leaks (after testing with a vacuum).

I'll update when/posts pics if anything weird.

leathermang · #11 08-17-2015, 03:10 PM

So you did not believe John and me when we said it needed to be checked under pressure ?
You have 'some R4 ' ? what do you mean by that ? your compressor is in pieces ?
I am going to predict that you have not done enough research to have a problem free AC for any meaningful length of time...
It is your car... so you can do what you want... I have tried to help you.

ambush276 · #12 08-17-2015, 05:30 PM

i have some cans of R4 that i can use to recharge. I used the vacuum pump as instructed to test for any leaks in the system. There were no leaks.

I then tested the R4 pressure and noticed that it is very very low (most of it is gone).

It held a vacuum fine for 1 hr.

did i misinterpret your instructions?

my buddy is an AC expert and will check the oil for me (but i didn't want to waste his time before checking if it held vacuum).

tyl604 · #13 08-17-2015, 05:39 PM

I believe what he is saying is that you should check for leaks under pressure, not under vacuum.

Mike D · #14 08-17-2015, 05:39 PM

R4 is the style of radial compressor. R12 is the type of refrigerant used in the older systems also known as Freon.

I'm with leathermang on this. Have someone else do the work.

engatwork · #15 08-17-2015, 06:27 PM

Quote:

for any meaningful length of time...

If it last less than a year you missed something during the repair

. Get someone on it that you trust to do a good job with it.