A little help getting my A/C to work?
 

This is for the '85 300D (girlfriend wants AC).

The A/C has not worked since I have bought the car. PO stated that he tried to have it fixed (at a reputable shop around here) with no success. This was 2 years ago.

- It has been converted to R134a (that's OK for New England - I dont plan to re-convert to R-12).
- The system has a charge of some sort- refrigerant escapes if I press the needle valve down on the low side fitting.
- The compressor will not kick on when set to Max Cool.

Can someone help point me in the right direction with where to go next?

I am assuming I need to either jump the compressor or check pressures to verify the pressure is high enough for the switch to let the system turn on.

I have a manifold gauge and willingness to learn, but I am a total AC noob. I have read up and understand the basics.

Thanks,
dd

Search "KLIMA"

It is a group of relays controlling when the compressor kicks in according to load, temp, etc. There are DIYs on here regarding diagnosis and fixes, some specifically for the 85s.

(edit) Here's a link: http://www.peachparts.com/shopforum/diesel-discussion/172032-klima-basics-missing-forum.html#post1344819

Does the car have the 134a fittings on the low and high side? You need to take a pressure reading. This system has a low pressure switch on the drier, so if the pressure is too low, the compressor clutch will not engage.

If the compressor was used for r-12 and then "converted", I can say the 134a probably took it out, due to higher operating pressures.

I would shoot electrical first, it sounds like his clutch is not engaging. Granted a seized and low pressure compressor would not engage, but it is due to the klima which regulates the engagement via a series of sensors.

I think at some point I read a good rule of thumb would be to supply the compressor directly with power with the engine OFF and listen for the clutch to engage. If it does, see if you can spin it at all. If you can, then your clutch is bad, if you can't then your clutch is not engaging and you need to trace it. Maybe one of the A/C guys on here can back this up.

We really need more info from the OP. Blown fuses will keep the a/c from working as well. And, yes, '85 introduced the Klima, but IIRC, it did not have as much control over stuff like it does in the w124.

This is very easy, jump the wires to the switch on the dryer and see if the clutch engages, now do you have any cool air? Look at the sight gauge on top of the dryer, you see bubbles? If so you are just a little low on freon. Go to the parts store and get the tall can of 134 that has a guage on it, it cost a little more but makes it very easy to see how much to put in. I used one of those and in 10 minutes I had checked and topped off all 5 of our cars. There is no reason not to have functioning AC, it hurts resale and in most cars the defroster uses it to defog the car.

Dan

To answer some of questions posed earlier:

RE: 134a fittings high and low

Yes, the car is fully converted to 134a and has the correct fittings.

RE: Compressor

I believe the compressor is a rebuild installed when the service was done a couple of years ago.

What additional info can I provide?

Should I just hook the gauge up to take a static pressure reading with the car off?

Can momentarily jumping the compressor with the system in an unknown state cause harm? Ill try jumping it with the car off to see if I can hear the clutch.

dd

make sure the center part of the compressor spins freely by hand.

turn the ac on max, jump the low pressure switch on the drier. one of the switches on the drier is the low pressure switch the other controls the aux fan in front of the condenser. if compressor comes on - likely low pressure - verify with gauges. if its low pressure you would now know that your acc and klima relay are functioning normal like.

if no go after jumping low pressure switch & pressures check out. you need to start looking into klima relay, OVP relay, and EGR computer. Does your tach work ?

Yes. My OVP relay was faulty and has been replaced with a new one. So I know that is OK.


When you guys say test the pressure, do you mean with the system OFF, or jumped??

Ill take the readings and report back.

dd

First see if there is pressure with the compressor off. So, yes a stacic reading would be good.

If there is a freon leak, keep in mind that oil will be leaking out as well, unless it is 134a freon loss throught the old R-12 compatible hoses. The old hoses are not the barrier type and over time the 134a seeps out, while the oil does not.

jumped. static pressure does not mean too much other than that some refrigerant is present in the system. does the compressor come on when you jump the switch ?

You can jump it with out the engine running. You do not want to make the compressor spin until you are sure there is enough pressure to make the switch close. Jumping the compressor is used to keep the compressor from cycling during charging. I was shown that when I worked at a GM dealership in the 80's.

Low pressure means low oil flow...not good for the compresor.

Did you perhaps type that backwards from what you were meaning to say ?

Jumping the compressor usually means temporarily making it run by ignoring the low pressure switch on the rec/dryer... thus allowing refrigerant to be installed in the usual manner enough that when unjumping it the pressure of the total refrigerant is high enough to start the compressor in the normal safe fashion.
One needs to worry though after any leak that the refrigerant may have taken oil with it... and there is no way to exactly know how much is in there without taking it apart, flushing, and measuring the amount put back in... and YES, too much oil can have a real detrimental effect on cooling just like not enough oil can seize up a compressor... particularly an R4 hockey puck since it does not have an oil sump like other types.... so all the lubrication is by the mixing of the refrigerant and the oil and it going around and around the system.

This is a good thread... so much fun when only safe legit answers are posted !

Yes, jumping, keeps the compressor on, bypassing the low pressure switch. That is what I meant. I did buy a low/high pressure switch for my MB, so that the compressor cuts out if pressure is too low or too high.

Answer
 

These links should help you.

Climate Control:
http://www.peachparts.com/shopforum/diy-links-parts-category/142408-climate-control.html

Automotive repair training book on line, you can download it.
http://www.tpub.com/content/construction/14273/
and
http://www.tpub.com/automotive.htm

Automotive Air Conditioning Systems
http://www.familycar.com/ac1.htm

Aircondition.Com
http://aircondition.com/knowledge_base/Home.html

Repair Tips, Short-Cuts
http://www.peachparts.com/shopforum/tech-help/59250-repair-tips-short-cuts.html#post360663

http://www.alldatadiy.com

MercedesShop Do It Yourself
http://www.peachparts.com/Wikka/DoItYourSelf




Have a great day.