The air on my 1991 300e (83K) just started blowing hot. It appears that the compressor is not engaging when I push the buttons on the climate control. I have searched on the related topics and it seems that there are various problems that can lead to this. Does anyone have ideas on what initial steps to follow when diagnosing this.

Thanks./

It sounds like the climate control unit has failed. A leak in the system (no or low refrigerant) would also cause the A/C to blow hot air, but the compressor would still be operational.

Still, do an A/C system check for leaks first to get that ruled out. I don't know of a good way to check the climate control unit, but they have been notorious for failure.

Check the serpentine belt. If it is loose, that may be your problem. If it is the original A/C system, chances are you have a leak. If the leak is in the evaporator area, that means big $$$$$
Have a pro look at the system if you are lost. It does not cost that much to diagnosed the problem. Most mechanics use a dye to look for leaks.
Good luck either way!

Actually, low or no refrigerant *will* cause the compressor to not engage. There is a hi/lo pressure cutoff switch screwed into the receiver/dryer. If refrigerant pressure falls below two atmospheres it disables the compressor.

Common causes of the low pressure cutout switch triggering include 1) It's cold out (below ~35F) 2) Low refrigerant charge.

Evaporator leaks are relatively common on Mercedes Benz cars. Dunno why. Because of the expense associated with removing & replacing the evaporator (>$1,000) I decided to gamble and use a leak sealer. The sealer is activated by moisture in the air. It is carried through the a/c system by the oil which circulates with the refrigerant. As the oil leaks out of the system, the epoxy sealer is activated and (in theory) seals the leak.

My 124 wagon would leak down from a full charge to the point where the a/c was useless in about 2 months. I installed the sealer early last November. When I checked the a/c in April, it was about 6oz down on refrigerant. Much improved! Since the beginning of April it has not leaked at all! The a/c works extremely well, at least for a 124 car...

Total cost was about $300 for refrigerant, oil, a new receiver-dryer, the sealant, and vacuum pump rental. I did all the work myself on a Saturday morning.

If you do have an evaporator leak you may wish to investigate an epoxy sealer. The participants over at the air conditioning bulletin board ( http://www.aircondition.com ) seem quite impressed by their performance. It saves having the evaporator removed from the dash, about 12 hours labor on a 124 chassis car.

You said it was blowing hot--do you mean ambient temperature, or heater hot? That caught my attention: If "heat" is coming from all vents, check for a vacuum leak to/in the heater monovalve. The repair kit is available for around $30. Just hoping you have a less expensive problem . . .

First of all check the pressures for the AC. If pressures are good and all pressure switches are good it's a good chance your compressor has failed. Check the pressures first.

An easy test for compressor function is to set the ACC to max cool, high fan and get under the hood and remove the two leads from the low pressure switch at the engine side of the receiver/dryer. Short the two lead together with a short piece of wire or a pigtail. I am talking about the spade wire ends you just removed, not the switch lugs. If the compressor comes up when you short the two leads and doesn't make any weird noises, then the best bet is you've lost your refrigerant charge. Which means you have sprung a leak somewhere and that is your main problem. If you don't get the compressor coming on, then there is a control problem which you will have to troubleshoot. Let us know..

Thanks for all of the good leads. I am probably going to have my independent test it. Is this something I need to take care of right away ? I might need to wait until next spring if the repair is $$$$$. I just replaced the headgasket,belt, tensioner, radiator,and some front end parts.

If you have to wait, don't use your A/C at all. It will kill your compressor (if still good).
In the mean time, you will have to switch over the 2-60 A/C system. You do know what that is, right? In case you don't, open 2 windows at 60 miles a hour. Sorry, I could not resist(I'd been there) ;^)

update
 

Well, I took the car in for an oil change and decided to have the A/C charged, dyed, and tested for leaks (at that point in time) . The climate control worked fine and did blow cold for about 24 hrs and then nothing. On my way to take it back.

Anyone have any words of wisdom?

thanks.

You have a leak!
 

Pray brother, pray that is not the evaporator!

Dealer replaced mine (on the 300E) a couple of years ago at the tune of $2800!!! Fortunately, the extended warranty I bought kicked in, so I was out about $250 instead.

Ouch !! I know it is 20hrs of labor. If this is the problem, should I replace anything else while I am under the dash?

When my OVP was in bad condition the A/C didn´t work.

Pelle