Air Conditioner Problems on a 300SL - PeachParts Mercedes-Benz Forum

BobH · #1 05-14-2002, 09:50 AM

Well, it starting to get a bit warmer now and my air conditioning problems have returned. After driving for about 30 minutes, the AC stopped blowing cold air. If I shut it off for awhile, it will start blowing cold again. Apparently, there are no leaks and the freon is charged. There is no evidence that the belt is slipping. The Mercedes dealer couldn't diagnose the problem last summer because it worked perfectly while the car was in the shop.In October, in Florida ,an AC specialist diagnosed a bad relay...cost me $400.00 and it stopped blowing cold air after an hour or two. It's been suggested that it could be the expansion valve. Does this sound like a likely cause?

Bob

euro 287 · #2 05-14-2002, 10:51 PM

Could be but to correctly diagnose it you need to have the pressure gauges connected while it happens. Normally a bad e-valve will show the low side pull into a vacuum. Check it with gauges first.

Gilly · #3 05-14-2002, 11:22 PM

I've seen bad evaporator temp sensors cause complaints similar to this also.
Gilly

euro 287 · #4 05-15-2002, 01:06 AM

One more thing , we had someone put that cryo-seal @$%^(% and it clogged up the e-valve . You didnt use that stuff did you like for a evaporator leak?

BobH · #5 05-15-2002, 08:16 AM

Thanks for the suggestions. Euro I've never used cryo-seal or anything to seal the system. If I understand you correctly, the pressure gages should be hooked up while the air conditioner stops blowing cold air. I do seem to recall that when I started taking the car to be checked, an AC repair shop said that it was pulling into a vacuum, but didn't diagnose the problem (maybe he didn't want to work on a Mercedes).I'm guessing he was talking about the low side.Gilly, would an airconditioner repair shop be able to check for bad evaporator temp sensors or is this something only a Mercedes shop would be able to check? Are they reasonably easy to get to or do you have to remove the dash board?
Bob

Gilly · #6 05-15-2002, 11:12 PM

Bob:
It's hard for me to know what the capabilities are of a particular AC shop. I would tend to think that it may need to go to a dealer to have the actual value read, although, you may be able to get that climate control panel into a diagnostic mode, so you can monitor the evap temp sensor yourself. Let me know, I can research if this is possible on your car if you want to know. The dealer could also check to see if there are any diagnostic trouble codes in the system.
Gilly

#7 05-30-2002, 12:01 PM

Gilly,

I have a couple of problems with the ACC on my 1990 500SL (R129).

1) I cannot seem to get it into diagnostic mode.

2) Air only comes out of the defrost and side vents

The compressor is operating normally, high/low sides are good and steady. Is it possible that the pusbutton unit is the cause of both these problems, or am I looking at replacing the switchover valve too?

Thanks,

-Dennis

Gilly · #8 05-30-2002, 10:10 PM

Dennis:
I'd have to look through the diagnstic manual to tell you how to get it into the diagnostic mode. I'm not too familiar with doing this, as we have the equipment at work to read the codes without any extra work like this, plus less analyzing the data like you have to using the pushbutton panel display.
I am unsure if either one of the items you mentioned are at fault, need to test the system to answer that. It may be that you are just missing a power supply to the panel, as this mode you are in may be a failsafe mode, so I'd check out the fuse box first.
Gilly

#9 05-31-2002, 01:11 PM

Thanks Gilly!

I will check the fuse box this weekend. The other thing that entered my mind is that I might have a vacuum leak to the switchover valve, so I intend to check its vacuum source as well. I do have the ACC Diagnostic Manual and there are two modes you can use to send this model into dianositc mode.

Test Mode A - With ignition on, press REST and within one
second, press blower speed button 4.

No Faults is indicated by an alternating in car
temperature and 02.

Test Mode B - Turn temperature wheel to white field

Turn ignition on and within 10 seconds press F,
RECIRC, and REST simulatneously for 2 to 4
seconds.

No Faults is indicated by 01 on the display.

I can't get either one of these to work. As I said I will check the fuses and see if I might have a problem there. I 'm a little concerned that I'm not getting diagnostics though.

Thanks again,

-Dennis

Gilly · #10 05-31-2002, 09:58 PM

Dennis:
I printed out the info at work, and yeah, you have the info word for word with what I have. Except there's 3 modes. I'm sure you have this info. I would concentrate on part B, the DTC's. That 2 to 4 seconds may be a critical time, so make sure you do that operation exactly, and if that doesn't work, try holding the buttons for a longer period, if neccesary maybe a minute. Maybe I'll get a pre-97 in I can practice on and tell you if I figure it out, or maybe there is a DD subject on this problem. Could still be a fuse both causing your operational problem, and keeping it from going into diagnostic mode.
Gilly

#11 06-07-2002, 12:29 PM

Gilly,

I checked both of the fuses and they looked good, I replaced them to be sure. I am still unable to get the pusbutton unit into diag mode, I do understand the concept of trying different wait times and spent the better part of an hour wearing out my ignition switch to no avail. My next step it would seem is to verify the vacuum source to the switchover valve.

Thanks,

-Dennis

Aija · #12 07-18-2002, 01:53 PM

I was just reading your thread, and wondered if we have the same problem. Please read the following thread:

Intermitent A/C Problem ?

I just called my mechanic and they are still running a series of tests on the car. They are considering replacing the climate control unit to see if that is the root cause. If it does not fix it, they will put in my old one back.

I would appreciate any comments from you.

Thanks.

BobH · #13 07-18-2002, 02:07 PM

Aija,

My symptoms were blowing cold air for brief periods and then blowing warm air. After shutting the car off for awhile, it would blow cold air. First attempt to fix was replacing the evaporator temp sensors. This didn't stop the problem. It turned out that I had a bad expansion valve. After replacing this, the air conditioner has worked perfect on some of the hottest days we've had and this was even with converting to R134A.

Bob

#14 07-19-2002, 03:18 AM

Bob

My 91 300sl has the exact same problem that your car had. I am taking it to the shop next week for the air conditioner and i also have an intermitant engine sputter and sometimes die during starting. I am going to copy you e-mail and bring it to my mechanic. Thanks for sharing the solution to your problem.

brian

#15 07-20-2002, 09:59 AM

Well, my ACC problems are solved, for now!! In my case the air would blow cold, but only from the defrost and side vents. This started intermittantly then turned into a cosistant failure over a period of three months or so. I am a very tolerant person.

I tried a replacement pusbutton unit, which I bought on eBay. Same result.

Next I checked the vacuum system. I started with the distribution block in the engine compartment, passenger's side, under the cover between the two firewalls. There is a block with four vaccuum lines connected to it, although it looks to support around nine. The topmost line is the engine supply line and is hooked to the intake manifold. The three bottom lines are for a resevior tank, the central locking system and the ACC switchover valve.

All of my lines checked fine, either holding vaccuum or in the case of the manifold line, supplying it, except the ACC switchover line (green/yellow stripe). If you test this line it should hold vaccuum!!! It may take a while to generate a sufficient vaccuum if you are testing with the system running. I suggest testing with the system off so that you can be sure you are just testing the switchover block, otherwise, the you could be experiencing the result of a leak downline somewhere in a vacuum element.

To get to the switchover valve block, you need to remove the passenger's airbag. This can be very dangerous and should not be attempted by anyone remotely prone to blowing themselves to bits. To avoid a lengthy lawsuit, I will not explain this procedure here!

Reach into the hole created by the now missing airbag and feel downward. You will feel a bunch of vacuum lines (12, I think) connected to what feels like a plastic brick. Push down on the brick and push the top of it towards the front of the car (away from you), it should pop out.

Once the switchover block is in hand, remove the vacuum lines by carefully removing the line retaining clip, it has two plastic tabs holding it in place. If your car is as old as mine, at least one of these tabs will break, relax, have a beer. It's better than all of that cussing you were about to do! Once this is done, remove the electrical connection to the block.

Now check the pin farthest from the electrical connection on the the block. It should hold vacuum. If it does, then the line from the distribution block under the hood is suspect. If it does not, then you need a new expansion block!

While all the lines are disconnected from this block, check each one to see that it holds vacuum. Here's the bad news, any one of these that do not hold vacuum may mean disassembling a good amount, if not the entire, dash assembly. I did find that one of mine was leaking, thankfully it was a side vent, which usually blow all the time anyway. So, I just capped its connection on the new expansion block I installed.

AC blowing nicely now, recirc working for when I pass the landfill.

-Dennis