A Taste of Summer & A/C woes - PeachParts Mercedes-Benz Forum

Dervman · #1 04-04-2006, 01:43 AM

Just returned from a week in Florida and a chance to exercise the A/C for the first time this season. Also the "perfect" time for the A/C to start acting up in 85-90 degree weather.... Grrrrr

I have tried a forum search but cannot seeem to find any entries relevant to my problem, so I'm hoping somebody here has some ideas.

The A/C will usually work just fine for a while with ice cold air from the vents, but then it just quits. No amount of adjustment or buttom pressing re-activates it. Sometimes after a stop for a while it will work again but always quits soon after. The length of time it works varies from a few minutes to about half an hour.

I have checked all the visible electrical connectors for possible bad connectivity and removed and replaced the #5 fuse in the fuseholder and made sure the contacts were clean. Thinking the Freon-12 may be low I had an A/C shop in Port Charlotte check the system. They said it was a bit low, but not too bad. They pulled a vacuum and it held fine so they re-charged the system and added some lubricant and tracer dye. Initially the A/C seemed fine after this but again after a few miles it quit once more.

The Compressor Clutch and Dryer were both replaced about 18 months ago and the system worked fine through the last two summers. The Compressor is the original (as far as I know) but the Hoses were repaired or replaced within the last 5 years by the PO.

I still thinking it might be electrical due to the intermittant nature of the problem, but I'm not quite sure what to look for next.

e300D97 · #2 04-04-2006, 08:29 AM

Just a thought but does your car have "cabin filters"? I know if the filters are dirty they can cause the system to ice up. This may be the case since you are getting ice cold air to start with.

Hit Man X · #3 04-04-2006, 08:49 AM

85-90°F hot?

That's nice compared to a summer here.

I know the SDL has some weird speed sensor on the back of the compressor also the Klima relay could have something to do with it too.

My compressor finally turns without the help of my hand

in the SDL now due to the proper (and new) Klima relay. It's low on refrigerant but big deal, I can see where it leaks (if it does) and repair it. It just hasn't been run in so long it may have just oozed out (no A/C run since '04). Then be nice and cool with my R12 in there.

The SD is going to take far more work. Basically everything minus evap core needs to be replaced. I have everything but three pieces.

I wish I knew more about that vac pods behind the dash to get air to come out of the center vents properly.

dannym · #4 04-04-2006, 10:24 AM

You said "Ice cold air". What exactly is the discharge air temperature?

Can you go to Ace Hardware and pick up a $2.00 dial thermometer and report back?

DAnny

Jmana · #5 04-04-2006, 10:30 AM

Hit Man,
If you want air out your center vent, I inadvertantly figured out a way to do it. Just take out your control unit, and look back in the cavity and to the right. You'll see a pink pod with a vac line coming out the front. Pull the line, and plug it up with a screw. Then pry the pink cap off (it's tricky, because there are 4 pins you have to try and hold in at the same time. Or you can just break them off). A spring will fall out. Pull out the cup with the rubber diaphram (it's attatched at the back with a lever). Now you should have center vent air, which can be controlled by the slider at the vent!

Mismost · #6 04-04-2006, 10:37 AM

Dervman....to me it sounds like your CCU, the push button unit itself. I too have been doing the search routine and I am basing my opinion on those results. But it works great, so the base system itself must be good, then it stops...stop & start are controled by the by the CCU.

Several threads about re-soldiering the connections and curing the problem. The thread I liked best was about swapping out the auto system with a manual version. I'm not as smart as the Mercedes enginners, but I'm pretty sure I can manually adjust the A/C and though it may tough, I can tell if it's too hot or too cold!

My problem is the center vents, no cold air....but great cold air at the defrost...bad vaccum pod/hose/leak???

Dervman · #7 04-04-2006, 11:34 AM

Thanks for all the responses.

No Cabin Air Filters (wish I had).

Where exactly is the "Klima Relay" and how would I check it?

I don't know the exact vent temperature but it was plenty cold when the system was working. I'm now back in Canada so A/C won't even be needed for a few months, but I want to get it fixed before I head south again in May.

I was thinking that perhaps the ACC/CCU might be the culprit, although it seems to work on all functions otherwise. It probably wouldn't hurt to pull it out and go through the re-solder process anyway as I'm sure it hasn't been touched before.

Rick Miley · #8 04-04-2006, 11:43 AM

The 87 has an RPM sensor on the compressor, and the Klima relay compares that to engine rpm. If the Klima detects a difference, it will disengage the clutch until the engine is shut off and restarted. This is to prevent a seized compressor from taking out the serpentine belt and leaving you stranded. So if your system is working fine for a few minutes and then shuts off, it is likely a slipping belt, faulty sensor, or Klima relay - not the PBU.

Hit Man X · #9 04-04-2006, 12:59 PM

Quote:

Originally Posted by Jmana

Hit Man,
If you want air out your center vent, I inadvertantly figured out a way to do it. Just take out your control unit, and look back in the cavity and to the right. You'll see a pink pod with a vac line coming out the front. Pull the line, and plug it up with a screw. Then pry the pink cap off (it's tricky, because there are 4 pins you have to try and hold in at the same time. Or you can just break them off). A spring will fall out. Pull out the cup with the rubber diaphram (it's attatched at the back with a lever). Now you should have center vent air, which can be controlled by the slider at the vent!

Hrm, does it always blow from there?

Jmana · #10 04-04-2006, 10:13 PM

Yes, but you can shut it off using the slider at the vent. The only bad thing is that it also blows out the defroster at the same time, so if you have the AC on and it's humid outside, you will get condensation on the window.

Dervman · #11 04-05-2006, 02:45 AM

Quote:

Originally Posted by Rick Miley

The 87 has an RPM sensor on the compressor, and the Klima relay compares that to engine rpm. If the Klima detects a difference, it will disengage the clutch until the engine is shut off and restarted. This is to prevent a seized compressor from taking out the serpentine belt and leaving you stranded. So if your system is working fine for a few minutes and then shuts off, it is likely a slipping belt, faulty sensor, or Klima relay - not the PBU.

Hmmmm... makes sense.

Where is the Klima relay located?
I have been experiencing strange Tachometer readings for a while that will change by about 800rpm from time to time. The Tach works fine (smooth and linear) but the reading reads low sometimes. About 2300rpm @ 80mph instead of the expected 3100rpm. I had put this down to a faulty tach amplifier, but could I have a bad sensor instead? If so where do I look?
The Belt is fairly new along with the tensioner and there are no signs of slippage.

Hit Man X · #12 04-05-2006, 03:16 AM

Right against the firewall beside the OVP relay (that's the red/silver one).

Dervman · #13 04-05-2006, 01:53 PM

Thanks.
Any ideas about possible Sensor Problems & locations?

Fuzzball · #14 04-05-2006, 02:21 PM

I'm not sure how to help without understanding what "just stops" means.

Does the blower stop or does the entire system stop? Is your compressor still engaging?

Can you define more completely what "stops" means?

This could be just brushes on the blower motor.

Dervman · #15 04-05-2006, 04:48 PM

The Blower works fine on all speeds and in Auto. position.
The A/C compressor dis-engages after a variable period of normal operation.
Air from the vents is nice and cold then just ambient temperature. Blower is still running but A/C is not.