Larry,

For what this is worth, my 92 300CE had some problems with the center vent not working. After I tested all the pods and found no leaks the head gasket let go after 150k. Long story short, there was a vacuum hose routed under the battery that had to be replaced. Air works great since then.

Cheers,
Steve

__________________
97 S500 Coupe 139Kmi.
92 300CE 150Kmi. Sold
89 F250 Diesel 220Kmi. Dead
98 GMC 1500 160K.

I don't want the recirc open. It must be closed to have recirculation and lower vent temps. I can hear it and I THINK it is opening, but I don't know any way to know that it is physically closing without pulling the instrument panel.

I know the center vent is opening because I can look through the vent and see it open. I wish there were a way to look at the recirc flap.

Thanks for the suggestions. Please keep them coming.

Have a great day,

I seem to remember the doors being open with no vac, but I'm no longer sure.

There is a blend door (the one inside the box) that might cause trouble by letting air through the heater core, but I'm not sure.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Larry -

You can see the recirc door close with the glovebox removed. Just a bit of the far right-hand side of the door is visible past the HVAC box. Shine your shoplight in there and use the mityvac. Normal (no vacuum) position is for fresh air. You should see the door swing away upward as you apply vacuum to the pods.

- JimY

Jim,

Thanks. I just put it back together because I have to go to Arkansas this week. I will try this when I get back.

Have a great day,

Well, I learned that the problem is intermittent.

I stopped for a little bit in Little Rock Tuesday afternoon and the cabin heat soaked. I started it up and it quickly took the vent temp to 38 degrees. Then yesterday afternoon, with it heat soaked, it struggled to get the vent temp to 60 degrees even after several miles.

Anyone have any ideas about what could be operating intermittently? I think I am going to take out the glove box for awhile so I can look at the edge of the recirc door as Jim describes when the problem occurs again.

Any help is greatly appreciated.

Hi Larry

Reading through the thread the only thing I can think of is it may be a switchover valve problem. Once the symptom occurs try monitoring the vacuum output to the indiviual elements. The swithover valves were known for chattering and or intermittent operation.

I like the suggestion of a sticky/failing switchover valve. With the glovebox removed you could try tapping it while it refuses to activate the recirc flap; that might help isolate the problem. With the glovebox removed a very distinct, single click noise can be heard as the switchover valve activates. Should be able to hear this when activating the recirc dash switch.

The only other idea is a crack in a vacuum hose or rubber elbow. Sometimes it'll hold enough vacuum to pull the element closed, sometimes not. I've seen similar operation from old vacuum elements with the diaphragm on the way out, but that can't be happening with brand new elements.

Mostly I'm clueless. I hate intermittents. My approach to solving intermittent problems is to wait until they become continuous problems...

- JimY

George Murphy, one of the technical directors for the MB club is the AC guru and even akes various ac parts. He would help you. I have heard him speak twice. phone 865-482-9175 george_murphy@compuserve.com

Larry
this is how my SL acted when the mono valve was going bad. I would get a little heat and the vents would get warm. Then go back to full cold. I replace the monovalve and the cars AC worked fine.
Isolate the mono valve hoses and see if the symptom disapears. Or open it up and see the condition of the unit. If it is junked up consider a rebuild kit for it. When mine went bad it was as I said intermitant.

Fresh air flap. Disconnect it from the vacuum element and "tie' it closed. Check the symptoms.

One note.
The 560Sl has a fresh air recirc button that applies vacuum to the fresh air flap for a few minutes. Closes the fresh air flap and then opens it after about 10 minutes.
This circuit may be able to be adapted to to your car.
If you dont' have the 560SL CD let me know, I check mine.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

Thanks for the suggestions. I will pay some attention to the switchover valve.

Dave,

Thanks for the monovalve ideas, but the 124 has a heater valve that is completely different from the repairable monovalve like was used in the 123's and some of the other chassis. Also I have checked the hose coming out of the heater valve several times during this ordeal and it is cool. Also, since it is not cooling upon start up while the coolant is still cold, that seems to eliminate that as a possibility. Thanks for your input and fly safe.

I may run out of hot weather to test it in.



One more possible clue. I drove to town late yesterday afternoon and the weather was raining and damp. I noticed sweat on the windshield caused by air from the defrost vents. I don't think air should be coming from there with the middle button of the climate control pressed. Is this a significant clue?

Have a great day,

Larry -

Condensation on the windshield means the defroster vent is not closing completely. The defroster flap uses a two stage vacuum element. Both sections must be closed to fully shut off airflow to the windshield. On older cars the small (upper) diaphram generally fails first. This is why you see older MBs (and occasionally BMWs) driving around with condensation on the windshield. On older cars like your '88 the foam around the defrost doors which seals the closed door is long gone. So you may get a bit of leakage (and condensation) even if it is working correctly. I can't remember if my wagon had the condensation problem after repairing the CC system or not - definately did before. The defrost element is readily visible with the glovebox removed, so it's easy enough to check whether it is closing partially or fully.

On my '87 124 the large, bottom diaphram would close immediately upon starting the car. This mostly shuts off the airflow. After about 30 seconds the small diaphram would close as well. I'm thinking it was designed to clear any condensation that occurred while the car was shut off.

I'm starting to think your car has a general lack of vacuum to the climate system. Perhaps it's time to check the main vacuum source to the switching valve - I believe this is the solid green line. Perhaps plugging it and then checking from under the hood with a mityvac. Also, disconnect it at the switching valve and check the vacuum reading with the engine running. The problem may actually be under the hood...

- JimY

Jim,

Thanks for the response. I did indeed check the vacuum source last week when I had the glovebox out. I did not measure it at the switchover valve with a gauge, but I did check at the firewall to ensure that the one way valve and the reservoir were okay. I was following the factory info and supplying vac at 400 mBARS as they indicated. I will measure at the input to the switchover valve and see what it is making.

Do you have any idea how much vacuum it should be supplying at the switchover?

I replaced the defrost pod of course, but I did not check the seal around the defrost door. I'm not so much worried about the condensation from the defrost vents. I was just posting this as a possible clue to the problem.

Thanks again,