Hi all,

Back after a few years between my old '89 300SEL and my new '89 560SL.

The climate control in my '89 560SL works as it should ONLY when the temperature dial is set on the Min. or Max. detents. Any setting in between would result in hot air blowing through all the vents including the two center vents. The pushbutton module appears to be working as it should when the temp. dial is set to either Min. or Max. with cold or hot air blowing from the appropriate vents.

I am thinking it could be the temperature sensors or the temperature setting module.

Appreciate any pointers on how to diagnose this or reference to where such information is available. The service manual provides a complete test procedure but I don’t have the Klima tool required to do the tests.

TIA

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Riadh
'89 560SL Rauchsilber
And what is good, Phaedrus, and what is not good -- Need we ask anyone to tell us these things?

A foam-like tube goes between the temperature sensor on your dashboard and the climate control unit. That tube usually deteriorates by the time the car gets to this age.

Remove the passenger side under-dash panel and look up in the direction of the temperature sensor. You will probably find that tube has literally fallen apart. Simply remove what is left and replace it. I did not bother looking for the correct replacement but simply used a piece ofr plastic tubing. Works fine and regulates temperature. The correct replacement may work better but I am satisfied with the plastic tube.

Thanks for the quick reply. Not arguing here, just trying to make sure I understand the climate control operation.

Why would the missing foam tube cause the climate control to blow hot air between the Min and Max settings? I understand the foam tube is there to allow sampling of cabin air by the cabin air sensor which draws air from the sensor grill just above the dash through the foam tube. If the foam tube is missing (I will take a look to confirm), then the air sample would be drawn from inside the dash not the cabin which would have slightly different temperature than the air just above the dash. If that is true, then the climate control error would be slight variation from what it should be not as drastic as changing from cold air at Min. to hot air just above Min.

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Riadh
'89 560SL Rauchsilber
And what is good, Phaedrus, and what is not good -- Need we ask anyone to tell us these things?

Foam tube was in bad shape. Replaced it, but as suspected, that didn’t fix the problem. Based on the climate control circuit diagram I thought it had to be either the pushbutton climate control unit (N22/2) or the Temperature Control Unit (N22/1 little box below the glove box). Had access to a good climate control unit. Installed the good unit. Problem persisted. Installed a rebuilt Temperature Control Unit. Problem fixed.

ps: Monovalve was ruled out because of the following diagnostic test suggested by Programa (the rebuilder of the climate control electronic units):
Referencing Left (L) and Right (R) terminals connecting to the pushbutton control unit (N22/2). Resistance between (L) pin 12 and (R) pin 4 should be 11 to 19 ohms. If not, replace Monovalve. When I performed the test, resistance was within range.

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Riadh
'89 560SL Rauchsilber
And what is good, Phaedrus, and what is not good -- Need we ask anyone to tell us these things?

thanks for updating the thread. I'm going through the same bits on my 300E.

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I'd rather argue against a hundred idiots, than have one agree with me. — Winston Churchill

I've seen these go out on the early W126's. A fellow member on here sent his in for me to take a look at and I found that a bunch of connectors were broken and a capacitor was loose! Another member on here, Hit Man x, has one with chunks missing from the transistors!

Check out the pics, they are from the first gen cars:
Inside the box: W126 Automatic Climate Control. First Gen.

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Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


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check the in-cabin temp sensor. if it's bad the CC will act as you described. there should be Ohm values for it somewhere in the manual