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  #1  
Old 07-20-2007, 08:04 AM
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Location: Columbus, Ohio
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1991 300SL Climate Control Problem

The center and side vents do not move when the controls are adjusted and the system appears to have defaulted to blowing air to the defrost and side vent ports. Had anyone experienced this problem and what was the solution and cost to resolve?

I did some trouble shooting for a vacuum leak and have determined that there is no leak and the 15 port switchover switch under the dash is not switching.

There are no trouble codes present. I get the proper voltages from the center and side vent control and feed back potentiometers, and all temp sensors. I have control of the fan speed, the AC clutch engages, the RST function works, The Auto and EC functions are OK, and I can switch from AC to heat OK.

I have the R129 Service CD and I can not find any reference to how to test the 15 port switchover value. Is there a way to determine the condition of the switchover valve?

To get to the switchover value I believe the passenger side airbag has to be removed. The is a procedure in the CD to remove the airbag.

Are there any precautions in removing and replacing the airbag?

Thanks you all in advance,
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  #2  
Old 07-20-2007, 10:13 AM
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Haven't ever seen a vacuum switch-over block cause that problem on a R129 chassis.

The center vent is a computer & is most likely the problem. Possibly un-plugged.
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  #3  
Old 07-20-2007, 02:16 PM
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Dan, take a look at this: Climate control diagnostics on '90 - '95 R129 SLs. It will tell you if the climate control unit is receiving the proper inputs from the thumbwheel controls.

Last edited by bobterry99; 07-21-2007 at 03:14 AM.
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  #4  
Old 07-20-2007, 02:43 PM
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Location: Columbus, Ohio
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Hi Bob,

I check all of codes and signals a few days ago and I have the correct voltage levels from all three thiumb wheel controls. I also have the feed back voltage signals from the vent doors.

Now I want to get to the Y11 valve so I can check the connection and check the vacuum on each of the lines leaving Y11.

The CD indicates that I have to take the airbag out with instructions to disconnect the airbag and remove the center retaining bolt. Is that all there is too it and wil i then have acess to Y11?

I found that I can buy a rebuilt control unit, the unit with the temp indicator and push buttons, but I have not found were I can purchase a rebuilt Y11.

sldan
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Old 07-21-2007, 03:28 AM
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Quote:
Originally Posted by sldan View Post
I check all of codes and signals a few days ago and I have the correct voltage levels from all three thiumb wheel controls. I also have the feed back voltage signals from the vent doors.
So if you dial the thumbwheels to close the side vents, then diagnostics tell you that the vents are closed; yet air still comes out?

Quote:
Originally Posted by sldan View Post
The CD indicates that I have to take the airbag out with instructions to disconnect the airbag and remove the center retaining bolt. Is that all there is too it and wil i then have acess to Y11?
That's all there is to it. The retaining bolt is 10mm.
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  #6  
Old 07-21-2007, 07:00 AM
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Bob,

When I checked the feedback signals they were all at the voltage that represented their position. Since I had the center vent unit out of the dash I was able to move the center vent door by hand while I was watching the fedback signal voltage on the temp display. The feed back signal tracked the flap position.

The vents do not follow the thumb wheel positions.

OK on the airbag.

sldan
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