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  #1  
Old 06-17-2007, 05:36 PM
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Location: Columbus, Ohio
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1991 300SL air vents defaulted to defrost

The climate control decided to direct all of the air to the defrost vents no matter if in automatic or not and not matter which manual button I push.

Has any one experienced this problem and what was the fix?

Thanks all,

sldan

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  #2  
Old 06-18-2007, 08:17 PM
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On my 126, it was a major vacuum leak. It's sort of a safety feature - if vacuum is lost, at least the windows will be defrosted.
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  #3  
Old 06-18-2007, 09:47 PM
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Thanks Fred,

I plan to look into this over the weekend.

sldan
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  #4  
Old 06-19-2007, 10:57 AM
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Hey Dan. Take a look at this post: Climate control diagnostics on '90 - '95 R129 SLs. It shows how to run diagnostics on your climate control system.
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  #5  
Old 06-19-2007, 05:37 PM
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Thanks Bob.

I purchased a Service CD but have had trouble using it since I have a too new version of Abobe and I have not bothered to sort that issue out yet.
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  #6  
Old 06-27-2007, 08:09 AM
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I do have a vacuum leak.

Any ideas on how to isolate the problem and can I replace a single actuator?

sldan
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  #7  
Old 06-29-2007, 02:09 PM
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You can isolate the problem with a tool that is a combination vacuum pump/gauge, Dan. One can be purchased at an auto parts store or borrowed from Autozone at no charge. To check for a leak you use the pump to apply vacuum and watch to see if the vacuum holds.

You can purchase and replace the flap actuators individually.
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  #8  
Old 06-29-2007, 03:04 PM
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Thanks Bob,

I finally can use my service CD and found all of the info in the climate control system.

I will muster up the courage to take the dash board off and get to it.

Ok on the hand pump. I will pick one up.

Thanks,

sldan
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  #9  
Old 07-15-2007, 07:56 PM
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Location: Columbus, Ohio
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Update on troubleshooting.

It appears that the 15 port switchover unit is not working.

Does anyone know of there is a way to tell if the switchover unit is defective or the climate control unit is not sending information to the switchover unit?

Has anyone had to replace a 15 port switchover unit or climate control unit?

If so what was the cost.

I have come to the conclusion that I do not have a vacuum leak but rather the switchover unit is not switching after checking for troubles codes and there were none. My vent controls are OK and I have good vacuum on the green/yellow tube to the switchover unit.

The CCU controls the fan OK and all other functions seem to work except that the vents do not respond to the position controls and all air goes out the defrost and side vents.

Tanks,

sldan
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  #10  
Old 08-18-2007, 08:14 AM
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How do I remove the carpeted curved piece that is directly under the airbag?

I have removed the two 8mm bolts and the bottom of the carpeted piece is loose but it appears to still be attached at the top or sides.

Thanks in advance,

Dan
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  #11  
Old 09-10-2007, 08:40 PM
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Finally found and fixed the problem.

The vacuum switchover valve located under the airbag was defective.

PN 129-800-06-78.

This device distributes the vacuum to the various vacuum elements which controls all of the vent, defroster, blending, and floor flaps.

I did not have any trouble codes in the main control unit and did not have a vacuum leak.

Thanks to all.

Dan
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  #12  
Old 09-10-2007, 09:04 PM
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Good detective work. Behind passenger side air bag? How did you finally decide it was this particular part? A bit of analysis please.
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  #13  
Old 09-11-2007, 07:14 AM
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It is located under the passenger side airbag and the cost for a new unit was$377 with dealer discount. I could not find a re-manufactured one. Apparently this part does not fail often.

Using the service CD, I used a process of elimination and luck.

First I did the trouble code, potentiometer, and sensor checks on the control unit and all was OK. There were no codes and all of the signals were within range.

I bought a hand operated vacuum pump and checked for vacuum leaks on all of the vacuum lines from the switchover valve to each actuator. This was the hardest part because I had to take out the airbag, stand on my head to remove the passenger side carpeted piece under the air bag which required removing the radio, the climate control control unit, and the center vent to get to all of the screws. I had no vacuum leaks.

I also checked the vacuum pump in the trunk and the vacuum distribution block. Both were OK.

The remaining component in the system is the vacuum switchover valve. It operates on voltage pulses sent by the climate control unit. Since there were no trouble codes from the Climate control unit and no vacuum leaks, I rolled the dice and ordered the switchover valve.

Then I had to stand on my head again to put things back together.

It took less than 1 minute to replace thje switch over valve and an hour to put thing back together.

sldan

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