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Old 12-09-2003, 12:41 PM
Registered User
Join Date: Dec 2003
Posts: 12
Temp... control '89 560 SL


I writing about my '89 560 SL, but I think this MIGHT relate to other models.

I just purchased my 'SL with 109K on the clock. I noticed that I was not getting ENOUGH heat.

My mechanic diagnosed that it was the MONO valve, and replaced it. Now I have the following problems. BTW the ambient temp has been between 40 and 50 degrees F.

1) I am getting MAX heat in all except the lowest temps on the dial (18-20) as soon as I move the dial near 22, I get full heat. My '86 300E would give me gradual heat as I turned up the dial.

2) When heat is on, center vents stay open, and very little heat goes to the floor.

3) When DEF is on, most air goes through the center vents and much less through the defrost vents.

Not sure of what to look at, and would love some ideas before I let my mechanic loose on the car.


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Old 12-09-2003, 01:59 PM
Registered User
Join Date: Oct 2002
Location: Colorado Springs
Posts: 481
I find on mine 91 560 sel similar settings. the system will blow hot max until the setting is reached then back down both fan and heat. so I think yours is okay. On the settings for defrost the bulk of air should come out of the defrost in the windows. on ec the air will come out of the center once warmed up, it does on mine. To get footwell heat i have to us the ac /heat button. Not sure if yours is the same but it sounds like it. i keep mine in the 18-20 range as that is around 69-72 , I find 75 or 22 to hot.
Martin Ingram
Colorado Springs
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)
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Old 01-03-2004, 01:14 PM
waylifeshouldbe's Avatar
Registered User
Join Date: Nov 2003
Location: Scarborough Maine
Posts: 57
This may sound obvious to the point of offending you but...I had a mercedes mechanic do some work to my 88 560 sel including the mono valve a few months ago. I had no heat and with the help of this forum purchased the valve. The mechanic was good enough to install for me. Upon return of the vehicle I had nothing but heat - could not regulate at all. I inspected the mono valve install and found they had not hooked up the electrical connection - all was well after I completed the "job". Could yours be related to a similar problem? My .02.....
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Old 01-04-2004, 06:50 AM
Registered User
Join Date: Jun 2003
Posts: 123
The mono valve is often one of the main problems in this car -- I have replace the insert a few times in the past 6/7 years.

You should not get heat from the center vents -- I wish that you could -- this would be great on cold nights with the top down. Only cold air from the center vents. When in "defrost" mode the center are close also.

I have had my climate control fixed many times -- the last time I also had the center vent blowing all the time -- It was the vac control for the defrost vents that was leaking -- it is inline someway with the center vent control.

I could not figure it out -- I have found that when it come to the climate control you need a dealer or someone who knows the exact model -- too many items can cause the same problem. Some are cheap to fix many are not!

Make sure that the inside temp sensor is clear of dust/dirt and that the air tube is connected and clear -- this will throw off the system if it is not right.
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