Temp... Control

[sdgrossman1]

Hi,

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.

Thanks,

Steve

[cbdo]

No great expert, but from what I know, I'd check the vacuum actuators, especially the ones that should close the center vents. The small grate on the center of the dash is the intake for air that's drawn onto a sensor for the feedback input; it tends to get gunked up, and is reputed to respond to a vacuum cleaner used to suck dust out of the opening. I'd also wonder about a) the potentiometer attached to the temp control wheel and b) the electronics that control the monovalve. The service CD does give test protocols.

[tan man]

i too have problems with max heat but at all settings. and my air comes out of all vents at all settings.
so i can recommend to you what my mechanic told me...solve one problem at a time.
i would start with heat being on at almost all settings. go back to the monovalve insert your mechanic just replaced. inspect it by pushing down to expose the rubber. if it is torn then you are lucky the fix is right there.
otherwise, you move on to the next part, the monovalve itself. is it getting proper current.
let me know if you need really clear diagnostic details, i'll pull some papers from my cabinet.