I have owned three different mercedes in my life, and i have never had climate control stay at a warm level.

I have had 81 300sd, 83 380sl, and now i have 85 300sd

I'm looking at the maintanence manual, and i trying to figure out hwere warm is.

Is it the heater exchange temp sensor???
I have changed the monovalve, i did the tissue test on the in car tempurture sensor(it sucking air.), I resoldered the climate control unit....The maintanence manual has that special tool Test Adapter Box...but i'm sure that it's impossible to find ...I mean what is the most como9on cause for no warm. I get hot and i get cold...

Oh two things i have noticed, that might be related... when i set my temp wheel to warm, i start smell a plastic being burnt smell coming from somewhere. Then i realized that when i touch the temp wheel i here like a electrical contact zapping noise faintly..which might be causing the intermitant smell..

anyways i have cold, and i have hot, i just wich i didn't have to adjust the tempurature with the windows.....

Maybe i'll just go through the maintance maual one day, it's just so long and confusing...beause they pictures are so dark and useless....and i hate taking the interior apart...

Dealer doesnt use the test adapter ether they test the old climate control with a dvom measuring how many amps etc something is putting out.

Ive fixed quite a few climate control systems and when I was done i had good heat a long time with no complaints from the people after going through and testing everything and repalcing everything bad.

your heat issue could ccu, vacuum related, monovalve, auxillary water pump, switch over valves, temperature sensors

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)