How long should it take to get warm air?
 

This mornings drive to work in my 81 300SD was a cold one for most of the way.Here are the stats,about 30 degrees out, 22 mile drive,temp gauge reads between 80-90, fan blows on all speeds,selector seems to work fine.I didn't get any warm air until about 15 miles into the drive.Is this normal? This is my first Mercedes diesel and have heard most diesel vehicles run cold in the winter.Should a piece of cardboard be put in front of the radiator or not?I did a search on monovalve, read a lot of posts but I don't think that is the problem. Any thoughts?

If your temp guage is between 80 and 90 you should be getting warm air. Not sure about 1980 model but my 90 starts pumping warm air as soon as the guage starts moving. If I set my fan to auto setting the fan won't blow until the coolant reaches a temp higher than interior temp.
Vacuum/clean the interior temp sensor. (long shot but easy to do)
Look into mono valve....If you didn't get any heat I would suspect this but you say you eventually got heat????

I wouldn't cover the radiator as your running temp is fine.

There is a monovalve repair kit. The monovalve gets 12v and open and closes the valve to allow coolant into the heater core. www.**************.com has the kit, partshop may have it too.

its the monovalve, get the repair kit. Make sure engine is cold, unscrew top (4 screws) insert new vavle, replace cover. Caution - do not overtighten screws as they may strip out
Good Luck

Newbie question, where is the monovalve and what does it look like???

Should be in front of second fire wall several radiator (small diameter) lines into and out. you only wan to replace insert
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1NP0SZ49Y1NP0SZFNO&year=1983&make=MB&model=300-DT-001&category=All&part=Mono+Valve+Repair+Kit

Should be a two prong connection going to the top or side. 4 small phillips head screws hold it in place.

C'mon guys, quit muddying the waters!
 

It's an 81, it has no monovalve.

there mite be a monovalve.
 

i don't know your 81 300 SD, but an 83 300D does indeed have a monovalve. At least part of it is a brownish type of color, the top plastic part, as i remember.

The four auto parts stores and the MB dealership I called today all said I do indeed have a monovalve.I will look tonight to make sure.

An '81 SD should have a monovalve.

Hmmm, I beg to differ
1147213007 is the part number at Mercedes Parts

Ok. All 126's with the OM 617 have a monovalve. Monovalves are easy to repair, here is a link to a nice write up/.

BUT -

before you jump to conclusion that the monovalve is faulty, tell us a little more. 126's don't turn on the heater blower motor unless the temperature gauge reaches 40C. The idea is not to blast the passengers with cold air. So, your blower probably is fine as long as warm air comes out when the gauge hits 40.

If it takes 15 miles for your temperature gauge to heat up that much, you probably have a bad thermostat. Test it and swap it if bad. If your engine reaches operating temperature normally, the next place I'd look would be the monovalve.

TJohn said: If it takes 15 miles for your temperature gauge to heat up that much, you probably have a bad thermostat
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My thoughts exactly......

The car heats to the 80-90 range in a few miles.My first 15 miles of driving are on the highway,the last 7 are stop and go. The last 7 miles is where I start to feel heat from selected vents.

I still say mono valve, 1-2 miles is proper warm up for the diesel, you have the ususal symptoms of monovalve failure.
Shoulda pitched this site, my bad:o
part number R3023-15174 right here on fast lane

Thanks all, I will replace the monovalve soon and post results!!!

I replaced the monovalve today.Was one of the easiest repairs ever. The valve that came out was in good shape.No tears in the rubber and no oxidation of the metal parts. I will drive it for a few days and post the results soon after.