I have a 1984 Mercedes 300 TD. The heater seems to take a really long time to heat, and when I do any highway driving (getting above 50 MPH or so), it stops working completely, blowing cold air.

Anyone have any ideas as to what could be the problem? Any help would be greatly appreciated.

Thanks!

-Brianne

Sounds like your thermostat is always open. What does the engine temp do during the drive?

It could also be the mono valve not functioning properly. Disconnect the wires to the mono valve and see if you get full heat. If so the mono valve is bad and should be replaced.

Third item. Check the foam tube in the dash connecting the temp sensor air flow to the blower motor. If gone or disconnected then it will give bad readings to the ACC controler. You have to remove the glove box interior to look at it.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

I have read somewhere that the defroster will work no matter what. If this is true, then you could use that in a pinch until you can resolve the heater issue.

__________________
" We have nothing to fear but the main stream media itself . . . ."- Adapted from Franklin D Roosevelt for the 21st century

OBK #55

1998 Lincoln Continental - Sold
Max 1984 300TD 285,000 miles - Sold
The Dee8gonator 1987 560SEC 196,000 miles - Sold
Orgasmatron - 2006 CLS500 90,000 miles
2002 C320 Wagon 122,000 miles
2016 AMG GTS 12,000 miles

Not true.

The "no heat above 50 mph" is a classic sign of a torn monovalve diaphragm. Does the heater start blowing hot air almost immediately when the speed is reduced? There is a repair kit that will fix that problem. The kit is much cheaper than a new valve and is easy to install.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=28D0KHXF228N0ON9WY&year=1984&make=MB&model=300-TDT-001&category=R&part=Mono+Valve+Repair+Kit

Guys I have a similar problem with my 91 350 SDL. The heater started to blow cold air regardless of the temp. setting and or the defrost switch button pressed. The fan runs at different speeds as it should, even remaining off till the engine comes up to temp. I ordered and am waiting for Mono Valve repair kit. This AM as suggested I disconnected the power to the Mono Valve. No change occurred. I will install the M Valve kit but is there any thing else I should be looking at?
Thanks in advance for your help.

Noodles

Most likely your monovalve is stuck closed. If not, there is an obstruction in the heater core and/or associated plumbing.