I have a 1991 300D 2.5, that really does not heat that well. After doing a search on this board, I found lots of information about the mono-valve, but I still have some questions.
1. What exactly, in detail does the mono-valve do?
2. Why would a bad monovalve give me limited heat?
3. What exactly does it look like? Is it electrical, mechanical, or other?
4. How do you replace it? Where is it located? Any special tools needed?

Thanks for your help,
Sincerely,
Cold in Maryland

On almost all MB's the mono valve is just an electric heater value. Power turns the valve OFF & its spring loaded OPEN! Under most conditions the valve is Pulsed on for a second or two & then should pop back open to allow hot water into heater core. After a while the coolant erodes the rubber diaphram & hydro-locks the opening & closing operation. If it sticks SHUT you have no heat! ON you car its right by the battery & easy to replace. Before you start the car try unplugging it, small brown 2 pin connector & then seeing if the heater will work.

CJ,

The "Infamous" mono valve is the one found on the late 123 cars with climate control. They are fraught with problems. The one found on the 124 cars, such as yours, are much more reliable.

On the 124 cars, particularly the ones earlier than yours, the achilles heel is the pushbutton control unit.

Best of luck,

------------------
Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

Thanks to both of you.
I will go out and test it. Is there a problem, if I ride around with it unplugged until Friday when the parts place is open? I have a 4 month old, that I prefer to take in my car, as opposed to my Wife's Mazda 626.