Why does my heat only work while sitting still?
 

Ok, so just to bring you up to speed. I have had a climate control problem with this car ever since i started driving it (2yrs) . Over that time I have fixed several heat/ac issues and the system is getting better every time I work on it. I have re soldered the push button assembly, replaced leaking vacuum pods, replaced some cracked vacuum lines, etc...
So now the A/c works almost perfect, but it is no longer summer :(. Now I want the heat to work. So After trying to nail down some exact symptoms I came up with this.
With the car warmed up, running in park, heat works perfect, fan changes speeds temp output changes locations full range from cold to warm. But when I drive the car on the road, the heat comes up to temp, then starts to blow cold air and the fan never changes speeds, it keeps blowing on 1 step down from max. So then to get the vent temp back up from cold I have to turn the temp all the way down past the detent for cold and back up to warm. The fan never changes but it starts blowing warm again until the process starts all over again.
Looking for Ideas..

You need a new monovalve insert.

Yep, check the monovalve, and also make sure the coolant level is good.

Odds are that your monovalve diaphragm is torn. You can confirm that it is by removing the four screws on the top of the monovalve and removing the enclosed solenoid. If you find that the diaphragm is not in good shape, one of these will most likely fix you up:
http://catalog.peachparts.com/ShopByVehicle.epc?q=1983-Mb-300sd-Climate--Control&yearid=1983%40%401983&makeid=MB%40%40MB%40%40X&modelid=300%2DSD%2D002%40%40300SD&catid=R%40% 40Climate+Control&subcatid=R3023@@Mono+Valve+Repair+Kit&mode=PA

I concur. I had almost the same. My heat would gradually disappear until I stopped the car, then I'd get it back. The pressure buildup inside the monovalve through the ripped seal will close the valve.

Check the parts section for used, tested monovalves- sometimes they pop up...:o

My heater's behavior is very similar. Does a 300CD have this infamous monovalve? If so, what's this thing look like and where is it located?

Look on the firewall just inboard from the battery. It's usually brown plastic, two heater hoses attached and an electrical connector on the top.

Has anyone mentioned a bad monovalve?
Depends on the year of the CD. If you have horizontal as opposed to vertical buttons on the heater control panel, it does.

I had the same problem.
 

it is the monovalve. just replace the insert. Done

Yup, I've got one then. Here's another question for you. I am taking the family for a couple of hour's drive in my 300CD on Thursday - going to my sister-in-law's for Thanksgiving dinner. Somehow I don't think I can get one of these repair kits and get it installed before then. Can the whole thing be temporarily bypassed to get some heat for the drive? Maybe with a plastic elbow and a couple of hose clamps for example?

IIRC, rather than do a bunch of re-plumbing, all that is needed is power and ground. There should already be power there, from the fuse F10 and the ground is switched on and off by the Pushbutton Controller. So grounding the one side of the connector will cause full heat.

However, once you aquire a monovalve rebuild kit it's about a 15 min. repair.

I guess I don't understand. From reading the previous posts in this thread, I understood that the problem is that pressure past a torn diaphragm inside the monovalve causes it to close - not an electrical problem. So I guess I don't understand how grounding and energizing it ("it" being what I presume to be an internal solenoid) is going to make it stay open.

And that is the plan - once I can get one. But as previously stated I don't see how I can get one AND find time to install it before around 10 am Thursday morning.

On the other hand, loosening two hose clamps, removing the hoses from the valve, and re-joining the two hoses with a plastic elbow - which is something I can pick up at any local auto parts store - seems more do-able to get the immediate result - until I can get the rebuild kit.

My monovalve was bad and here are some pictures of it.

http://www.peachparts.com/shopforum/...1&d=1227707000



http://www.peachparts.com/shopforum/...1&d=1227706269


http://www.peachparts.com/shopforum/...1&d=1227706997