1985 M/B 300d climate control problems
 

I have a 1985 300D with a malfunctioning climate control system. The blower will only come on before engine start when the ignition key is in the on position. After engine start the blower will almost always shut off (while controls are still requesting the blower to run). If the blower continues to run after engine start the heater will blow warm air as long as the engine remains at low RPM. Once the engine RPM increases above 2000 RPM the heater will blow cold air. What is the problem?

Thanks

Pilot

I think that you have two problems. The first is the ignition switch. It is not very expensive and pretty easy to replace.

The second problem is not so easily diagnosed, but I expect that you have a vacuum problem of some sort under the dash, but there are several things that could be causing this. I would start by pulling the glove box and ensure that the foam hose is in place that goes from the horizontal pipe behind the glove box to the temp sensor in the center top of the dash. I would also make sure that the monovalve is not flooded. These are not likely for your symptoms but very quick and easy to check.

I've never had to mess with the vacuum pods on your model, so I don't know exactly what to tell you on that except to check the lines near the oil filter that go through the firewall. That is where the vac source is applied to the underdash climate control.

Good luck,

I think the mono valve has a torn diaphram. With the increase in rpm you get a increase in water flow through the valve. With a torn diaphram the valve will not work properly. The valve is easy to rebuild. Replaced mine on a 1984 300D.

To get the blower to work with the engine running just barely turn the key towards the off position and it will come one before the engine shuts off. It will be a very slight nudge to the left and it will come on. I had the same problem in my SDL, it was the electronic portion of the key switch. Is what's happening is the spring inside is worn and not pushing the key fully back after engaging the starter. On these cars they cut off all power so the starter has more for starting.

Thanks
David

Monovalve and key switch

300d climate control
 

I appreciate all the suggestions, my wife and I will start on the suggestions soon. It appears that it is protocal to list the equipment that own's you?

Pilot

M/B 300D
AC CA
JD 3020
1952 Bristol Bus
BE-18
RC-3
CE-140A
260Z
1970 C-10

All the above are used and not in cherry condition but highly prized.

and to be really "in" you name them :D

Kudos
 

The suggestion about moving the ignition key for the heater/AC blower was steller! We now have air circulation in the auto. Can you suggest a shop manual that would help with the location of climate contol sytem and its componets? I have a Haynes that is of no help in this area? In regards to naming equipment the only ones that we are intmate with are noted below.

Pilot
AC,CA-Lorraine
1970 C-10-Golden Goddess
BE-18-Twin *****
Bristol Bus-Battle Wagon

Does moving the key back so the fan works fix the other problem as well? Now that you know the little trick, you could just live with. I still naturally bump my key back even though I've replaced the key switch. It still moves ever so slightly so I like to think I'll make this one last at least 300k miles instead of the other one failing at 250k. These old MB's are junk. :D

Thanks
David

if you are getting all hot or all cold, then your monovalve should be changed as a first step. Easy DIY, 4 screws, lift out replace. Behind 2nd firewall
Could be other components but doubtful if things are turning on and fan speed is changing

monvalve and shop manual
 

Where do I find the monvalve on my 1985 300D? Also can you recommend a shop manual that will describe the climate control system? Thanks

Pilot

The mono valve is mounted on the firewall.
Look up ACC Mono Valve in Fast Lane for your car so you will see what it looks like. It is moumted with the shiny parts up with a electrical plug on top.
It has 2 water lines. One in the bottom and the other in the side. It is mounted high enough so that you will not lose collant.

The only shop manual that I have found helpful in these older 126 models is the AC MB shop manual for the year and car. They are as rare as hens teeth but occasionally come up on ebay

fast lane
 

Can you be more specific on an internet site for "fast lane"? When I did a search I was unable to ascertain what was the appropriate site. Thanks.

Pilot

look at the top of this forum page for the parts sponsor