Fan works on defrost only
 

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Just replaced all the actuators and the vacuum distribution module. Had a young apprentice working with me and after everthing was put back together, we took it for a ride. Had great heat, all the vents seemed to be working correctly. Cycled from max heat to max cool and turned on the AC and got ice cold air. Cycled back to max heat and it would not heat, just cold air. Fan was still working though. Let it sit for the night, came back out this morning and messed around with it and found the connector for the fan speed select was pinned out wrong. Fixed that and the fan worked fin but still no heat. After a few minutes of messing with the system the fan stopped and would not work at all UNLESS I put it on defrost and it worked like a champ. Only air was cold and not warm.
What would cause the fan to work in defrost but not on any speed selected in any other mode?

bump. Somebody has to have seen this before.

Speculation: on my 300SDL, there is a temperature switch that will not start the fan until the heat exchanger is up to given temp, to ensure no cold air in the face on start-up. Exception, if you push the Defrost button.

Your 190E may be different, but maybe the heat exchanger temp switch is bad, or maybe it got unplugged by you or your apprentice, fooling the system into thinking it's too cold to blow..

Best Regards,
Jim

Make sure the vac line to the thermo valve is on and the valve itself is working. Fan speed is manual on the 190, so I would guess something is wrong with the pushbutton unit UNLESS it also does automatic fan speeds (I don't know -- will find out when I get the one I have a new starter).

Peter

Vac leak
 

Hey man, I have a 79 300D and I have the same problem. I just dont' worry about it now because I know what it is. The vaccuum system most likely has a leak. My whole vaccuum system has multiple leaks that I have to lock the driver door to seal the vaccuum so the engine will shut off when I turn off the key. I am pretty sure you have a leak somewhere as well. I can only use defrost, but that's enough for me. Replacing the whole vaccuum system would just be costly and not worried about it really. I have gotten estimates at about 100 dollars a door plus more for the other areas. I hope this helps, check your vaccuum system.

the default for the ACC is defrost mode to work under all circumstances, as MB designed this for saftey.
If it is all cold air , have you checked monovalve? I am not sure yours had one but this is a classic symptom of failure . And yes it can occur ....just like that , I know :)

I have a 1989 190E 2.6 and the fan would not work except on defrost also. I replaced module in dash that had all buttons and temperature wheel and did not fix problem. Then by accident I discovered that when I turned key to crank car, the key went about 1/8 inch too far. I carefully turned key backwards 1/8 inch with the motor still running and the fan worked perfectly. I put old module back in dash, cranked the car, turned key backwards, and the fan worked perfectly. This sounds crazy but its true. Now I have to make sure I turn key backwards so heat and fan will function properly.

Great job finding why the fan wouldn't work. Sorry we weren't more helpful!

In my opinion, you will need to budget and schedule a new ignition tumbler, since it sounds like the start return spring is getting weak.

I had an Audi that broke this spring, and I had to return the key to Run from Start, or the starter stayed engaged. It was no big deal at first, but that game got old really soon. So, the first chance I had, with $$$ in hand, I had it replaced, and never looked back.

Best Regards,
Jim