Fan works on defrost only

[190edude]

Searched and found nothing on this issue:

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?

[190edude]

bump. Somebody has to have seen this before.

[Jim H]

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

[psfred]

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

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.

[meltedpanda]

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