heater issue
 

I need some help my car is a 1984 300SD (126)

the AC does not work and the heater use to work.

what happen was one day it went out, and so i looked under the fuse box and changed fuse #11. and it worked again. a couple days after that it would turn itself on and off by its own (i press the defroster button). sometimes it would come on, then go away, then come back on in random intervals. when it doesn't work its not just the defroster button, but the whole heater system.

and it blew a couple more fuses. now it doesn't blow the fuse, but it does work either. could some one give me an advice on what i could look into? its started to get really cold here and i need the defroster to work in order to drive. thanks for your advices.

check your mono valve, although blowing a fuse is not the usual symptom. Worth a look as it is an easy diy and check if the rubber is broken replace it.
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heater issue
 

My '89 190E is doing the same thing...the heater blew a fuse, I replaced it. While driving the car, the heater blower will run for a while, then it will completely shut off for a few minutes...then it will come back on again like a central unit in a house. Is it the valve or the climate control unit? The car has 213000 miles on it.

heater issue
 

also....when I'm sitting at idle like at a red light, I can hear something clicking like a relay on the passenger side of the vehicle under the hood...this is my first MB so I'm not sure what's normal and what isn't.:D

check your mono valve assembly it is a solenoid so ti can cause a clicking

in my car there are 2 fuses that are blowing. one is an 8a and number 11 is a 25a. i actually got some heat now although the fan is not on all the time.

i have to look into this monovalve thing. can someone explain what it does exactly? thanks