The climate control blower on my 1972 4.5L is manually controlled from the interior with a 4 position dial switch. The blower speed will often go to max even though a low blower position was selected. I first suspected that the switch was faulty. Since I happened to have a spare used rotary switch I swapped it but the problem stayed the same. Next, I checked out an electrical schematic. The blower resistor has an overheat bypass switch incorporated in it. This switch shunts the resistor coils if they are getting too hot. The blower goes to full speed until the resistor coils cool. I'm convinced this is where my problem comes from. Question, can it be repaired? It is housed in a cage next to the blower under the cowl and is connected to the blower only by wiring. I will take it out to inspect when I have a chance but wonder if anyone has experience fixing the bypass switch in the resistor cage. I called the dealer and they said that the blower resistor is not available separately. I must buy the entire blower assembly ($330).

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1972 450SL
1982 300D Turbo

The switch is likely fine - the resistor is overheating due to too much current, or too little airflow. The motor bearings may need a clean/lube, or windings in the motor may have shorted. Restriction in the airpath due to dust bunnies or else might also reduce natural cooling.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

I dug in and checked the blower and it is fine. I oiled the bearings while i was there. One of the resistor coils was bent too close to the bimetalic switch. I bent it away and now all is fine.

Thanks for your help sbourg.

Side note: the blower resistors are available separately if the parts guy is willing to look through all the footnotes. it costs ~$200 so i was lucky that mine was repairable.

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1972 450SL
1982 300D Turbo