1999 E300 blower motor runs when off
 

For a while now I've been having issues with my blower motor sporadically going on and off while it should be on full blast. Tonight I driving to work and turned off the A/C system and the blower was blowing full blast out the defroster. I shut the car off and the blower motor is still running full steam. I've tried restarting the car, locking it etc. and it won't shut off. I'm sure I'll have a dead battery 8 hours from now when I'm ready to leave work. What would cause an issue such as this?

Most probably a failure of the controller (essentially a big transistor on a heat sink) that acts like a relay. It is probably connected to the battery all the time and depends on the climate control unit to shut it and the blower off. If the controller shorts internally, the blower runs all the time. In high speed it can draw 20 to 30 Amps and yes, it will run down your battery. To prevent that, disconnect the battery (access panel in the right rear seat to access the negative cable). Long term, you must replace the controller.

Just checked the wiring diagram

The regulator is connected straight to the battery with no fuse :eek::eek:
I will never understand these guys, I do industrial controls and we would never get away with stuff like that...

Only way to kill it is to disconnect the regulator which requires taking the lower dashboard cover off and dropping out the fan assembly.

Went out this morning and the battery was dead. Just as I figured the Fan won't turn off even if I unplug the Control Unit. I just read that it's wired directly to the battery. Wonderful. So basically I have to wait till the new unit I ordered from California on Ebay gets here. I just went under and disconnected the blower motor so at least I can still drive it for now.

Blower motor resistor.

Not in a W210. Last E-class with actual resistors in the blower motor circuit was the W123.

Lol......do me a favor and look up "w210 blower motor resistor".

I should know. I have two of them, and I've replaced the resistor twice.

I guess I'll be buying one of those now. It looks pretty straight forward and easy to replace. Is that true?

I think this is a misunderstanding of the word "resistor." What I meant was the classic physics/EE definition of "resistance" from Ohm's Law (I = E/R). The component I had in mind was the resistor pack used on older MBs like the W123 (photo attached). Later cars (W124, W210, etc.) use a transistorized regulator or controller that is often called (by both users and sellers) a "resistor" although it isn't. A search for "W210 blower motor resistor" pulls up only references to this transistorized regulator or controller. See post #2 in this thread.

Jeremy