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Cabin fan on max, even after key has been removed

On my 2001 SLK 320, the cabin blower is running on high, regardless of the settings. It is on even when the ignition is off, even when the key has been removed.

It only stop after I take out the fuse in fear of running out of battery.

It started happening when I was driving on the freeway. It just suddenly started blowing hard.

If you are pulling fuse #36, a 30 amp fuse, that feeds the blower regulation unit that controls the cabin blower motor. That unit gets signel from the enviornmental control unit. Likely, the power transister in the blower regulation unit has shorted out and is feeding unregulated battery voltage to the motor. It is rare that the internal schematic of a unit is available; replacement parts are usually not available; and the only replacement is the entire unit. You probably could locate the module (unit) by following the blower motor wires back to their source connection.

Maybe Bentzman will reply and we will get further education on this matter.

Two things that bother me, transistors more commonly go “open” when are the source of the trouble and the technique of varying voltage as opposed to varying the ground to control speed is diffefent to me, but I have never cared very much to learn Mercedes, a look at the schematic would show whats up.

it will be cheeper to put in a toggle sw. on the power line to the blower than to replace the control module, but then you will only be able to have it off, or on high.

Yes, it is fuse #36. Anyone can help me locate the “blower regulation” unit. Is it on the driver side? Is it next to something that is easily identified? I don’t know what to look for.


If it uses a fan relay for the high speed, the relay contacts could be fused together or sticking. A relay replacement might do the trick. If it’s an electronic control that manages the whole thing and there is no relay, just ignore me.

To find the module you should be able to just follow the wires back from the blower motor. They should tie to the speed control module and it should be mounted on the ducting. It is fairly common for the in series transistor to fail in the shorted mode. It would be a good idea to check the blower current to make sure it isn’t drawing too much current and damaging the speed controller.

I ended up going to my mechanic, who replaced the blower regulator (part. W0133-1716561)

I cost me about $500 including labor.

You could buy the same part for about $150 on this website:

The part looks like this closed up:

Glad you got it fixed, though it wasn’t cheap. Thanks for the update and info.