Interior fan runs when ignition is off
 

Initially I had a bad blower regulator (2002 CLK) I replaced that and fixed the initial issue (erratic fan control), but now I have another problem: the fan runs all of the time.

I tried another regulator from a second vendor and it does the same thing: works great when the key is in the ignition then fan restarts about ten seconds after taking the key out of the ignition.

It restarts right after I hear a relay "click" somewhere in the dash.

Monitoring the three pin connector to the regulator, one pin is the 12v power and is always on. The second pin seems to be a control voltage which goes from 0 to around 6v at full fan on. This pin stays at zero volts when the key is removed.

The third pin would appear to be a ground pin but it is NOT zero ohms. while the key is in the ignition, it stays around 15ohms. Once the key is removed, and the "click" occurs, the resistance falls to 3ohms. This coincides with the fan coming back on, but I have no idea why!

Does anyone have a service manual that can tell me what this third connector is and what would be controlling the resistance? The wire is brown. Is there a relay somewhere for this circuit that might be stuck on?

thanks

I just wanted to follow up on this in case anyone finds themselves with the same issue. I went through three of these regulators then finally went and ordered a MB certified part. Yep, that was it, cheap Chinese regulator. Both suppliers said that had never heard of that problem before. Sure!

Live and learn.