Blower Motor Works Perfect With Engine Off
 

OK, Here's one for you guys.

My 1990 300e's blower motor works fine until I start the engine then it seems to have a mind of it's own. With the engine off I can controll the speed , with the engine running it goes to low. Sometimes it will run where it's set(high) for awhile then drop to low after say 40mph.

I've done the CCU solder joints repair, and even traded out the CCU with the 260e and no change.

Any Thoughts at what might be causing this, Is there a vacume control aspect?

I once had a problem with an erratic blower motor on a 124.

It turned out to be the connector that plugs into the firewall up near the expansion valve.