'big transistor' . . . .
Vinamg, tnx again!
Did a search but never came up with that thread. But I do understand completely how it operates, as an "emitter follower" amplifier. And how it can fail!
Here's how it can. Since it's controlling the power for the fan motor, it's must withstand a lot of power dissipation, thus the required heat sink to the blower's frame. Roughly here's the scenario: 13.8V (terminal 15) and 15amps (when the transistor is about 'half' on, yields about 90 watts dissipation. The transistor must dissipate the energy not transmitted to the blower. Thus when the fan is on MAX, the dissipation is minimal since the transistor passes all of the power to the blower.
Most people operate their cooling controls around halfway on so it's constantly dissipating a large amount of energy. So I can see how it can easily fail.
Thanks again for the great info.
One other point: recently somebody suggested using pictures to describe problems and/or their solutions. This to ease problem solving by making it easier for knowledgeable people to 'see' your problem. We all should do more of it.
Regards . . . . JimF
'94 S500 Cpe
Visit my Mercedes Web Page