If there is anyone still fighting this problem, (heater fan runs even if the ignition key is out) i might have solved the problem. In my car it did anyway.
Inside the resistor pack there is two semi conductors (transistors) IRFZ 44R (large one with tree legs) which I have replaced. And everything seems to work again!
It was my friend (a electronic engineer) who helped me. We did not find the exact one so we replaced them with a couple IRFZ48 and as I said, it works, so far! Don´t forget the white grease underneath the semi conductors, it helps to keep the components cool, they get very warm.
I also have a thought about the A/C function, my aircond. is currently not working (leakage I think) maybe this speed´s up the life time of the semi conductors?
Any way, Good luck!
That is SUPER information!
I am qualified in electronics too, and was wondering whether this 'resistor pack' actually was resistors (as it would be in manual-switched fan systems) or whether it is a speed controller. You have proved it to be the latter.
A quick Google search confirms my suspicion that the IRF Z44R is a Power MOSFET (MOS = Metal Oxide Semiconductor - sensitive to damage by static electricity, FET = Field Effect Transistor) - see data sheet http://tec.icbuy.com/upload/database...c6d84bb0d0.pdf
- what does all that mean, well basically it is a device that is insulated by design, but can possibly fail to a dead short (arc across) and therefore leave the fan running all the time. Static electricity damage may have shortened its lifespan at any stage. Or, depending on installation, its insulation from the heatsink might be a factor as well.
In operation it is, in effect, an amplifier: it amplifies a small input current to switch the larger current of the fan motor. The names of the pins source, gate, drain are named for the flow of electrons, with the gate being the controlling signal. You see MOSFETs featured all the time in audio amplifier circuitry, so an audio repair shop can probably help with replacement parts. By the way, for anyone wondering why you'd bother, the cost of the MOSFETs is probably a pound or two each, bit less than the replacement Alfa part
The nice thing about a field-effect semiconductor is that it has a large zone of operation where it is not 'fully saturated' (fully turned-on) and therefore becomes a resistive element to control the motor speed. BUT when operating in an unsaturated mode (partly-on, fan running slowly) there can be a great deal of heat dissipated. I think you are right in your assumption that if the A/C is working, the MOSFET will last a little longer due to the cooler airstream.
Anyway, there will be many substitute parts possible for a 50A Power MOSFET, I must have a look through some other spec sheets - the unsaturated/partly-on operation curves are probably the most critical characteristic to match up.
Also, I should test the output with an oscilloscope (while mine works!) to see if it is performing a PWM function (pulse width modulation), i.e. whether it is pulsing the motor to reduce the speed. That would be much better for the life of the MOSFET, but it would probably cause a mechanical resonance/whine in the motor, so it is probably not the case. But if it is, that would broaden the range of possible substitute MOSFETs.
Again, thanks for resurrecting this thread, great information to know. And welcome to this forum