On Thursday, February 6, 2014 11:04:17 AM UTC-5, wrote:
The original fsu board was tied down only on the edges by that black plastic which was clipped by some small fins of the aluminium radiator .It did have a thermal paste i also used a paste .

The +12 i took from a main +12 and used a 2.5 amps fuse .

The transistor i think disipates about 50w to 80w of heat at low speeds that makes the heatsink to small to dissipate that heat and the airflow of the blower at that speed is too little.Its true the transistor is made for 250 degrees but lets be true they dont make like they used.

I think it's probably generating more heat than that. The transistor
is rated at 300W. In any case, I don't think it's the power transistor's
power handling capability that is
the problem. It's that the heat causes other parts to fail. I'm sure
you've seen pics of ones where the solder joints came undone. IDK
what causes the one here to basically shutdown once it gets too hot,
but suspect it's something to do with the Elmos chip, either by
intentional design or that it just shuts dowm by malfunctioning from
the heat. Whatever the max operating temp is for that Elmos ASIC,
I'll bet it's being exceeded when these things fail.

The overall design is probably just enough if everthing works like
it should. If the blower winds
up pulling some more current than normal, for whatever reason,
then it pushes it over the edge. I'm thinking as Ralph suggested,
it's probably the blower that's the root cause here. And to fix
that requires pulling the dashboard in the X5. Another great
design. Not something I wanted to do 30 years ago, but today
with all the air bags and God knows what else crammed in there,
it's got to be a lot worse. Wish I could get some oil to those
motor bearings and see what happens.

Going back to what Ralph said about bearings, I'm not convinced
that the blower is putting out full RPM even when it's connected
to 12V directly. There ia a lot of air coming out, but I think
it may have been even more previously, hard to tell for sure.
But if it is putting out less at full, then that could be a sign
of the bad motor bearing thing. Another sign I think is that
it seems even when you first start it up, with a cold FSU, it
seems to me that the airflow at say mid speed is more like
airflow at 1/3 speed, etc.

Good luck with your project at let us know how it works after
you've had it running for awhile.