"There was a bunch of white gunk under the old transistor that I thought was corrosion from time and heat. Is that incorrect? I cleaned it off. Was that wrong?"

That stuff was the original heat sink compound or thermal grease to make heat transfer to the case better. You need to use this stuff when you install the mosfet, so that heat generated can be better conducted away to the case.

"The new transistor is screwed directly to the heatsink - metal to metal. that is the only difference I can find."

I have a query about that - can you determine if the metal case (of the blower controller on which the mosfet is mounted)is connected to either battery negative (ground) or battery positive? Have you got a multimeter to use?

Phil

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'98 N/A S70 196000 Klm