ted emailed me some photos of the Fan Control Amp in his '02 Quest.
Bottom side of the Fan Amp.
Top side, with enclosure removed.
This is a much more modern design than on the Q45's - It uses pulse-width modulation to adjust the fan speed by rapidly switching a pair of MOSFET transistors (Q2 and Q3) on and off. Longer pulse on-time = faster fan speed. Shorter pulse on-time = slower fan speed.
The heat sink is attached to the transistors with a metal spring clip. Popping off the clip allows the heat sink to be removed.
The fact that Ted's fan runs at full speed all the time is a clue that Q2 or Q3 might have an internal short between their Drain and Source terminals. Since they're both connected in parallel, a short in either one would effectively prevent the other one from working normally.
On a hunch, I suggested that Ted replace both transistors. They're available at Mouser Electronics for around 2 bucks apiece (link
here.)
Interestingly, the PC board has provisions for connecting a third MOSFET (Q1) in parallel with the other two. Apparently, some engineer decided later that having just two MOSFETs would suffice (which in hindsight might have been the wrong decision!).
Along with replacing Q2 and Q3, Ted decided to also install a third transistor in the unused position where Q1 is on the PC board, on the assumption that spreading the fan motor current among 3 transistors would reduce the chances for another transistor failure.
(shown with Q1 Photoshopped into image)
Ted reports that the Fan Amp now works normally - so the repair worked.