I had an opportunity to scope the signal line on a 203 fan today. It is a PWM signal, 10Hz, ~3.5V.

It isn't exactly like I was thinking but doable none the less. 10% PWM equals fan off and the activation allows you to step up 10% PWM at a time up to 90% PWM. The car I tested today had about a 3.6 to 3.7 volt signal and the service literature says over 3V is a good signal so it should be pretty easy to output a PWM with the PCM and use a voltage divider to get it around 3.5 volts.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff