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My daughter's 1994 CA-spec 940 (Bosch LH2.4) triggers the fan from the ECU and from the pressure switches on the air conditioning. The ECU gets info from the coolant temperature sensor in the cyl. head and can switch the fan to low or high speed. IIRC, it has always shut off with the ignition, although the wiring diagrams show that the A/C switches could keep it running if high pressures are detected.
As for keeping your engine cooler after shutdown - I would say this simply doesn't happen. I have a 740 wagon with a 5.7litre Chev LT-1 V8 (aluminum heads, iron block). It uses a fan and shroud from a 960/S90, and a custom, all-aluminum 3-inch rad. The fan is triggered by a temperature sensor in the rad fins. I have a mechanical (Bourdon tube) temperature gauge with its sending unit screwed into the left cyl. head.
Now - to your question...after shutdown my fan sometimes runs for about one minute. While this is happening, I can watch the cyl head temperature gradually climb, because there is no coolant circulating. The radiator may be getting cooled down, but not the engine. Just hot air being blown around under the hood. After about five minutes the head temperature shows approx. 220F. The instant the engine starts, this drops quickly to the normal running (thermostat) level of about 185-190F. So my conclusion - based on these observations - is that having the fan run after shutdown does nothing for engine cooling.
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Bob (son's 81-244GL B21F-M46, dtr's 94-940 B230FD, my 83-244DL B23F-M46, 89-745(LT1 V8), 98-S90, 77MGB and four old motorcycles)
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