As I briefly mentioned a couple posts ago, it could be the fan motor itself. I am unsure what the current draw should be, but I'd at least find out if it seems too high or low. After a brief starting spike, based on the fact it runs off of the 40 amp Maxifuse, i'd suspect that when jumped to a battery it should run at around 15-20 amps. When the fan motor is jumped to battery power, is it "howling" fast, or just *kind of* fast? You seem to be a very articulate person, sorry if this is too "unscientific" for you. This does sound an awful lot like one I mentioned in a prior post. Tech "A" ordered a control unit (pulse box) for this particular car and Tech "B" installed it only to find that when the fan was tested via activations that it didn't fix the inop fan, ended up being a motor. I also recall on that job a similar "1 sec" activation of the fan (about 2 revs), then quit, so maybe the controller actively monitors the current consumption of the fan motor and will stop the attempt if the current consumption is incorrect? It was a confusing problem because like you. I also tried jumping battery voltage to the motor connector and it did run, just not as fast as a healthy motor. Fast enough to make someone not familiar with the system to not suspect the motor is about shot. If it's a good motor there is a distinctive howl to the aux fans at full battery power, sounds kind of cool.

Gilly

OK, OK, tutorial time on the fan noise:

Motor getting bad, hooked to good battery:
Blaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

GOOD motor howl:
Waaaahhhhh-ooooooooooooooooooooooooooooo!!!!