I know i'm late entering this thread but what the hell i'll throw in my $0.02 anyway.
i just had this same problem in a my 98' xj. however when moving the wires that went to the fan it would try to start spinning and then that would be it. i took out the fan assembly, pulled the motor and disassembled the motor. there was corrosion EVERYWHERE on all the copper tracks that lead to the 4 brush's as well as the soldered connections for ground and power were shot. i scraped off as much of the corrosion as i could. broke off the now brittle wire ends where it was soldered and resoldered them. coated all the exposed wire/solder/copper tracks w/ dielectric grease. and installed. bingo works like a champ.
as was already stated, in mine when the coolant gauge goes a tick above 210 the electric fan turns on (reguardless of a/c position) and is on all the time any time the a/c is on.
knock on wood its been working great and saved me at least $100. so in the future it worth breaking the fan open and spending the hour or so to try to fix it. my jeep has 130k on it and there was plenty of life in all 4 of the brush's FWIW.
figgure this, it dosent work. so if you fark w/ it and it works great, if not you didnt lose out on anything but a bit of time.
