I have a 2004 XJR, my cooling fan wasn't coming on but would thump repeatedly like it was trying to (with car running and with key in run position but not running). Replaced the complete fan thinking the motor was bad and replacement fan does exactly the same thing. I hear and can feel the thump thump thump as it continues like it's trying to come on when can is at temp but never does. Checked the 80 amp fuse which seems to be in tack. For a loss on this one any information or direction to another form would be much appreciated!!!

 

Sounds like a faulty relay.

If the thermostatic switch is direct-wired to the fan, then that would be your most likely problem, with it cutting out whenever it is supposed to pick up the amp load of the fans.

Otherwise, if there is an interim relay then that could be it.

In either case, double-check your wiring connectors to your source for the fans.

With any luck, you might get off with spraying some electrical contact cleaner on them and re-connecting them, being sure to apply some silicone grease to keep moisture from getting back in and messing things up.