Cooling Fan Issues

hi there, can anyone shed any light on my problem, i have just changed the coolant rad on my 1999 3ltr auto S Type, everything was fine before i did, apart from the rad was split in two at the bottom, i had to remove the A/C rad to be able to remove the coolant rad, installed a brand new rad, put the A/C rad back in, put the cooling fan back in, filled the system with water (only water just in case there was any leaks) started the car, no probs or issues, took it for a drive, but when i got back i noticed that the cooling fan was on, didn't really think anything was wrong, i let the car cool, then went out to check the water level, which was fine, but when i went to start it the battery was dead, i put the jump pack on the car, & the cooling fan started, i managed to get the car going, but the fan wont turn off unless i disconnect it, why is this??? it didn't have this fan problem before, has anyone else had these sort of issues, or know how to fix it, i really hope so.