why does my overload light come on?

Check 12 volt lighting if ok I doubt it's a new battery job. As for the pump fresh water? or toilet one?.
Turn pump upside down to access the impeler turn it with smaill screw driver and see then if it works as they some times doo stick through lack of use on winter storage. The other thing would be if plug in pump to check power to the pump with mulitmeter.
Is the overload light for 12volt system or mains?
You could do search for manufacturer of you caravan if you don't know already Just enter manufacturers name followed by website and see if they can shed light on it. Refer back incase some one has had the same problem and replied with a solution.

bob
April 2009