Here is the problem I been trying to solve....... When I bought the truck, the guy put a used alternator on. Well after he put that one on, the water temperature gauge started pegging into hot about a minute after the truck is started each time. The truck is running fine though. Also, now if the rpm's reach above 3500, the battery light will flash on. I know I need a new alternator but what could be causing the gauge to peg out? I changed the gauge sensor already. Some tell me it could be a bad ground....some say I need a whole new cluster which is $350..... Can anybody help on this one??????

 

I have the same problem with my 1998 F150. did you ever get this resolved? My problem started a few weeks after I replaced the alternator. Any help on this will be appreciated.
Thanks.