Replaced altenator, problem with fan not kicking in when it should, help???

Ok, our 1996 Corsica, 4cyl. Replaced altenator yesterday, and now the coolant gage is hitting 3/4 before the fan will kick on, and it only stays on till the 1/2 mark. Had same problem last year before we replaced the heater core. Could it be the bad altenator fried the resistors to the fan and it's not getting a correct reading from the engine temp? What about belt tension being to tight or to loose? Any hints would be great, the car has 175,000 miles on it but runs great, gets me around on my paper route each am and has for 4 years now.
Thanks a bunch
Mark & Donna

My 1995 fan doesnt even work, and the highest temp I will get to is 1/2 at idle..and 1/4 when moving. Corsica's are notorious for having head gaskets blow. Look at your engine where the head meets the block. take a look at the right side of the head. If you see a moist area there, you have a head gasket leak. Next check your coolant level. Make sure you dont go "bone dry". If you have a leak you need to repair it asap. If you havent already cracked your head (due to extreme temps) your asking for trouble by not getting it repaired while they can still grinding the head down.