Spring is here and summer temps are on the way. I have read all about over heating problems that C4's have so I decided to head them off at the pass. I installed a cool fan switch [On at 200 off at 185 degrees] in my brothers 1987 corvette. I installed it on the passenger side rear cylinder head so it would control the primary cooling fan [the one behind the radiator]. After installation I started the engine and watched the temp gauge. After a few minutes the gauge went from reading low to 299 degrees in about 3 seconds. To make a long story short the hypertech tech advisor told me that the cool fan switch could only be used to control the secondary fan [the one in front of the radiator] I have already installed a switch to manually override that temp switch. I would like to know how to turn the primary fan on at a lower temp around 200 instead of 228 degrees. My 1988 corvette that I bought last September will turn the primary fan on at 175 degrees and it looks like a factory temp switch in the passenger side cylinder head. I am not sure why it's doing this I haven't looked at the prom to see if it looks like an aftermarket piece. My question is what is the best way to make the primary cooling fan come on at a lower temp?