OK I just replaced the alternator from the stock 66 amp to a rebuilt 94 amp. No more fan motor running low at stop lights ect. All was good right? Until today I went through an ATM and the fan motor got really fast the volt meter went all the way to 18+ put it in drive got on the street everything went back to normal just over 13 volts where it normally sits. then at the next stop light it went back up to 18 took off and the volts dropped to about 10 fan slowed way down and felt like the truck was going to die. So I shut off the ac crippled it to Autozone, tried to explain to the guy what happened he would not here it hooked up his machine and told me he can not test the alternator because it is not hooked up? I told him to look at the wires that it is hooked up and he told me sorry he can't help. I asked him to test it with a load on it ac, lights he said no it will not help? Is this guy right what would cause it to spike like that?