I have to shut down the truck as the volts continue to sit at over 18 if i continue to drive it than it will fry all the electrical in the truck ! !! I guess shutting it off just resets it , it oniy jumps up every onice and a while ! as for the alt i am not to sure because i only shut down the truck for seconds (while i am still rolling) that woudent change anything in the alt so that is the oily reason i am thinking it doesent have to do with that !!
But i dont know!

 

Well, some 18V spikes are not going to fry all the electrical in your truck. If that were the case, then you would already have burned out bulbs and blown fuses. The reason you don't want to run with an overcharge that high is that it will damage the battery over time. Before you start assuming it's the PCM, run the instrument cluster check. Hold down the odometer button and turn the ignition switch to "on." You will get an indicator light bulb check and each guage will sweep. If that checks out, I would get a cheap voltmeter and connect it direct to the battery and see if your problem re-occurs. If it does, then you'll know the problem is either the alt or the PCM. When you're dealing with electrical systems, you have to trace it to the problem. Simply replacing parts is never the answer.