charging question

I'm doing an install on an 88 isuzu pickup and have a few issues. I dont have all the info i need yet since he's getting the subs tomorrow, all i know is they're soundstream. This is the amp. i upgraded the wiring to 4 gauge with a 150A fuse inline with the alt wire. Depending on if the subs are SVC or DVC will depend if u run at 2 or 1 ohm, i've heard alot of good stuff about the MTX amps being run at 1 ohm...would it be necessary to use a cooling fan when running at 1 ohm even if its in the open? this is the type of fuse i have, i only could find 150A at the local stores, but i noticed i can order them online from 60A to 300A The biggest concern for the truck is that when i read the voltage output, which is fine with truck off (12.8V) and at idle (14.8V) but when i rev up the truck the voltage increases quite a bit...i managed to get it up to 17 volts and i could have revved it higher and possibly got 18V. He's been driving the truck around for a while with it like this, I'm curious if the voltage regulator would be after the battery for this truck, I'll be testing the instrument panel wires to see if i still get such a high reading, but that doesnt matter as much to me as the fact that my amp is still directly effected (head unit works fine with no volume change with RPMs). Would going from 14V to 17+ V cause overheating of the amp or volume increase when the engine is reved? If so is there something i could use to keep the voltage at a steady 14-15V?

Having the alternator run at 17 volts will make the amp run more efficiently if it CAN handle it, , however the izuzu's electrical wont last too long at 17 volts, stop running that vehicle till you can get that figured out, the regulator is most likely fried in the alternator. 15 volts is the absolute max you would want to be running to your vehicle.2003 Chevy Avalanche,Eclipse CD7000,Morel Elate 5,Adire Extremis,Alpine PDX-4.150, 15" TC-3000, 2 Alpine PDX-1.1000, 470Amp HO Alt.

Yes, I was going to ask if it's an upgraded alternator or the factory alt, and if it's an upgrade was it one designed for that vehicle since it sounds like you don't have a voltage regulator in the system.  If it's a factory alt you need to take that vehicle to the shop. Support the12volt.com

the guy put a new engine in it not too long ago (couple weeks i believe) the alternator is a new stock alt. I'm surprised everything still works inside the truck. I did discuss getting a new alternator with the owner before which is when he told me that this one was brand new. I dont believe he can afford a new HO alt at the moment, so i'll see what it costs to get this one fixed then..if its cheap enough i'll pay for it myself. Thanks for the input, i'll report back if the voltage for the instruments in the truck are normal, cause that would be wierd.