What should the voltage be for a alternator voltage gain control be for 1998 Isuzu trooper 3.5

What makes you think you have a problem? Make sure battery has a full charge? Were these components tested on or off the vehicle? The systems should be tested on the vehicle. If there is a vehicle wiring problem for either component, testing them off the vehicle won't show wiring problem. Testing off the vehicle is good as far as it goes. I'm old school, If I suspect a problem, I use a digital multimeter across battery posts, everything off, the generic for a full charge is around 12.6 volts. Note the reading, start the vehicle, the second reading across battery posts should be higher than the first, if charging system is working. You should be able to do same test at alternator bat terminal. The battery and generator connections are important. Any applicable trouble codes?

Tested both on and off battery not charging ,have it down to the wires off alternator just needing what voltage should be at the voltage gain control

What voltage is there? I don't know? I did look at your wiring diagram. My thought is charging system voltage? I can't be positive. I'm not an Isuzu expert. You didn't explain your testing in your first post. A lot of charging systems the generator needs excite voltage before the generator will charge, That is the generator L terminal, the charge lite circuit. Hope you get it going your way. I got nothing else.