I swapped my 89 2.8 V6 out for an 89 L98 TPI. Mounted everything, car runs. Problem: Battery will not charge from alternator! I have replaced alternator three times regardless of the test result, and it still will not charge the battery. I added an extra ground from the engine to the frame. I added a ground from the back of the alt to the frame. Still nothing. I am assuming that the charging system (wires) is/are the same from the V6 to the V8. I have tested continuity and power in the lines to the alt and everything is as it should be. Still no charge! Checked the fuse, it's good. pulled the black wire off the back of the alt while it was running, put a meter on the alt and it is only putting out 1.2 volts. raise the RPM's (4500) and it goes up barely. The red wire going into the alt from the side has 12v from the battery. (tested with meter). This has me so confused, annoyed and pissed I have honestly considered selling the thing. Any ideas whats wrong?

Never mind, problem solved!

my is having the same problem how did you fix it?

On mine, the tan and white wire that plugs into the alt with the red wire didn't have 12v because I changed the fan set up and discarded the original relay. Stupid me didn't realize that the fan fuse also is incharge of the alt via the fan relay. So, I just wired to the tan and white wire 12v that are only on when the ignition switch is on. So, pull the plug, flip the ignition on and see if the tan and white wire has 12v and let me know because I have a few of other ideas that I tried.