So I replaced my alternator on my truck this will be the third one on it in the last 3 days. It’s reading 15 volts when I put my tester on it. The battery is brand new. When I drive it the gauge shows ab 12 volts, the battery light is on and almost doesn’t crank after a drive. I’ve read a bit about it but I haven’t tracked down an exact cause. The connector to the alternator is plugged in and seems to be in good condition.

 

If you measure from battery - post to + post a voltage and measure a higher voltage from battery - to alternator output bolt, the heavy power wire is bad. I read your post again and suspect output wire from alternator to battery+ is bad.

Here is trouble shooting sequence:
You should measure 0 volts from output bolt on alternator to +12 volt battery terminal with engine running. Then check for 0 ohm resistance, engine off. More than 0 volts means bad output current wire. Not bad alternator.
You should measure 0 volts from case of alternator to -12 volt battery terminal with engine running. Then check for 0 ohm resistance, engine off. If not 0 volts & 0 ohms, engine ground wire is bad, so run a new heavy wire from alternator case to battery - terminal.
4 pin alternator jack should have brown wire from alternator to charge indicator per Chev. wiring diagrams.
Wire on next terminal should be gray wire from PCM to control alternator per Chev. wiring diagrams.

My 2002 diesel truck has only a black wire on second terminal of 4 pin jack. Signal should be PWM or pulsed DC signal. My charge indicator is fed by PCM, not alternator.
Total Power Parts on ebay sold me same 130 amp alternator for 2000 & 2002 trucks, a 112853 part #. You have wiring failure or wrong alternator. Expect to need longer belt if you put this on a gas engine. Not pushing this seller on you. Price is up $21 to $90 in 2 months.