GMC Jimmy Alternator Not Charging

My Jimmy Does Not Charge The Battery When Running.it Reads Low Voltage And Has Had 2 New Batteries And Also 2 New Alternators.

After Having Several People Look At This I Thought I Would Try The Manufacture. I've Already Gone Through 3 Alternators. What Happens Is...

96 Jimmy S15 V6 Auto. I Noticed Last Week That When I Pressed The Gas The Voltmeter On The Dash Went Down And Headlights Dimmed. All...