So this is for a buddy of mine he has a 99 blazer and first start up in the morning everything works fine but when he drives it and stops for just a few minutes then goes to start it again it starts but the battery light comes on and it will die shortly there after this only happens when its hot outside when its cold he has no problems new battery and alternator anyone know what this might be cause he keeps getting told its the computer not telling the alternator to charge

 

Does the system voltage increase once the engine is running? If not, then the alternator isn't charging.

Others have had problems with the PWM alternator controls that these trucks come with from the factory. If this is suspected, it is often easier to rewire the alternator to system reference control. The link below runs through how to accomplish just that:

Convert Alternator from PWM to Battery Reference