99 grand am alternator

My son has a grand am that the battery was hooked up backwards on. we replaced the alternator 2 times but it keeps blowing them. I am getting him a new battery just to try that, but am not sure why it keeps killing the alt. I have looked for blown fuses but found none.you can jump start it and it will run as long as the battery has a charge but it kills the alternator within a few seconds. Anyone know what would cause this?

I suspect you blew a fusable link with the backwards hookup, and that blown fusible link may be creating an open circuit for the output of the alternator. I would need to see a wiring diagram for your car to confirm.

My point is an alternator needs a battery in the circuit to absorb/dampen out the voltage spikes that an alternator normally produces. If there is an open circuit between the alternator output and the battery, you can blow either the diodes in the alternator or even some electronics in the vehicle.

Look to see if you’ve blown a fusible link. You may want to also verify that you have continuity between the output of the alternator and the positive post of the battery (and if you don’t, that may be the fusible link).

The point that JM brings up about the line between the alternator output and the positive battery post being open may be correct. If the line was open then the alternator regulator would try to boost the output voltage to try to compensate for the open condition and that may be taking out the alternator. To see if that is true you could see if there is 12 volts at the alternator output lead while the engine is off. That lead should be hot at all times.