What would cause my cars electrical system to be strained when the engine is idling with the AC or heater on?
This usually indicates that your blower motor has either tight or dry armature shaft bushings or the armature itself has shorted windings causing excessive current (amperage) draw to the blower circuit, This can be determined by removing the blower motor fuse (usually a 30A fuse) and probing an amp meter in series to each blade terminal and turning the blower on high with the engine about 1500 RPM and observing the current reading if it is higher than 12-15 amps you need to replace the blower motor!
It's best if you test your alternator output by touching the battery terminals with an inexpensive hand held volt meter with the engine running. You should see between 13.5 - 14.2 volts if the alternator is charging properly. You forgot to list exactly what your volt meter is reading. One other thing to be aware of. Your replacement alternator may not have the same amp-rating as the original alternator. The more electrical systems you switch on your car, the more amp load is placed on the alternator. An auto electric shop, private mechanic or new car dealership should tell you for sure exactly how many amps under electrical load your alternator is putting out. I'll bet money something is not quite right with the alternator.
It's going to be another bad alternator. GM had a problem with your era and model of vehicle as well as other models where they would keep ruining alternators due to excessive heat in the area of the alternator.
