re: OT Mechanics: Battery/Alternator Question

Just to clear some things up in this thread... auto parts stores can not properly check an alternator in the car with a bad or low battery. Their tests go through the battery and can give false readings... most now have equipment that will say "bad battery, replace battery and then retest for alternator". When alternators go they usually take out the battery too. That means you will probably need new battery AND alternator. Be careful not to kill a new battery with a bad alternator

Alternators can be bench tested out of the car which is something you should do before replacing.