Customer complaint was that his vehicle will not crank over after it was running for an hour. He vividly explained that it will not restart after shutting it down and leaving the doors opened for a few minutes, it almost seemed that he was backtracking his steps and attempting to recreate his problem. He had already replaced battery and performed his own test on the alternator by unplugging red and black post and turning the lights on while car was running and was sure it was good. He insisted it was an electrical problem and suggested not to check the charging system. By the way, the vehicle will restart imminently after jump starting it. All this information was accounted for and i was concerned of a severed parasitic drain after shutdown. The parasitic drain test checked out and i ruled out any electrical problems. I was concerned of anything that will be binding the engine while it was trying to turn over, but a low amperage check while cranking quickly determined that wasn’t the case. So i proceeded on checking the charging system, what i saw was what i wasn’t expecting, the alternator was charging at 14 volts.

So i decided to measure the diode reading on the back of the alternator and this is what i saw. Huge peak to peak swings measuring over 1 volt. The spec that i use is no more than 300 mv from peak to peak. I quickly determine that the alternator wasn’t charging properly and after a few minutes when the temperature started to increase the diode readings became worst and the same time the alternator was charging at 7 volts. It was hard to convinced the customer that the alternator was bad, especially if he already’ had someone else check the alternator output voltage while the vehicle was running. The alternator was replaced and the problem no longer exist.’