Triumph Repair: battery, gm cars, diods

Question
QUESTION: My husband pulled the cables off the battery while the car was running to make sure there was no problem with the alternator. We took the alternator back since we did not need it and the sales man said my husband should never have done that to a 2003 model car. He did not explain the ramifications as he appear rather disgusted with what my husband had did. Can you tell me what future problems doing this could cause?

ANSWER: Hi Kari,
Some GM cars you could do that to see if the alternator was charging but on most other cars it can blow the diodes in the alternator the instant the cable is disconnected. It is a very bad practice as only a very few cars it can be done on. The proper procedure is to put a volt meter across the battery and read battery voltage and then start the car and set the engine at a fast idle and read the battery voltage again. It should be from 13.8 to 14.5 volts if the alternator is working. If you don't have a volt meter they are not expensive and can be purchased at Radio Shack or even Wall Mart.

---------- FOLLOW-UP ----------

QUESTION: What could we expect to happen when the diodes blow in the altenator? Would it be an instant problem or one that could take time to present itself?
Thank you for your timely response.


Answer
The instant you disconnect the battery. If it blows the diods, it will not charge any more and driving the car will run the battery down shortly after as you use up the battery power. That will happen very quickly if the lights were turned on and wipers and a heater or AC fan was on.
If you put a volt meter like I said to do, you would not see 13.8 to 14.5 volts at the battery while at a fast idle.