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August 18, 2005
Good to know
Having just purchased an Acura TL (the nicest car I've ever owned), which also boasts an unbelievable amount of hi-tech wizardry, this piece of advice, courtesy of mechanic-columnist Douglas Flint at the Car Connection, seems worth passing on:
Discharged with prejudice
Every now and then a customer will come in with an electrical problem, either starting or charging, and tell me he knows the alternator is working because he unhooked the negative battery terminal while the car was running and it didn't shut off. Apparently the look on my face must be sufficiently horrified because after a few seconds they usually say, uh, did I do something wrong?
I suppose this alleged test dates back to the Model T Ford and probably wasn't valid then. It is rather the equivalent of setting a woman on fire to determine if she's a witch or not. The answer won't much matter. As the negative battery cable is pulled away, a tremendous voltage arc occurs. The alternator starts to both overcharge and burn with nowhere for the voltage to go, and every circuit in the car is desperately hunting for ground. Think of all the computer processors, diodes, solid state circuits, relays, and modules all getting an enormous voltage spike. You may as well fly a kite into a thunderstorm and hook it up to the back of your PC. Heck, BMW says you can blow the radio in their car with just a jump start.
So please, if you believe nothing else I've written, do not disconnect your negative (or positive) battery cable while the car is running. It is not a valid test, it tells you nothing, and is likely to cause future electrical problems.
The rest of his Mechanic's Tale columns are worth a read, too.
Posted by Mike Lief at August 18, 2005 07:28 PM