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Old 02-17-2008, 04:01 PM   #1 (permalink)
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Removing Cables + Killing Alternators?

Common wisdom says don't pull the positive battrey cable while the engine is running, correct?

Why is this? What happens, and why does it kill the alternator?

Back in the day this used to be the way you checked to see if the system was charging (Pre-alternator stone age). Nowaday's It's considered a good way to fry you alternator, although many people do this and suffer no ill effects. I'm just looking for a clear logical answer to this question..
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Old 02-17-2008, 04:40 PM   #2 (permalink)
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good question.

i think i always dis-co'd the neg cable, but for the same reason, to test the alternator. if the engine kept running then the alternator was working.

i have also "heard" that doing this is not a good idea anymore although i still do it
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Old 02-17-2008, 06:42 PM   #3 (permalink)
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It doesn't kill the alternator. It used to be a way to test if an alternator was working or not. No affect on the alternator what so ever

Pulling a positive cable and then hooking it back up can send an electrical surge through the system, thus burning up an ECU in newer cars.
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Last edited by gustav129; 02-17-2008 at 06:44 PM.
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Old 02-17-2008, 07:00 PM   #4 (permalink)
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Quote:
Originally Posted by gustav129 View Post
It doesn't kill the alternator. It used to be a way to test if an alternator was working or not. No affect on the alternator what so ever

Pulling a positive cable and then hooking it back up can send an electrical surge through the system, thus burning up an ECU in newer cars.
I think that's what i'm thinking, ECU and assorted electronics. Is it possible that this surge on re-conection would also blow the voltage regulator, thereby rendering the alternator non-charging? Just curious.
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Old 02-17-2008, 07:55 PM   #5 (permalink)
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Quote:
Originally Posted by gustav129 View Post
It doesn't kill the alternator. It used to be a way to test if an alternator was working or not. No affect on the alternator what so ever

Pulling a positive cable and then hooking it back up can send an electrical surge through the system, thus burning up an ECU in newer cars.
At least one alternator builder disagrees with you: http://www.powermastermotorsports.co...ternator_.html
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Old 02-18-2008, 04:22 PM   #6 (permalink)
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Yes William, it can happen, but it's more of a protection of a couple hundred $$ ECU rather than replacing a cheap regulator. ECU protection is also the same reason why it's not recomended to jump start a newer car anymore. Now your supposed to only use a booster pack.

Quote:
Originally Posted by Albin View Post
At least one alternator builder disagrees with you: http://www.powermastermotorsports.co...ternator_.html
Sorry I didn't get back to respond. "Switching an alternator off" is a different scenario than what is being discussed here.


Basic alternator wiring (please forgive my horrible MS Paint skills):
Unhooking the positive battery cable doesn't switch the alternator off.


What you are talking about look like this:


I've seen this is used in mud racing (and probably drag racing), along the same thoery of less drag on the motor. Turn the alternator off when you get to the starting line and there's less drag on the motor to charge a battery, similar to switching to an electric fuel pump or electric water pump. then turn it back on on the way from the pit to the trailer.
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Last edited by gustav129; 02-18-2008 at 04:25 PM.
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