Battery disconnected with engine running
Thread Starter
Instructor
Joined: Jun 2008
Posts: 233
Likes: 9
From: Marietta Ga
Battery disconnected with engine running
So in an attempt to troubleshoot a "jumpy" amp gauge, I disconnected the negative battery isolation switch with the engine running. Reconnected and thought that may not have been a good idea.
The engine didn't stall and I'm wondering if that can potentially fry something? Did I do something stupid to my alternator by doing that or is it no big deal? Car runs OK.
The engine didn't stall and I'm wondering if that can potentially fry something? Did I do something stupid to my alternator by doing that or is it no big deal? Car runs OK.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
So in an attempt to troubleshoot a "jumpy" amp gauge, I disconnected the negative battery isolation switch with the engine running. Reconnected and thought that may not have been a good idea.
The engine didn't stall and I'm wondering if that can potentially fry something? Did I do something stupid to my alternator by doing that or is it no big deal? Car runs OK.
The engine didn't stall and I'm wondering if that can potentially fry something? Did I do something stupid to my alternator by doing that or is it no big deal? Car runs OK.
When you disconnected the battery with your engine running the alternator kept it running. No big deal.
Pro
Joined: Apr 2014
Posts: 615
Likes: 187
From: Orlando, Dundee, Val-des-Monts PQ
"Disconnected the NEGATIVE battery isolation switch" not the positive. We all take off the positive to check alternator working or not. I have no clue if the method original poster used would adversely effect anything. Mark
Melting Slicks
Joined: Mar 2008
Posts: 2,989
Likes: 198
From: Meriden CT
So in an attempt to troubleshoot a "jumpy" amp gauge, I disconnected the negative battery isolation switch with the engine running. Reconnected and thought that may not have been a good idea.
The engine didn't stall and I'm wondering if that can potentially fry something? Did I do something stupid to my alternator by doing that or is it no big deal? Car runs OK.
The engine didn't stall and I'm wondering if that can potentially fry something? Did I do something stupid to my alternator by doing that or is it no big deal? Car runs OK.
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Thread Starter
Instructor
Joined: Jun 2008
Posts: 233
Likes: 9
From: Marietta Ga
That didn't help with the negative disconnected. Its very random and when it takes a large move it even drags down the idle. Other times it just sits at zero rock solid on the amp gauge. I have a volt meter on the cig lighter and the voltage jumps around from 14.2 to 14.6 volts at idle.
I think I'm chasing a random short....
I think I'm chasing a random short....
Thread Starter
Instructor
Joined: Jun 2008
Posts: 233
Likes: 9
From: Marietta Ga
Voltage erratic -Problem Solved
That didn't help with the negative disconnected. Its very random and when it takes a large move it even drags down the idle. Other times it just sits at zero rock solid on the amp gauge. I have a volt meter on the cig lighter and the voltage jumps around from 14.2 to 14.6 volts at idle.
I think I'm chasing a random short....
I think I'm chasing a random short....
Last edited by Joebrick; Mar 27, 2016 at 11:07 AM.
Tech Contributor
Joined: Jun 2004
Posts: 20,905
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
It's not a wise thing to do. If the alternator happens to be putting out a decent amount of amperage charging the battery, and you open up the battery line, the alternator doesn't have the ability to ramp down the output current instantaneously. You end up with a big inductive voltage spike on all the harness 12v lines. This is a bit abusive on any electronics in the car. Thankfully our antiques have a minimal amount of sensitive electronics content in them. The electronics are designed to deal with this abuse, but there's no up side to pushing one's luck.
Burning Brakes
Joined: Aug 2008
Posts: 878
Likes: 55
From: Deep South Denmark
It's not a wise thing to do. If the alternator happens to be putting out a decent amount of amperage charging the battery, and you open up the battery line, the alternator doesn't have the ability to ramp down the output current instantaneously. You end up with a big inductive voltage spike on all the harness 12v lines. This is a bit abusive on any electronics in the car. Thankfully our antiques have a minimal amount of sensitive electronics content in them. The electronics are designed to deal with this abuse, but there's no up side to pushing one's luck.
I would also worry about frying the bridge in the alternator.As for positive or negative battery pole, there is no difference, once either of the poles are disconnected no current runs to or from the battery.
The only reason why it is generally recommended to disconnect the negative is that if you short the wrench you use to loosen the connector to the metal body in the car (not an issue in the Corvette) then nothing happens if it's the negative.
Last edited by Danish Shark; Mar 28, 2016 at 10:17 AM.
Instructor
Joined: Nov 2013
Posts: 220
Likes: 17
From: Pasadena MD
So in an attempt to troubleshoot a "jumpy" amp gauge, I disconnected the negative battery isolation switch with the engine running. Reconnected and thought that may not have been a good idea.
The engine didn't stall and I'm wondering if that can potentially fry something? Did I do something stupid to my alternator by doing that or is it no big deal? Car runs OK.
The engine didn't stall and I'm wondering if that can potentially fry something? Did I do something stupid to my alternator by doing that or is it no big deal? Car runs OK.