Automotive

Battery light comes on when idling and goes away when accelerating?

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[OP]
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Aug 23, 2013
1244 posts
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Vaughan

Battery light comes on when idling and goes away when accelerating?

Hey everyone my 08 ford 350 super duty gasoline work truck has been giving me problems as of yesterday morning. When I got downtown for work, I noticed my battery light came on. When I rev up to about 3k rpm or begin driving, the battery light disappears but as soon as I am coming to a stop or fully stopped and idling, the light reappears. I went to my mechanics yesterday after work and he tested the alternator with full load (wipers, radio, light, etc.) but said the alternator was fine. He said I needed a new battery. When I took the truck to him, the battery light wqs not coming on until I left his shop (I guess it was charging on the highway all the way down to his shop) and now another friend of mine who happens to be a mechanic said he still thinks it's the alternator or a loose wire setting off the indicator. What do you think it is? I don't want to buy a new battery and it ends up being the alternator as I'm still warrantied for the alternator (changed less than a year ago)
75 replies
Deal Expert
Aug 22, 2011
33707 posts
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Ottawa
Your mechanic should have tested the battery also.
[OP]
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Aug 23, 2013
1244 posts
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Vaughan
vkizzle wrote: Your mechanic should have tested the battery also.
I think he did and that's why he said it was weak. He said it should be fine until it gets colder outside

Edit: but he rushed because he was busy and this isn't the first time he misdiagnosed
Deal Addict
Nov 30, 2011
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GTA
It could be a corroded wiring connection. Try disconnecting the battery terminals; clean them with a wire brush and re-connect. If you can access the wires coming out of the alternator do the same to them and see if it makes a difference. I've had a car just die from a bad battery connection (and then purr like a kitten once they were cleaned).
Deal Expert
Aug 22, 2011
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Ottawa
marcodt wrote: I think he did and that's why he said it was weak. He said it should be fine until it gets colder outside

Edit: but he rushed because he was busy and this isn't the first time he misdiagnosed
It may very well be your battery in this case, as idling doesn't produce enough charge versus actual driving.
[OP]
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Aug 23, 2013
1244 posts
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Vaughan
So many mixed answers here haha and I don't really want to spend the money on a new battery.. Any other ways to test this?
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Jan 27, 2006
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Woodbridge
^ My guess as well, this happened with my Acura RSX.
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Nov 24, 2013
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Kingston, ON
Your battery light is probably coming on when the voltage dips while idling, and then charges just fine (gets the voltage up) from the alternator while actually driving.

I'd just replace the battery, clean the terminals, and see what that does for you. Batteries are ~$100 (Costco has good deals) and tend to have issues after only a few years. It's a cheap consumable item that's not worth hours of troubleshooting with alternators unless the problem persists after a change. You say the alternator was changed a year ago, but I imagine your battery is older than that?
[OP]
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Aug 23, 2013
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Vaughan
Mike15 wrote: Your battery light is probably coming on when the voltage dips while idling, and then charges just fine (gets the voltage up) from the alternator while actually driving.

I'd just replace the battery, clean the terminals, and see what that does for you. Batteries are ~$100 (Costco has good deals) and tend to have issues after only a few years. It's a cheap consumable item that's not worth hours of troubleshooting with alternators unless the problem persists after a change. You say the alternator was changed a year ago, but I imagine your battery is older than that?
Thanks for this, Mike.
That's correct the serpentine belt and alternator were changed together last winter and the battery has not been changed for a few years.
Does this need to be changed immediately or can I wait until late fall?
It's actually for a catering coffee truck so when I get to my stops, I rev the engine to make the light go away and shut off the truck that way when I turn it on again, the light is gone until I become idle again.
[OP]
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Aug 23, 2013
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Vaughan
I also noticed that if I shut the truck off with the battery light and then turn it on, the light disappears for a few seconds (as if I revd or something)
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Jul 14, 2009
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GTA
So your willing to risk being stranded in your truck rather than spend $100 on a battery for peace of mind.

I really dont understand some people and their train of thoughts.
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Jun 12, 2007
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London
Goto partsource. They can test your battery for free while its still in the truck
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Nov 24, 2013
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Kingston, ON
marcodt wrote: Thanks for this, Mike.
That's correct the serpentine belt and alternator were changed together last winter and the battery has not been changed for a few years.
Does this need to be changed immediately or can I wait until late fall?
It's actually for a catering coffee truck so when I get to my stops, I rev the engine to make the light go away and shut off the truck that way when I turn it on again, the light is gone until I become idle again.
$100 business expense. Seems like a no-brainer to change out now, in my opinion. You could wait until it gets colder, but if the light's coming on now, why risk being stranded? I don't know about you but my time's worth more than that.
[OP]
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Aug 23, 2013
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Vaughan
What if that's not the problem? My boss won't pay and I'm already paying him $500 rent per WEEK and I'm supposed to fix HIS truck that HE owns? Lol.

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