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Old 01-29-2017, 06:10 PM   #1
LagunaSeca
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Voltage drop when starting

Hello, my son's gal has a Ford five hundred sedan and had been told by a "shop" that her starter was bad. I showed my son how to clean the battery terminals and then tested the voltage with my electrical tester. It showed 12.60 v at the battery which is good. When cranking the motor to start, the voltage typically dropped into the mid 11s, once into the high 10v range. As a comparison test, we did the same test to my 4 cylinder 1994 Camry and it went from 12.65 to the mid 11s as well when cranking, with about a year old Bosch top of line battery. Question - is a drop into the mid 11s or high 10s unacceptable when cranking the engine? Everything else seems unremarkable, nothing weird, and it starts within a few cranks. Thanks for your input.
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Old 01-29-2017, 07:06 PM   #2
aleekat
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Re: Voltage drop when starting

Normally 9v or less is an indication of battery issues. Clean terminals, tight, etc.
So why did she take it to the shop and the diagnosis of needing a starter?
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Last edited by aleekat; 01-29-2017 at 08:16 PM.
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Old 01-30-2017, 01:59 PM   #3
LagunaSeca
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Re: Voltage drop when starting

I gather she stopped by Pep Boys to get her battery checked. She probably saw some ad or free promotion and they told her that the starter needed to be replaced as it was being hard on the battery. 12.65v at the battery looks good to me, and it seems to crank over fine. I just wasn't sure what the magic number was for voltage drop at a battery, what is acceptable for voltage when under starter load.
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Old 01-30-2017, 08:44 PM   #4
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Re: Voltage drop when starting

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Old 01-30-2017, 08:58 PM   #5
LagunaSeca
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Re: Voltage drop when starting

Yeah, I wasn't so much concerned about the current drop between the battery and the starter. Everything I've read online says 0.2, or 0.6, or 1.0 volt drop under a variety of conditions. If that is correct I should not see less than 11.6 amps at the battery when cranking the engine on the starter. And all my cars must be screwed up. I think that unlikely. It's strange that I cannot find this simple number online somewhere as to an acceptable voltage under starter operation.
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Old 11-08-2017, 12:32 PM   #6
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Re: Voltage drop when starting

When you are measuring voltage drop, dont forget the negative battery cable is just as important as the positive one, and often runs down low in the engine bay, close to water, salt, etc. often the negtive battery cable does not connect directly to the starter, but depends on another length of wire from frame to starter, or frame to bell housing, etc. Some of these connections can be really feeble.


When a franchise shop tells you that you need a starter, I wish I could say that I would believe them. There are honest mechanics and service techs out there, but my wife's sisters have gotten lot of B S from mechanics.
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