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Brake light comes on when it starts running rough

1216 Views 12 Replies 5 Participants Last post by  AmerAirSusp
Battery light comes on when it starts running rough

Sorry if this has been covered before, but after searching for a couple hours, I really need to get going on this.

I've got my buddys 2003 Mustang GT Conv. that runs great when it's cold, but skips, surges, bucks & everything when it gets warmed up.

I know, I're thinking the IAC, but the kicker is when it starts acting up, the red battery light flickers on. After doing this awhile, sometimes, the light will stay on for a little while even though you've let it go back to idle.....sometimes it goes off as soon as you get out of it. Sounds like a ground problem or something, but before I tear into this thing, I thought I'd ask the one's "with the skins on the wall" so to speak. Any ideas?
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Disconnecting the alternator 2 pin connector proves nothing other than it disables the dash charge light.

Low battery voltage WILL make the motor run rough as virtually every subsystem on today's cars is affected by low voltage.

IMO it's especially short sighted to ignore the possibility of an alternator problem because it's so easy and cheap to have an alternator and battery tested. Many auto parts stores will do it for free.

Note, no battery also equals no alternator. It is possible for a weak battery or weak battery connection to affect the alternator.

This is so important that IMO every electrical problem should START with a through review of the battery and charging system. Do this before replacing a single part.

Bottom line. Today's cars simply will NOT run right without a strong battery and charging system. Cut corners here at your own risk.
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If $$ are an issue consider a salvage yard alternator. The Alternator on the 4.6 is stupid easy to change. I've done it in a parking lot with nothing but a socket set and 15 minutes.

Because the alternator is so easy to change it's not that big of a deal if you get a short service life. The last salvage yard alternator I bought for $35 and is going on 3 years and counting <knocks wood>.
and I've never heard of any way to test for bad diodes.
FYI, a purpose built alternator tester (the kind a professional would use) will test for a bad diode.

It is possible for the home DIY'er to test for a bad diode. The usual way is to use a VOM set to the AC scale (yes the AC scale). Measure the AC "ripple" at the battery. The value should be low say less than 0.2 volt.

The problem with this method is that it's affected by the quality of the VOM meter used. A cheap meter will not read the AC ripple correctly. Hence the reason why I always recommend just having the alternator tested with a purpose built tester.

IMO the odds do indeed favor a bad alternator. If $$ aren't a serious issue, then I agree with just replacing with a re-man unit.
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