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Brake light comes on when it starts running rough

1214 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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Thanks for the help. My reasoning behind it being something electrical, is because of the rpm's that it acts up at. If it was acting up bad while idling, I could see how the low rpm would effect the charging. The puzzling part is that it can be 2,500+ rpm, spitting & sputtering and the battery light on. I checked the charging system with a battery tester I have and it showed it was charging at 14 volts(my tester won't do amps) and this was while the battery light was on.

For S & G's, thinking maybe the alternator was shorting out, I disconnected the 2 pin connector on the alternator. The ONLY difference then was that the battery light wouldn't come on at all, never helped with the running bad.

I would go through the normal motions of cleaning the IAC, MAF, etc., if it weren't for the battery light coming on at the same time as it running bad. I'm having trouble trying to understand the connection between the two.
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.

Thanks for the help. From what I understand, the battery is a few months old tops(looks really new) and I did check the charging system while it was running and while the batt light was on. Tested at all different rpm's and it never left 14+ volts. The battery light would go off occasionally, but you can bet, every time the engine spit & sputtered, the batt light was on!
I had a similar problem a while back. Battery light would come on intermittently regardless of engine speed. Engine ran fine though. Voltage test at battery terminals while the light was on still yielded +14V.

I went ahead and just purchased a new alternator anyway, as 98% of the time that light means "new alternator time". And with the drive I was making at the time I really couldn't afford to chance it and be left sitting somewhere. Sure enough, when I removed the belt and spun the pulley by hand it was apparent the bearings were worn and there was also a little endplay in the shaft.

Notice I said "98% of the time".... The other 2% is either a bad or loose ground or bad battery, or bad diodes which cause excessive AC ripple - which may also explain the driveability issue.
Yeah, I've heard of an alternator making "noise" to interfere with electronics, but not enough that it would freak out the pcm......but replacing it does make the most sense. Thanks for your help guys. I'll let you know what happens.
Thanks for the input guys. I've never been a big fan of throwing parts at a problem, especially when it's somebody else's money. Most of the time there's some sort of test for the part in question. I've already tested the charging part of the alt. and it's doing it's job.....and I've never heard of any way to test for bad diodes.

Seeing how I don't have any codes to help the search, I've told my buddy "this is what I would do". He's quite a bit younger than me, so I gave him my opinion/experience on the cheap "lifetime warranty" rebuilt alternators. I told him to get the best he could afford. We'll see what happens.
Problem Solved!

Replaced the alternator and that fixed the problem! Apparently it was shorting out the pcm, causing it to fall on it's face.
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