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Alternator going bad and rough idle

Tiny
WHITELAWR42
  • MEMBER
  • 2000 MITSUBISHI DIAMANTE
  • 188,200 MILES
I have replaced my alternator 2 times in the last 10 days. The first time I replaced it, I did not replace the battery at the same time. I bought a new battery and then it went out same day. I got another Alternator and it worked well for about 5 days then today, the emergency brake and battery lights come on. I also notice a rough idle when those lights are on. All of this happens once the car (and alternator) are warmed up (about 5-8 minutes after start up). For a while, they just flicker but then they just stay on. During this, the voltage is usually normal when revved but really low when idling or coasting to a light. I do not remember the exact numbers. I am hoping I have just been sold another bad alternator. It is under warranty so it will be free. Can someone help me?
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Thursday, October 13th, 2011 AT 3:04 AM

13 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
It sounds like a bad alt. If it is a remanufacturerd alt, some are poorly done.
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Thursday, October 13th, 2011 AT 3:12 AM
Tiny
WHITELAWR42
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It is remanufactured. I purchased it at O'reilly Auto Parts. It has a life time warranty but this will be the 3rd alternator I receive from them. I am having it replaced tomorrow but lets say that don't help, would you have any suggestions?
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Thursday, October 13th, 2011 AT 3:15 AM
Tiny
JACOBANDNICKOLAS
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Have the alt checked at the parts store before you take it. With the engine running, it should be putting out around 14 volts. Also, have the one you return checked to see if it is actually bad. If it isn't, my first guess is you have a bad fuseable link between the alt and the battery. If the alt is no good when they check, chances are you are getting bad ones.
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Thursday, October 13th, 2011 AT 3:43 AM
Tiny
WHITELAWR42
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Alright, well earlier today, I had Midas and O'reilly check using multimeters. When Idling (brakes engaged, in gear) or in park, I was getting about 12.5 with a lot of fluctation. When in park and revving to around 1700 RPM, I had 14V. When the motor gets warmer though, performance seems to dip. I am hoping all it is is the alternator.
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Thursday, October 13th, 2011 AT 3:48 AM
Tiny
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You should get around 14 at an idle. If it wasn't steady, chances are the voltage regulator is bad in the alt.
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Thursday, October 13th, 2011 AT 3:50 AM
Tiny
WHITELAWR42
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Well there is a lot of fluctuation. That did seem odd to me. Sounds like you are right.
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Thursday, October 13th, 2011 AT 3:52 AM
Tiny
JACOBANDNICKOLAS
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Let me know what you find. I'll look for your reply tomorrow.

Joe
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Thursday, October 13th, 2011 AT 4:00 AM
Tiny
WHITELAWR42
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Well, my battery consistently is at 100% charge when I have had it checked. I have noticed that lately I do have a rough idle. Also, the ANTI LOCK light has been on since I purchased the car. Not sure if it has anything to do with the Alternator but brakes work fine. But along with the rough idle, the exhaust is extremely rich. I am missing a Catalytic Converter but its even richer than usual. Also, the ECU has not been reset since the Alternator was replaced twice. Not sure if it has much to do with anything though.
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Tuesday, October 18th, 2011 AT 4:27 AM
Tiny
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If you are missing a converter, the computer is in a closed loop. It is causing the rich mixture because it doesn't know what to do.
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Tuesday, October 18th, 2011 AT 5:03 PM
Tiny
WHITELAWR42
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Its richer than usual though. And the idle has never been so rough. Its occasional though. When I bought the car, the mass air flow sensor was disconnected and I had it reconnected recently. Would that possibly have something to do with it? My gas mileage increased significantly after the sensor was reconnected.
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Tuesday, October 18th, 2011 AT 5:09 PM
Tiny
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If you disconnect the MAF, does the engine smooth out?
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Tuesday, October 18th, 2011 AT 7:47 PM
Tiny
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No, Just bad gas mileage. The extremely rich exhaust and rough idle have only begun to come up in the wake of the alternator replacements.
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Tuesday, October 18th, 2011 AT 8:29 PM
Tiny
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I will note that I have not replaced it since I began posting on here. I am trying to find out if the alternator is the main culprit before I spend more money on labor.
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Tuesday, October 18th, 2011 AT 8:41 PM

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