Focus Fanatics - View Single Post - bad alternator causing rough idle ?
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Old 02-14-2013, 03:39 PM   #7
SVTF 03
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Join Date: Jun 2012
Fan#: 99859
Location: Canton, MI
What I Drive: 2003 Pitch Black SVT EAP ZX3

Posts: 473
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I did a test with 3 local well known auto part store chains with my alternator. Had it tested at the first store and it was good, then took it to another store of the same name and it failed. Then took it to the other 2 different chains and said it was good. Watched these noobs try and hook it up and I had to tell them how to do so and how to even mount it in the machine they had. I hate taking my stuff to people that are ignorant. Ended up doing my own test with a voltmeter and determined it was bad after all. The power post was shorted to the case. So the positive cable post on the alternator was acting as a ground. I actually ended up finding all of this out after overnighting a 230amp alt to go in my SVTF (I always am looking for the best bang for my buck). It worked for 2 minutes and died (covered under warranty). That is when I did all the testing. Now I'm not sure why the 230amp alt died. But I did notice on the all-in-one harness (Negative, Positive, and Starter Cable) that it has a 175amp inline fuse in the factory wiring. I have a brand new harness with the same rated fuse. My guess is the factory wiring can't handle the 230amp alternator or vise-versa. But that's just a guess. But the original bad alternator was definitely the culprit of my engine running issues.

I have yet to do a Big 3 upgrade. But from what I've read it would be a definite improvement. My car ended up dying on my way home from work one day with no real indication of any issues. I had replaced the coil and wires before all this happened, just as preventive maintenance and it just needed a tune-up.

You should have 13+ volts when the alternator is charging. Just put a voltmeter to the battery when the car is not running. You should see 11.5-12v depending on the battery and it's charge. Start the car. The voltmeter should show 13+ - 14.5v. If it's not charging, check the 3 prong connector. It's been said these go bad. Mine was in great shape. I remember talking with Tom (Focus-Power)(Turbo Tom) and he had mentioned the RPM in which the alternator can start charging can be changed IIRC.
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