Ford Focus Forum, Ford Focus ST Forum, Ford Focus RS Forum

Ford Focus Forum, Ford Focus ST Forum, Ford Focus RS Forum (
-   General Technical Chat (
-   -   bad alternator causing rough idle ? (

beatmaniac1 02-09-2013 09:53 PM

bad alternator causing rough idle ?
Hi everyone !
I have a bad alternator and/or voltage regulator that i've identified with the in-dash test mode ( voltage was under 12 v till 2500 rpm ). I'm also having a rough idle : motor shakes and listening to the exhaust gave me impression of an engine misfire (airflow isnt steady at the tail pipe like pof-pof-pof-pof... ). My question is : since voltage at idle is slightly under 11v , could this lead to an engine misfire ? Like if the coilpack would not have enough ''juice'' to make a good spark ?

Thanks [ffrocks]

stromberg2 02-10-2013 07:19 PM

That's a great question! I've got rough idle as well.


LittleRedSVT 02-11-2013 12:41 PM

My alternator failed a diagnostic test and I also have rough idle... I cleaned by IAC valve, didn't do anything.

Help would be appreciated.

bd8134 02-11-2013 11:43 PM

If you think you have a bad alternator you should either get it checked out for sure or replace it. Autozone or Advance Automotive do this for free. It is a more comprehensive test with it removed.
Yes low voltage can cause a bad idle and you may end up breaking other parts if you drive it like that.

beatmaniac1 02-12-2013 12:54 PM

I checked the voltage with the in-dash test mode, which I believe is pretty reliable (is it ? ) . The lcd wouldnt display 12 v till I hit 3000 rpm and the battery is less than 6 months old so I figured the problem had to be the alt. I havent run the car since the problem appeared. Its gonna get repaired tomorrow morning probably so i'll know for sure. But thanks for the answers, i'll let you know if the alternator was guilty of the bad running of the car

thanks !

beatmaniac1 02-13-2013 05:02 PM

So we changed the alternator today but we also found that the problem was a bad coilpack ( seems like they happened to both fail at the same time ), it's gonna get repaired tomorow so i'll finally have my car back and runing again ...But its going to cost a small fortune [scream] it sucks that I had to go to a garage ...

SVTF 03 02-14-2013 03:39 PM

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.

MarkTheShark 12-28-2014 02:47 AM

My vehicle runs fine until I turn on the headlights or flash high beams which both cause engine to shake, cut out, and cause gauges to go crazy like an alien space ship is nearby. Could my issue be an alternator?

Milton 12-28-2014 12:54 PM

bad alternator causing rough idle ?
Alternator, or battery or wiring or connections or any combination. Go over all wiring and connections. Then have the battery charged and then load tested. Then have the alternator tested.

Mandroid 12-28-2014 01:32 PM

I think you need to seriously look into the alien space ship theory...

But really, what Milton said is correct. However, the alternator is a common failure part on these cars. But don't go replacing things without testing first. Do you have a multimeter?

All times are GMT -5. The time now is 12:31 AM.

Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.1
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.