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Discussion Starter · #1 · (Edited)
Folks,
I’ve done some searching and haven't found a thread on a situation quite like mine. My battery light has been going on and off now since maybe April. However, the conditions under which it’s coming on and off are atypical, and I’ve had a hard time finding patterns in the car’s behavior. Here’s what I know:
  • Car is a 2005 Focus ST.
  • Both the battery and alternator I replaced myself in October 2009, about 42k miles ago. The alternator was a remanufactured one.
  • The battery light DOES NOT come on at idle. [This suggests to me that it is not likely to be an undervoltage problem]
  • The battery light ONLY comes on when moving and in gear.
  • If the battery light comes on, pressing the clutch pedal and/or putting her in neutral and returning the car to idle almost always turns the battery light OFF.
  • Revving the engine to cruising speed (~3000 RPM) in neutral does NOT bring the battery light ON. [This suggests to me that it is not likely to be an overvoltage problem]
  • I drive with headlights on 100% of the time.
  • I have a pair of Hella FF75 floodlights covertly mounted behind the grille that I use as auxiliary high beams when I’m driving in rural country or in the rain. These are controlled independently of all other lights and are properly fused/installed with a relay.
  • I have never been able to get the battery light to come on when the car is stopped and in neutral, which is also the only time I’ve been able to get a multimeter on the battery to measure voltage.
  • With a multimeter on the battery and the car in neutral, battery voltage is within proper operating range under a variety of tested loads and RPM ranges, again leading me to believe that the voltage regulator is OK.
  • The battery light tends to come on with greater electrical loads, but it’s difficult to pinpoint exactly how much.
  • It seems as if the battery light is more likely to come on when it’s raining.
  • I have had a trusted shop look at the car for more than two weeks and they haven’t been able to pinpoint the problem. They tested the alternator and battery and both tested OK. Charging system tested OK.
  • On one evening the car repeatedly stalled when the battery light came on. When this happened, I turned the key to restart and the starter motor was quick and strong as usual, suggesting that the battery was not run down. When I was still rolling, I was also able to roll-start the car without a problem.
The Question that is yet unanswered: What is the actual battery voltage when the battery light comes on?

Any thoughts on what’s going on?

Thanks in advance for your help.

[*]UPDATE[*] 22 August 2011

Ok so I just put a new alternator in the car. Paid a small fortune to have a brandy-new FoMoCo alternator installed instead of spending all day putting in a Mexican remanned myself like I did last time. And on the way home, about ten miles on the new alternator, the battery light came on again.

Any ideas what's going on here?
 

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Get yourself a cigarette lighter plug-wire your voltmeter to that

Others will suggest how to turn your cluster into a volt reading display
 

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You can pull up voltage on your odometer.

Press and hold the trip reset button, turn the key forward to the run position while holding the button. Continue to hold until it enters diagnostic mode (you will know when this happens).

Release the trip button and press to change what parameter you are wanting to find. If I remember correctly battery voltage with come up as Bat 123 with 123 being the voltage the computer is reading, just put a period inbetween the 2nd and 3rd number to get your voltage reading.

Drive the car in diagnostic mode and watch the voltage when your battery light is coming on. My alt just died on my 07 and I used this to diagnose and finally just watch the car die on the way home from work lol.
 

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Discussion Starter · #4 ·
Thanks for the ideas.

Does the car reset itself OUT of diagnostic mode when you shut it off and restart it? Are there any limitations the ECU places on the car (decreased rev limit, etc) in diagnostic mode as a self-protection feature?

Thanks.
 

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DTC P0606
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Does the car reset itself OUT of diagnostic mode when you shut it off and restart it? Are there any limitations the ECU places on the car (decreased rev limit, etc) in diagnostic mode as a self-protection feature?
Yes to your first question and no to the second.
 

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Discussion Starter · #6 ·
UPDATE 22 Aug 2011

Ok so I just put a new alternator in the car. Paid a small fortune to have a brandy-new FoMoCo alternator installed instead of spending all day putting in a Mexican remanned myself like I did last time. And on the way home, about ten miles on the new alternator, the battery light came on again.

What is going on here?
 

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I donno if you've solved this problem yet or not, and I can't swear this is the answer, but my car is doing the same thing, and I'm thinking the serpentine belt is bad..... unfortunately, no one around here carries them? so I have to order one and wait to find out (then I have to wait for it to rain again, cause it only does it when it rains, or on some mornings when there's dew on everything....)
 

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Discussion Starter · #9 ·
Since this thread came back from the dead, here's a quick update.

The new FoMoCo alternator I had installed in the summer of '11 was judged to be defective by the installer and he replaced it under warranty, free of charge.

Fast forward to March of 2014. The second alternator from summer of '11 failed and I just had another remanned unit installed in the car. That's the FOURTH alternator I've put in this car since I bought it. I've still never been able to identify what is failing such that the alternator is unable to charge. What's interesting is that the latest one died at the very END of a 300-mile run without stops. It seems odd that it would quit after hours of steady-state operation.
 

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I've had bad luck with any re-manufactured alternator when I was working at the dealer. Replaced 3 on 1 vehicle in 3 months. 1 diode will go and then it won't fully charge under load. Best bet is to have a local electrical shop rebuild it for you.
 

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Vince your Moderator
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^This
 
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