2002 Ford Taurus battery not always charging

Paladin3

Diamond Member
Mar 5, 2004
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Hi folks,

Our 2002 Ford Taurus died on my daughter with a dead battery while she was driving it. Long story short...battery was tested and replace, alternator was tested and replaced, all connectors checked, cleaned and tested with volt meter. I also could not find any drains on the batter when the car is off.

Playing around with voltage meter to try and diagnose:
  • New battery shows fully charged around 13.6 volts after overnight charge.
  • Good connectivity between new alternator and battery positive (with neg disconnected) at 0.3 OMS.
  • All fuses confirmed good.
  • Car starts and at idle voltage to battery jumps to 14v+ so the alternator is charging initially.
  • After 10-15-30 minutes of driving (depending on lights, radio, wiper, etc.) battery warning light comes on. Eventually even the ABS warning light will come on because of low voltage.
  • Checking battery with car running after 15 minutes shows voltage slowly dropping as if alternator is no longer charging battery.
  • If I rev the car up enough the battery light will go off, but eventually comes back on at idle.
  • If we continue to drive the car or let it idle long enough car will eventually die with a dead battery and not start.
  • Again, alternator has been confirmed good and charges battery at least sporadically and/or high RPMs.

If I put the car on a charger overnight, it is good for limited driving the next day, but the battery light will eventually flash and it will die if driven too much and have a completely dead batter.

So this tells me there must be something keeping the working alternator from charging the working battery sporadically, but not full time like a bad connection or broken line fuse would.

Any ideas on what to check next? I'm guessing the voltage regulator is the next suspect to replace. That means the entire PCM/ECM computer component. Anyone have any thoughts on what else I might check before I pull it?

Any thoughts or ideas are appreciated.
 
Last edited:
Feb 25, 2011
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If you're doing the work yourself, I'd at least try putting another alternator in there before I replaced the (much more expensive) computer.

Sort of a prayer move, I guess, but unless you were checking voltage the entire time, it's possible you have a defective (new-defective happens) alternator that is only conking out as it warms up.
 

RLGL

Golden Member
Jan 8, 2013
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Ohms test means nothing. You cold have corrosion in the cables. Do a voltage drop test.
 

Paladin3

Diamond Member
Mar 5, 2004
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If you're doing the work yourself, I'd at least try putting another alternator in there before I replaced the (much more expensive) computer.

Sort of a prayer move, I guess, but unless you were checking voltage the entire time, it's possible you have a defective (new-defective happens) alternator that is only conking out as it warms up.
Thanks, but the alternator has been tested good.

I've got a source for a new computer for $179+ shipping with two new uncut keys so no having to get it programmed at the dealership. If I install it myself it won't be too hard on the wallet.

Anyway, I just lost access to a nice, heated garage and have to do the work outside. It's currently snowing here and below freezing during the day, so I'm in no hurry to get this done, hehe. Thanks for the help.
 
Last edited:

deadlyapp

Diamond Member
Apr 25, 2004
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It kind of sounds like the diodes are defective, but usually this would be tested in an alternator test. Perhaps they are giving some strange behavior once the car has warmed up.

Checking the resistance of the diodes while on/cold and on/warm may help diagnose that issue
 

Paladin3

Diamond Member
Mar 5, 2004
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Didn't mean to abandon this thread, just been a little busy.

I did a drop voltage test with car @ 2000 RPMs with high beams, wipers, radio and interior lights all on to put a load on the system. Results were the same cold and after the alternator/engine heated up:

Between battery positive and alternator positive: .04v
Between battery negative and alternator case/ground: .01v

Checked serpentine belt and it's not slipping. Checked every fuse with multimeter and all tested good.

Brand new alternator isn't charging the battery at all. No jump in volts after starting, and battery voltage slowly drops the longer the car runs. Revving car doesn't make the alt kick on and start charging battery.

I'm going to pull the new alternator and have it tested. If it tests good I think the only thing left to look at is replacing the PCM computer which regulates alternator output. Or any other thoughts?

Thanks for all the help, folks!
 

PottedMeat

Lifer
Apr 17, 2002
12,365
472
126
If it tests good I think the only thing left to look at is replacing the PCM computer which regulates alternator output.
seems like you've covered everything and that's all that's left

edit: are there any control signals you can check between the PCM-Alternator?


i thought dodge was the only one that had a non integrated regulator
 

Paladin3

Diamond Member
Mar 5, 2004
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I finally solved the problem and it was something silly. The alternator wasn't grounding well where it was bolted in. I pulled the alt and sanded down all the contact points where it mounts until they were shinny. Then reinstalled the alternator and the problem is gone.

O'reilly Auto Parts put the car on battery load tester for me and all parts of the charging system showed good. Alt was kicking out 14+ volts. Took the car for a long drive with lights, wipers, radio, heater and rear window defroster going. No battery warning light and no battery drain at all.

I feel a little stupid, but lesson learned. Thanks for all the help, folks! We're a two car (and one motorcycle) family again!
 
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Feb 25, 2011
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I finally solved the problem and it was something silly. The alternator wasn't grounding well where it was bolted in. I pulled the alt and sanded down all the contact points where it mounts until they were shinny. Then reinstalled the alternator and the problem is gone.

O'reilly Auto Parts put the car on battery load tester for me and all parts of the charging system showed good. Alt was kicking out 14+ volts. Took the car for a long drive with lights, wipers, radio, heater and rear window defroster going. No battery warning light and no battery drain at all.

I feel a little stupid, but lesson learned. Thanks for all the help, folks! We're a two car (and one motorcycle) family again!
Awesome!

Free fix is best fix!
 

mizzou

Diamond Member
Jan 2, 2008
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not a huge 2002 ford taurus fan....ibwas kind of rooting for you to lose it lol

;)


good job on the fix!
 

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