1992 Ford F-150 Voltage at battery posts

Tiny
DUSTMAN
  • MEMBER
  • 1992 FORD F-150
1992 Ford F150 V8 Two Wheel Drive 157000 miles

My gauge on the dash started showing a very low charge rate. The needle went to the low side. Stopped by one of those auto stores. The battery was tested, it's fine, and the alternator was tested, it was the problem. Bought a new one and installed. Checked the volts at the battery terminal with my multimeter. Engine not running about 12.8vdc. With engine running, about 12.8-13.8. "A guy" told me it should be about 13-14vdc point something with engine running.
Is this true?
Where to go from here?
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Monday, August 23rd, 2010 AT 6:26 PM

13 Replies

Tiny
WRENCHTECH
  • EXPERT
Yes, that does seem a bit low. It could be a quality issue with the alternator. You might want to try another one but don't be surprised if you get a whole run of the same brand with the same problem. Those discount stores sell some real junk.
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Monday, August 23rd, 2010 AT 6:42 PM
Tiny
DUSTMAN
  • MEMBER
Prior to taking the alternator back and getting a different one, I'd like to check the voltage at the alternator. The alternator has 2 plugs.

Which plug do I unplug for voltage check?

Which terminals do I place my multimeter probes?
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Tuesday, August 24th, 2010 AT 10:21 AM
Tiny
WRENCHTECH
  • EXPERT
You can run an alternator without a load attached to it.

Your already measuring the output because it's connected directly to the positive battery post.
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Tuesday, August 24th, 2010 AT 10:55 AM
Tiny
DUSTMAN
  • MEMBER
I wanted to check the voltage at the alternator just in case a problem might be at the plug(?)What do you think?

What do you mean when you say "you can run an alternator without a load attached to it"?
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Tuesday, August 24th, 2010 AT 11:02 AM
Tiny
WRENCHTECH
  • EXPERT
The output wire for the alternator is connected directly to the battery terminal. That terminal has to remain connected to the battery as long as the engine is running or it can spike all the modules in the truck.
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Tuesday, August 24th, 2010 AT 11:18 AM
Tiny
DUSTMAN
  • MEMBER
Ok, here's what I'm finding.
On 8/25/2010 with engine cold and outside temperature about 90 degrees F:
13.32 vdc at battery posts engine not running
14.56vdc with meter at battery posts with engine running @ 1500 rpm
14.8vdc with battery negative cable off, engine running, with meter at positive post to a ground
14.5vdc with battery negative cable back on but loose, engine running, with meter at positive post and to a ground

With engine up to operating temperature (idling @ about -1k)
14.12vdc with meter from ground to positive battery post
13.8vdc with engine running, and a/c on maximum
13.08vdc with engine running, a/c on maximum, and headlights on
12.9vdc with engine running, a/c on maximum, and headlights on high beam
12.9vdc with engine running, a/c on maximum, and headlights on high beam and radio full volume(cool tunes play'in)
With engine still running this time rpm @ 1k+
13.5vdc a/c on maximum, and headlights on high beam and radio full volume(cool tunes play'in)
13.79vdc a/c off and high beam and radio full volume(cool tunes play'in)

Is my alternator doing what it should or should the voltage still be higher?

Is my battery ok?

Noticing when the a/c is turned on, the rpm drops below 1k, also the voltages lowers. With a/c turned off, rpm rises to 1k+ and voltage rises as well. Could I adjust the idle on the throttle body to keep rpm @1k?
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Wednesday, August 25th, 2010 AT 12:25 PM
Tiny
WRENCHTECH
  • EXPERT
No, that's pretty much normal. Your not supposed to have any accessories on when testing it.
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Wednesday, August 25th, 2010 AT 12:41 PM
Tiny
DUSTMAN
  • MEMBER
Ok, I didn't know about not having accesories on when testing.

So my alternator is not bad?

Can I adjust the idle higher to keep my voltage at 13.8+? Is that enough to keep my battery charged?
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Wednesday, August 25th, 2010 AT 5:15 PM
Tiny
WRENCHTECH
  • EXPERT
No, the computer controls the idle. It's not adjustable.
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Wednesday, August 25th, 2010 AT 5:47 PM
Tiny
LOGGER9409
  • MEMBER
DRIVE IT. IF YOU EVER HAVE A PROBLEM IN THE FUTURE, BUY THE 180.00 BRAND NEW NEVER USED ALTERNATOR. DON'T SWEAT THE VOLTAGE DIFFERENCE NOT A BIG ISSUE UNLESS YOU PLAN ON HAVING ACCESSORIES LIKE SNOW PLOW OR A BIG SYSTEM (RADIO WITH AMPLIFIERS ) OR POWER INVERTER.
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Wednesday, September 1st, 2010 AT 4:54 AM
Tiny
DUSTMAN
  • MEMBER
What is the difference? I do recall the guy telling me the one for my truck was for the extra stuff that was needed. This one was about $40.00 less. Are you saying mine was a rebuild?
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Wednesday, September 1st, 2010 AT 8:45 AM
Tiny
WRENCHTECH
  • EXPERT
Did you have to give him your old one? If you did, it was a rebuild.
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Wednesday, September 1st, 2010 AT 9:22 AM
Tiny
MATTHEW.ADELMAN
  • MEMBER
[quote="Dustman"]1992 Ford F150 V8 Two Wheel Drive 157000 miles

My gauge on the dash started showing a very low charge rate. The needle went to the low side. Stopped by one of those auto stores. The battery was tested, it's fine, and the alternator was tested, it was the problem. Bought a new one and installed. Checked the volts at the battery terminal with my multimeter. Engine not running about 12.8vdc. With engine running, about 12.8-13.8. "A guy" told me it should be about 13-14vdc point something with engine running.
Is this true?
Where to go from here?[/Quote

Actually, if your last alternator was bad, it could just be that the alternator is having to replace the amperage your battery lost during the period that you alternator was faulty. An alternator is only designed to replace the amperage that it takes to start the engine. That's why your alternator is usually rated around 95 amps, even though your battery is around 800 CCA. If your alternator is charging the battery at higher than non-running battery voltage, your system is good. I'm willing to bet, though not necessary, if you replaced your battery or put it on a trickle charger, your problem will be eliminated.

If it ain't broke, you ain't gonna find it, so don't fix it.
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Saturday, September 11th, 2010 AT 3:36 PM

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