Battery Light Came On Yesterday
 

My battery light came on yesterday while cruising at highway speed and sometimes at traffic lights. The battery light came on intermittently and stayed lit while it was on. The battery light did not flash or blink. The truck acted completely normal yesterday other than the battery light coming on at times.

I plan to put a voltmeter across the battery while the engine is running . Voltage should be 14.2 volts. Lower than 14 volts or higher than 14.4 volts could mean an alternator problem. I'll then turn on the lights and accessories. The voltage should stay up around 13 volts. I'll have someone to rev engine up to 2000 RPM. Voltage should go up to 14 or so.

Q1: Does this sound like the correct procedure with correct voltage ranges?
Q2: What kind of re-manufactured alternator would you buy?
Q3: Would you buy a Motorcraft alternator? Below is the 'query by VIN' data that was returned from my internet search:

<table id="ctl00_ModalPopupContent_pcMain_ucViewIllustrat ion_rptrNonCallouts_ctl03_gvNonCallout" border="0" cellspacing="0"><tbody><tr class="gridViewHeader" valign="top" align="left"><th scope="col">Part Number</th><th scope="col">Part Description</th><th scope="col">Price</th><th scope="col">Your Price</th><th scope="col">Availability</th><th scope="col">Quantity</th></tr><tr class="gridViewRow"><td style="width:16%;" valign="top"> 10346
(GL-417-RM)
http://www.fordparts.com/Images/Logo...Motorcraft.gif
</td><td style="width:26%;" valign="top"> Alternator
4.6L; With A/C; 115 amp Reman (same as 130 amp Original replacement requires new pulley #F65Z-10344-AA)
</td></tr></tbody></table>
My cost = $237.98

Requires a new pulley?? Should I just slap a remanufactured one on that is 130A or as close to that amperage as I can get? Thanks in advance! Keith.

I had much the same symptoms with my XP last year. I did basically what you propose to do and ended replacing the original 11 y/o alternator.
I got mine from the local Autozone because I was in a hurry. I purchased a new (not remanufactured) Duralast Gold 130A with a lifetime warranty for $165, it came with a new pulley. Took 20 minutes to change.
Tip, if you do this job, don't take the serpentine belt all the way off, just from the alternator.

Thanks Boat Tug!

In the last four days I've had the battery and alternator tested in the car three different times. Each time, both components checked out OK. The Charging System light is now staying on fairly regularly. Today I noticed the AC fan motor would run proportionally to the vehicle's rpms. All input would be appreciated.

This usually indicates a bad battery or a bad connection to it. The blower is varying RPMs with the engine because it's main source of power has become the alternator and not the battery.

Thanks alloro. I had the alternator replaced this morning and all seems to be well. It got to where the CS light stayed on constantly and felt I had to act. I purchased an O'Reilly's Ultima (new) at $165. It has a lifetime warranty which I hope I'll never have to use. My battery is 10 months old TODAY.

Same boat here too- sorta.

Battery light came on last Friday while driving home. I flipped the ScanGauge over to read voltage and it was 11.3-11.5. Truck drove home just fine.

I parked it in the garage and drove my Explorer for a couple days because I didn't have the time to tackle it until Monday. I pulled the alternator off and took it to Oreily's for check. Their fancy machine tests the alternator and the voltage regulator. Both passed successfully. Twice.

I went home and checked fuses, fuseable links, wire connections and anything else I could think of. I followed the Haynes diagnostic procedure for the "engine off" condition and then reassembled everything and fired the motor up to start checking the "engine on" condition. Now I was getting the correct voltage of 14-14.5 while idling. I didn't do any further diagnostic since it seemed like my problem was fixed.

I drove it to work today. Voltage was fine. Drove it to lunch. Voltage was fine. On the way home I put the air conditioning on and the voltage dropped to 13.6-13.7. Shutting the A/C off had no effect on voltage- it stayed at 13.6.

Is 13.6 with the A/C on and blower on OK? Or should I not worry about it?

Should I do a voltage drop test?

I believe my battery is 12/2012 and the alternator has "Motorcraft" cast in the housing so it could be original.

When the alternator was out, a noise could be heard when spinning the pulley by hand. It was like a bearing grinding a little or maybe the brush rubbing the armature.

Unfortunately from what I recall lots of folks here have pointed out the scangauge voltage reading is inaccurate. Why this would be the case I don't know.

In any event to be sure measure voltage directly at battery terminals at idle, and also at a high idle with headlights and heater on HI, etc, to test charging system under load. Voltage will sag a little bit with all the accessories on, but still want to see 14.x under most conditions.

That's fine, but at the same time it wouldn't hurt to do some follow up testing by taking readings when the alternator is cold versus hot to see if there's a difference.

Interesting. I'll compare when I get home with my multimeter and battery terminals.

Had a similar problem with a 1998 5.4. Turned out the fuse holder on the passenger side by the front door wasn't holding one of the fuses for the alternator tightly. It's been years so I don't remember which one it was. The fuse was fine if you looked at it or just checked resistance across the two sides, and most of the time when you checked voltage through it, it was also fine. If you bumped it just right one of the blades would stop making contact. I'm guessing the wrong bump on the road loosened it up and the alternator stopped charging, but then at some point another pot hole would move it back in place. We fixed it with a small goober of sodder on the fuse blades.

I chased this one for about a month, replaced the alternator and battery in the process. We found it by pure luck.

To alloro's point, alternator brushes wear to the point where it stops charging when it gets hot.
In the morning when cold, all seems well but after a bit of driving, you can find the alternator stops charging again.
Also, if you hear or feel noise or grinding, the bearings have gone bad and grinding or the armature is making slight contact.
To ensure the best reading, use a vom between the rear large alternator post and the battery ground. Expect to see about 14.4 volts just after start up.
With mine, I'd get part way down the road and if I didn't notice the gauge, it would dip to zero charging once warm (no light).

Light came on again today. Didn't notice it at first but for sure after 10-15 minutes.

Replaced the alternator with a new one. Now I'm getting 14.4.

The ScanGauge and multimeter are also a bit off. Nothing too major. .5 volts or so.

I've looked for this passenger side alternator fuses. I'm not sure my truck has this.

0.5 volts doesn't sound like much but it's a hell of a lot when it pertains to auto start batteries and state of charge. Tenths count here. For example 12.8 volts is a fully charged "maintenance free" ca/ca type battery at 70F, but 12.3 is something like 50% charged. So long as the amount of error between scanner and battery posts is known adjustments could be made, I guess.

It's surprising how many trucks, old and new, don't really have a reliable method of measuring battery or charging voltage on the fly. Idiot light only. Just picked up one of these that use 12 volt power port, or cigar lighter receptacle, Innova 3721, about 14 bucks. This will at least allow one to observe trends and get a heads up that maybe the battery is starting to choke.

edit: Depending on temperature and state of charge you'll want to see higher than 14.4, closer to 15 volts. This is at subzero temps. Remember everything with batteries is temperature dependent.