I just noticed today that the battery gauge in my 99 F150 is dipping way down. In the past it would stay a little above half way between H and L no matter what I was doing. Now it won't make it to half way with all the accessories turned off. With the AC or headlights on it is almost down to L. It comes up a little bit above idle, but not a whole lot more.

That sounds like the alternator to me, but is there anything else I should be looking for? Its a 5.4 with 140k on it. The battery isn't even 6 years old and was one of the 8 year Ford ones. It seems like if it was the battery it would just crap out. Oh, and I haven't had a battery light come on or anything like that.

 

If the battery has a shorted cell, I think you would see what you are describing.

When you shut down, open the hood and then, about 3 hours later take a voltage reading across the battery terminals. I suspect it will show well below 12.7 V, which is what a fully charged battery should show. (Please don't open a door before you do this reading or it will throw the whole thing off.)

Now, if your battery voltage is way below that 12.7 mark, it means either your battery is dying (and I think that IS the problem), or that your alternator is not good. But, if your battery has a shorted cell, your alternator will not be able to do what it's supposed to do, and you'll see the symptoms you describe.

Honestly, if you've had the battery for 6 years, I think it is dying, regardless of its warranty. You can also get the battery load tested to confirm this, but do it soon. A shorted battery will destroy an alternator and then you'll have to replace both.

- Jack

 

I appreciate the help. On the drive home I ran out of power and had to quickly park it. Unfortunately I'm 1000+ miles from my tools so I guess it's just going to have to go to the dealer.

 

The dealer isn't the only place that can fix it - just the most expensive.

 

Well, dealer says alternator, and of course found some other stuff. They did say my battery tested "weak" so I'll be running it to costco (thought it was a Ford batt- guess not) as I found its just over half way to its 100 month "life". Hopefully they will prorate a replacement.

$350 for the alternator installed doesn't seem that bad given what Ford gets for the part alone. The other stuff, well whatever.

 

Costco is VERY good about honoring battery warranties, but, you MAY need your receipt for it. If you don't have that, well, make sure you save the new receipt.

As I said, a weak battery can take out an alternator. I learned this way back in the late 60s and 70s, when alternators replaced generators. My Ford vehicles then would ALWAYS suffer alternator AND battery failure at the same time.

This was before I learned how delicate alternators really are, and how important it is to keep healthy batteries in your vehicles.

- Jack