cyoder

Hello all,

I'm new here with my first truck that Ive ever owned. I have 1999 F-150 w/ 475k on it, but engine and transmission rebuilt at around 365k miles. The truck ran and drove fine when I first got it. Parked it for a few weeks due to it throwing a code for cylinder 3 misfire. Went out to move the truck one day and it fired up and ran fine. Moved everything I needed then went to move the truck back and it wouldnt start. Just clicked. Tried jumping it with the explorer I have and still nothing. Put the explorer battery in and it fired right up. Battery was almost 6 years old and with sitting during the cold I just bought a new one of same size. Truck fires up instantly, but upon driving to work this morning the battery light came on halfway to work. Still had full power on lights, heat, radio, etc. Voltage on the gauge never dropped. Left work and the light stayed on the whole way home. Put the meter on at home when I pulled in with truck running and its at 14.5 volts. Turned off truck and the battery says 12.4 Volts. Am I right thinking that the alternator isn't bad? Wouldn't it have died if the alternator wasn't charging? What else could be the issue? Ive checked fuses 14 on the inside and 11 under the hood and both are good, and I have cleaned the battery cables and terminals. Truck didn't have this issue when I drove it the first few weeks.

RLXXI

Sounds like the alternator to me, the brushes get worn and/or the regulator inside begins to fail and typically cause these issues.
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cyoder

Thanks for your response. I was thinking along those lines at first, but I have never seen something like this happen. I have always seen everything die when the alternator goes out. So I was hesitant to spend the money on a new one. I figured if it was the alternator a 40 mile drive to and from work would surely kill the battery if it wasn't charging but it never even acted dead.

RLXXI

I've seen this exact thing happen many times. Of course I've been doing this kind of work for a living for almost 40 years now so it stands to reason I might lol. One minute it's charging, the next it's not, let it cool off it charges, heats up it stops. Battery light on and charging is also common. Even in 99 they had electronic regulators inside them.
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moparado

You need get a voltmeter to troubleshoot whats going on.
Here's the possibilities:
-The alternator could indeed be bad.
-The battery could be shot not taking a charge.
-The battery clamps could be shot or corroded.
-The alternator (+) terminal connections could be corroded
-There could be a bad ground somewhere especially the battery (-) to chassis ground.
-Parasitic battery voltage leak somewhere to ground.
(Shot alternator diodes are a prime suspect which could also cause a battery charging problem.)

First thing i would do is get a decent DC voltmeter and measure the voltage with engine running at the battery terminals not the battery clamps.
Report back if you're still stumped.
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cyoder

Thanks for your response..
-was thinking alternator first but wasnt sure
-the battery is brand new so shouldnt be shot and it charged with my charger before i put it on the truck
-the clamps were cleaned thoroughly, so they are no longer corroded, but problem still happens
-i have unplugged the two plugs on the alternator and looked to see if there was corrosion, but did not see any
-how do I test for a bad ground to the chassis?
-not sure what a parastitic leak is?

Put a voltmeter on the terminals when I got home today and after a 35 minute drive home, the battery tested at 14.5 volts while running and tested at 12.4 volts while off. Terminal to connection has no voltage drop either.

moparado

A parasitic voltage leak to ground means some device or short circuit is drawing amperage from the battery when the engine is off.
The only sure way to measure this is to put an ammeter in series with one of the battery terminals with all lights, radio, everything off..
Depending on the vehicle and on board electronics, i'd think a current draw of more than 150ma could be problematic.
As i said, a shorted diode in the alternator is a one of the usual suspects.

Here's a somewhat tricky thing about a bad diode in an alternator.
The voltage regulator will try hard as hell to compensate for it but the tell tail sign is to measure the alternator's voltage output with AC set on the voltmeter instead of DC. That will measure the AC ripple voltage. How much AC ripple is good or bad? You need to measure on a comparable vehicle for comparison.

Far as checking grounds terminations, the best and surest way is to remove the ground terminal from the battery's (-) cable to wherever it connects to the chassis or engine even.
Burnish the terminal and its mating surface on the chassis with fine sand paper, a clean wipe of rubbing alcahol and re-bolt it back on.

My bet from what you've been saying is something is intermittent, be it a terminal connector, voltage regulator or likely the alternator itself.

cyoder

Ok that all makes sense. thanks for the explanations. Everyone else I have talked to seems to think the alternator is the culprit as well. I hope this truck doesn't become a money pit. I only paid $500 for it though so I guess there is room to spare.

moparado

Alternators don't last forever.
The carbon compound brushes wear down enough as to not making or intermittently making contact with the slip rings.
Bearings wear out along with the diode or the diode bridge going bad.'
Not much more can go bad except the rare armature or rotor windings either open or shorted.
I'd say on hi mileage alternators the brushes are the usual suspect.

To possibly avoid a money pit, first see if the brushes are shot.
On some alternators the brushes can be removed and replaced easily saving a bunch of money.

Mike's2006F10

Funny, I'm having the exact thing happening with my truck. When it first happened, I took it back where I bought it, and they said because it was not a Ford Alternator in there and that Ford's don't work really well with aftermarket parts. Never had any real problems, the light would come on, once or twice, or not at all for months. Except, I drove up to my brother - in - laws on Monday, snow is melting, Big puddles, which I did my best at missing, but hit a few. On the way home my battery light was on more than not. I put a brand new battery in it a month ago. Wife had the truck out yesterday and the light didn't come on at all. But still going to get the alternator checked. Thanks for all the clues to check!