Question? Should i be worried or drive as normal?
<hr style="color:#D1D1E1; background-color:#D1D1E1" size="1"> OK, cruising back from a day at the gun range, i notice my "battery light" flashing on the dash. So i shut off the A/C and radio and monitored the volt meter as i drove.

The volt meter seemed to be reading slightly high but other than that no issues.

Made it home after driving some 40+ miles. But the "idiot light" continued to alternate between flashing and staying on, to going off altogether while stopped.

The truck is a 2000, F-150 XLT extended cab 4x4 5.7 v8 w/137k miles

First, where do i start checking? Second? is this something to get major worried about or do i just drive as normal.

I had planned on driving another 90 or so miles tomorrow to do some trout fishing before summer classes start on monday.

HELP!!!!!

 

Get a voltmeter and check the battery with the truck off. See what kind of voltage you get. Then, start the truck, leave all your accessories off and check the voltage again. Now, cut on all the accessories and check the voltage a 3rd time. Sometimes a weak battery can strain the alternator and make the light flash. Could be the Voltage regulator which in internal to the alternator. Could possibly be that it is not putting out enough amps. Does it do it going down the road or sitting idle?

 

At 137k the alternator brushes likely are worn down and comutator worn.
Voltage check is ok but won't tell you much about the condition of the alternator.
Basicly if you do several starts, then measure the running voltage at the battery you should see something in the area of 15.6 or more indicating the alternator sees the lowered battery voltage and is trying to put a charge back in it.
As the battery recovers it's charge the voltage drops back to about 14.5 +/- a tenth or so.
All these actions depends on battery health and alternator health for how long it takes and it's capacity to recover the battery charge.
I would test the battery capacity from a full charge to see what it's age is and test the alternator for ability to charge.
This still may not see the brush/commutator condition.
Better to get this fixed and go fishing some other day. The fish will wait for you.
Good luck.

 

Will do the checks prescribed.

The battery light stays on at 50-70 mph...but seems to turn off sitting at red lights at idle....then as your coming up to sped starts to flash, then stays on at speed....but when i got back into town, the last mile or so of the trip it stayed on constant.

As to the battery health...it BETTER be good, considering i just bought a new one 3 months ago.

I, myself was kinda leaning towards ole/worn out alternator.

Thanks guys. I will mess with it today and check back with the results of tests.

 

Well yes, at higher RPM if the light comes on the brushes can no longer follow and stay in contact with the comutator from either or both being worn and the comutator surface is rough.
Replace the alternator.
If your happy the fish will be happy and have a nice worry free day on the water..
Good luck.

 

With that many miles on it I would just replace the alternator. It's worn out. It's cheap insurance! It's better to jump the fish then need a jump.
You might try cleaning all your ground connections. Any black wire bolted to the body or engine. Take them loose and sand the connection areas and the wire terminals till shiny. No sand paper? Make one of your Wifes cardboard nail files disappear. Protect the connections with dielectric grease (tune up grease) or use Vaseline jelly. Just enough to protect surfaces from corrosion. Same for your battery. Clean the battery connection to the engine block too. If you don't have a repair manual for your truck the library does. You may have to order it from a main library. Can't check it out but your can read it and take notes.

 

Thanks guys!!

I have the haynes repair manual. Its looking like i will be doing an alternator job come pay day.

Finally got out and did some checking. Took the voltmeter and turned off, just sitting, battery was reading 12.3 V...fired it up, checked it and it was reading 12.1 V. Looked in the cab and sure enough the battery light was on.

Off the top of my head I seem to remember that the alternator should be cranking out 13.5 V when working properly. Correct?

Anywho, thanks again for the advice!

 

Yes, on a running motor the battery voltage will be higher depending on the battery state of charge.
Some 101 on charging system:
The alternator has a regulator that controls the field winding hence the output.
.
How does the alternator know when and how much to charge?
.
The regulator has a "reference circuit" that all actions are referenced to.
If the battery voltage is lower than this reference, the battery's higher resistance will accept current forced from the alternator into it to reverse the chemical reaction that took place upon discharge.
The difference between the battery voltage and the reference determines the charge current value.
As the battery voltage increases by charge restoral, the difference between it and the reference value keeps reducing the charging current until a level is reached to only supply the ongoing load from the motor and anything else drawing power to operate.
At this pont there is stabilization so the voltge remains nearly constant with smaller changes in load current and the battery only accepts enough charge to keep it at it's desgn float level without overcharging.
.
From the above, you can see that for instance an overcharging condition boils the water out, damages the plate structure, wears the alternator brushes/comutatior, overhreats the alternator and causes extra loading on the motor above the normal.
.
Finally, another condition that runs the battery down and causes excess charging is a 'diode' stack low resistance or short failure. This will discharge the battery after shutdown depending on how low the leak resistance is vs time.
Good luck.

 

A static not running battery should be at 12.6 volts. Running it should be at 14.5 volts. After cleaning and protecting all your connections and you don't get these levels. You have a problem with the alternator or battery. I have snow plow trucks. Charging is a big issue with us! Everytime you move the plow is like running the starter. So I know charging issues and amp flow. An alternator only puts out 1/3 it's rating at idle.

 

ooops -- just saw your last post. So much for my overvoltage theory.

Just for the books, you should get a voltmeter and read what's going on when --especially when the light comes on. In dash voltmeter is probably not too accurate. Easiest way to do it is to wire up a cigarette lighter connector ( or whatever they call them now), so you can read it safely while driving.

I'm thinking that it might be reading too high and that's causing your light to go on. Overcharging will damage your battery (undercharging will also cause damage because they only recharge so many times).

A defective regulator can cause overcharging--but these days they're pretty much internal to the alt, so it's still an alt problem.

It would just be good to know what it's doing now, so you can be sure that it's fixed.

12.6 is pretty pretty much normal for the battery sitting there, 13.8-14.5 when it's charging. Over 15 while charging is not good.


good luck,

hj