1999 f-150
5.4 80,000miles

whenever I'm driven crazy like, ( i know, i shouldn't do that ) and I'm go'n about 75mph and still in 3rd (od auto), my headlights dim and the battery light comes on. As soon as the engine drops to 2000 rpm, the lights brighten and the icon goes away.

what do ya'll think?

need new battery (I think it is the factory)

or

new aternator (factory i'm sure)

what route to do you think i should try?

tim

 

Im going to do some guessing here.

At higher rpms the coils have to fire more often and draw more current.

The old generators produced more current at high rpms but alternators allow more current to be produced at low rpms.

The battery cannot supply enough current at the high rpms because the ...

Oh, what the hell! Just take the battery in and have it tested as a first step...

Also make sure that the battery and alternator connections are clean...

 

I had exactly that same problem on a 1976 E 350 with 90A alternator. Over time it got worse till I replaced the alternator. Never had a problem for the 3 years after that I had the truck. BTW, it had a 4.10 LS and no OD.

 

Maybe your belt is slipping at high rpms......

 

For a 1999 with a digital Odometer, put it in DTM and press the button until bAtt is in the display. ( at least I think the 99 F-150 is a digital odometer ?? )

Check the values of what is show both below 2,000 RPM and above 2,000 RPM to see what is going on.

This is what can be shown under that section :
Displays the code (0-255) for the battery voltage input to the HEC. Battery voltage gauge will indicate present battery voltage. 93-102 6.2-9.1 volts, low voltage
115-124 8.5-10.7 volts, Normal band start
215-225 15.8-18 volts, Norm band end
230-241 16.9-19.1 volts, high voltage

Good place to see if the icon is from low voltage condition.

If the voltage is in the normal band both below and above 2,000 RPM, then the next most likely area is the amp output is falling on its face ( V * A =Watts ). If the voltage is good the amp output is falling off, causing the headlights to dim ( could be either or causing it ). To test the amp output and amp draw on the system you would need a meter with an external current loop, which you might not have access to. Easy test for this, take it to the store and have them test it.

For the full text on the HEC DTM :
http://www.mustangworld.com/ourpics/News/99hec.htm

Good luck

 

I ALSO HAVE A 99 F150 WITH THE 5.4, 72,800 MILES AND FOR THE PAST 4 - 6 WEEKS THE BATTERY INDICATOR LIGHT WOULD COME ON UNDER HARD ACCELERATION. AND SOMETIMES MAKING SLOW TURNS. TOOK TRUCK IN TO DEALER THEY CONFIRMED BAD ALTERNATOR AND REPLACED IT. STILL UNDER EXTENDED WARRANTY. OR WOULD OF COST ME $500.00. IF MINE HAD OF NOT BEEN UNDER WARRANTY I WOULD OF CARRIED IT TO AUTO ELECTRIC SHOP THEY WOULD OF REPLACED THE DEFECTIVE PART(S) AND BEEN MUCH CHEAPER. THE REMANUFACTURED ALTERNATORS ESPECIALLY NON OEM FORD, ARE NOT VERY RELIABLE. AND STILL COST ABOUT $265.00 FOR THE 110 AMP WHICH IS WHAT I HAD.

 

my '00 4.6 had the same problem, was bad altenator. had replaced under extended warranty. only had about 50,000 miles on it.

 

Just last night as I was driving home I notice as I accelerated my lights would dim and the battery light would flicker on. Hooked up a meter and ran it from the battery terminals out under the hood and inside the passenger door. My wife held the meter and when I accelerated the voltage dropped from 14.45 vdc to 11 vdc. replaced the alternator today with the 130amp unit from AutoZone for $151.00. Lifetime guarantee. Problem solved!
98 Expy 104,000 miles 5.4 motor.

 

I had this exact thing happen over a year ago, my first thought was belt was slipping. That wasn't it, then about a month later my alternator completely died. Replaced with a quality re-man unit (70K miles) now 1 year later (92K) the problem is starting to happen again. Going to replace soon. As the RPMs go over 3500-4000 the headlights dim and the battery light goes on. Is this about the same RPM range everyone else starts to notice the problem?

 

Today I did, as someone suggested, put it in DTM mode (which I never even know you could do, very cool!!!) With the engine off the number displayed was around 150. Then after I started the car the numbers are around 190. I quick check with my voltmeter shows that the car makes 14v at idle with the car on. So I assume 190=14v and 150=12v. Then when I drive the car, the number stayes at 190 up until about 3500-4000 RPM, and then the number drops all the way to around 150 as the battery light goes on. So after having done this I think that my alternator is producing little, if any, current at the higher RPMs. I would suggest anyone else with this problem try this simple test.

 

i had the exact same problem on my 98 4.6 at around 3500-4000 rpms and ford put a new alternator under warrenty luckly. but im sure that your alternator is bad.

 

Replaced alternator and the problem is gone just as I figured. Luckily the re-man unit i bought a year ago had a warranty.

 

Great thread, found it while Googling! Is it OK for a Ranger owner to post in an F150 board?

I had the alternator replaced last week in my '03 Ford Ranger, $340 bucks later that seemed to solve the problem - at least initially. The shop also recommended I change the battery, not because the charge was low, but because it was nearly a 7 year-old battery, I declined.

This morning, the battery light came on again!!! I noticed when the battery light comes on, the ABS light also comes on. Both lights seem to only come on when I accelerate to about 3,000 RPM's.

I replaced the battery myself this morning with a Kirkland/Costco battery to rule out the battery as being the issue.

The battery light and ABS light still come on. Again, it seems to be when I accelerate to about 3,000 RPM's.

Neither light stays on. As soon as the RPM's come down, then the lights go out.

It seems suspiciously similar to the problems in this thread. Could the new alternator be bad?Nearly $400 bucks later, I still don't have a 100% resolution. I'm probably going to bring it back to the mechanic this week.