Alternator or battery or something else?

[GMA]

I've got a 2001 F150 with a 4.6 and 180k on the clock.

Recently i've noticed an unusual behavior. Under full throttle at higher speeds, the battery light on the dash flickers. If it's dark outside, the dash lights and head lights dim a little. It only does this if I'm running at hwy speeds (60+) and put the pedal to the floor to pass. Also, only when it downshifts twice, to really bring the rpms up into the higher end of the power band. It's easily replicated in this situation. The needle on the battery gauge on the dash does not drop when this occurs. And it will not do this under full throttle accelerating off the line, like 0-60, but i have not tried to take off from a dead stop and hold it on the floor all the way up to 80 or 90 mph.

First thought is alternator can't keep up with the load when the injectors and coils are firing at that high of an RPM, but every time i've seen an alternator fail in the past, they lacked the ability to keep up with demand at the low end of the rpm spectrum and the issues cleared up as RPM's climbed. This one seems to do the opposite.

Thoughts?

 

[atikovi]

Don't know but not sure how smart it is to floor a 20 year old 180K truck, let alone run it into the higher end of the power band . Beat on it often enough and you'll find the engine fits loosely in a 5 gallon bucket.

 

[Hall]

Check the alternator pulley

 

[GMA]

Don't know but not sure how smart it is to floor a 20 year old 180K truck, let alone run it into the higher end of the power band . Beat on it often enough and you'll find the engine fits loosely in a 5 gallon bucket.

Not the way it is normally driven. And it had probably never been floored before i got it last year. But the issue was first noticed when the cruise was set at hwy speed and hills were encountered. The cruise would advance the accelerator to maintain speed up the hill. On steeper hills, that meant more throttle. First few times it happened, i immediately turned off cruise. But sometimes the best way to diagnose an issue is to identify the scenario that makes the outcome repeatable.

 

[GMA]

Check the alternator pulley

That thought did cross my mind. Tensioner, maybe?

 

[tasch72]

Sounds like a serpentine belt length (too long or stretched) or tensioner issue. Would be worth a stop by a parts store to for a free check on the battery and alternator and to check the part number on the belt.

Do you have intermittent issues with the power steering, as well?

 

[GMA]

Sounds like a serpentine belt length (too long or stretched) or tensioner issue. Would be worth a stop by a parts store to for a free check on the battery and alternator and to check the part number on the belt.

Do you have intermittent issues with the power steering, as well?

Power steering works like new. Never an issue there.

 

[stangguy]

slipping belt?

 

[CharBaby]

I blame it on something to do with the alternator…bearings…brushes…belt…internals whenever the engine reaches the upper range of its RPM, thus the alternator RPM as well. Otherwise, leave it alone til you need another alt.

 

[GMA]

Think I'll slap a new belt and tensioner on it. Not sure when/if the previous owner ever had that done. Then go from there. I guess it makes sense that the belt could be slipping at the highest end of the rpm range/under the heaviest load.

 

[Marcozi]

With that many miles I'm betting the alternator's voltage regulator brushes are bad. But I recommend you replace the alternator if the new belt and tensioner don't fix the problem.

 

[meep]

Is it possible that the engine drops alternator fielding at WOT to allow maximum engine power, then resume when it’s no longer WOT? Similar to A/C compressors?

 

[swampdodo]

Long shot...but are you located up north...could it be a grounding problem (corrosion)???

 

[Chris142]

Brushes are short. At higher speeds they are losing contact with the commutator.

 

[Donald]

Brushes are short. At higher speeds they are losing contact with the commutator.

+1

Bring it to an alternator shop. They can replace the brushes and turn the commutator on a lathe.

 

[Asterix]

+2 The brushes are worn down and too short and/or the copper slip ring is no longer round and the brushes are skipping over the lumps at high rpms.

BTW, it's good to do a regular Italian Tune-up even on a 20-year old truck.

 

[GMA]

I put a new alternator and belt on it this morning. Issue is gone..... runs like a new one.

 

[Accent Abuser]

Don't know but not sure how smart it is to floor a 20 year old 180K truck, let alone run it into the higher end of the power band . Beat on it often enough and you'll find the engine fits loosely in a 5 gallon bucket.

Nonsense!