I have a 2000 f150 H.D. edition and I just replaced the alternator because at anything about 4000 rpm's I'd get the battery light to come on and lights would dim. Well I replaced the alt today and when I drove it, weirdness happened. From a stop I gave it full throttle to see if the battery light would come on. It did but something new happened.. Now at 4500 rpm's the battery light will come on steady, AND the ABS light comes on as well. When it shifts into 2nd and rpm's drop, the battery light goes off and 2 seconds later the ABS light will turn off as well. I also felt a quick shudder, like what a missfire would feel like, when at very low rpm's in 4th and lockup has occurred. Why would replacing an alt cause not only the battery light to come on but now the ABS light as well? Both lights go back off when normal driving, its just at the 1-2 shift point at anything over 4500 rpm's. The alt is a 130 amp from NAPA and I had to re-use my old pulley. Thanks in advance!

 

First check your drive belt tensioner and your belt for slack. At WOT the belt could be slipping. As for the ABS maybe a scanner with ABS capabilities can see whats up.

 

The light goes out almost immediately after the battery light goes out. Maybe I got a bad ground or battery, the battery is about 4 years old and its a Walmart one But my lights are back to being bright even at hi rpm's so maybe its battery or a ground. I'll re check those things first. My ABS works prefect, even after it turns itself back off with the battery light when motor reaches lower rpm's. ABS light never is on steady and I never saw it come on till yesterday when it came on with battery light. Got a gremlin in there hahaha

 

I would check the grounds for sure.
Adding a ground cable from the battery to the chassis on my 84' completely fixed all of my electrical nightmares that I was experiencing.

You can get the battery checked for free at most auto stores, and 4 years seems to be the average lifespan of a battery from what I've heard and seen... so that would be worth getting checked out as well. Even if that doesn't fix the problem, its better to find out its going bad and have it replaced then to have it die on you randomly.

Even without a new battery yet, just cleaning the battery terminals and cable connections really well and applying some dielectric grease wouldn't hurt. Make sure to get all the corrosion off that you can and see if it helps.

Not sure about the ABS light though since I don't have one in my truck... due to the lack of ABS in general .