Out of ideas for electrical issue

[Friborage]

So I just bought a new explorer to replace my old one. It has 125k on it, looks to be in excellent condition. I've had it for about a week now, drives good but a little weak. Replaced air and fuel filter, cleaned throttle body and maf, did a seafoam treatment and ran a half can through the gas, definite improvement but Im pretty sure it still needs plugs. Everything works great now except this electrical thing.

On my old 95 explorer, the voltage gauge would show normal.. like smack dab in the middle of the battery picture on the gauge, but at idle when I turned headlights on, blower fan, rear defrost, hit the brakes, really any major electric draw it would bog down on the gauge and everything would dim. Then soon as I would rev it a little it would recover right away. Eventually the belt started chirping and even revving it wouldn't recover it, so I replaced that and problem solved. It was solid, I could turn on every electrical device on that vehicle on high and the voltage gauge would not even budge.

Well, this new one (2000, 4.0L) is doing the same thing. From the time that I test drove it I noticed the voltage gauge was reading low, even while driving so I made a mental note of it. I ended up buying it that day and took it right to my work and used our charging system tester. Alternator showed bad, battery showed bad. I wasnt really upset, I expected to have to put money into it right away, so I changed out the battery and alternator right there in the parking lot. The belt tensioner came apart when I took the belt off so I threw a new one on while I was there anyway. The gauge looked way better now when there is no load, and while driving. It would still bog down under electrical load at idle, though... the gauge drops low and everything dims, but recovers the second you rev it. The belt started squeaking, thought for sure that was it, but alas, it didnt help the drooping gauge.

So that's where I am now. New alt, new belt, healthy battery, clean battery terminals. It was a reman alternator so Im maybe thinking I got a bum alt? Voltage gauge droops when I turn on electrical stuff, blower fan slows down, headlights dim... everything recovers the second you blip the gas pedal. No aftermarket electrical devices at all. Any hints?

Edit: Also I've noticed with a dome light on that the dome light dims and brightens very slightly rhythmicly... like with the rotation of the motor.

 

[RomeovilleIL]

Most likely the battery cables are corroded under the insulation, leading to lower current flow.

 

[BustedKnuckles24]

Pulsing dome light tells me the alternator is bad. A blown diode in the built in regulator will do this. It also kills the output of the alternator.

 

[DjDom]

Corrosion, bad connections, or bad wires.
I would start there.
The ground wire for the block/body is probably corroded at it's contact point.

 

[koda2000]

I would start by checking the voltage output of the alternator (with a VOM) at the "B" terminal. with engine running and all accessories off, you should see around 14.6 volts. also check the voltage at the battery. it should be around the same, perhaps slightly less. then turn a bunch off stuff on and check it. i'm thinking bad alternator or voltage regulator.

 

[boominXplorer]

I agree on replacing the cables but if that doesn't work look into a new idle air sensor. You might be dipping below 500rpm for a few seconds and loosing amperage causing the dimming under load.

 

[Friborage]

It was the alternator in the end. Replaced alt a second time and now it runs strong electrically.

Just something to keep in mind that people sometimes forget, and as s parts professional I sometimes don't like to admit: brand new products from well known names can sometimes be dead in the box!

 

[D.R.Ball]

So , is every thing working now ? After all things (never fail in the box)......New battery cables should never be ignored either....