Symptoms of bad alternator? | Ford Explorer Forums - Serious Explorations

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Symptoms of bad alternator?


Well-Known Member
November 6, 2002
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City, State
Edgewater, CO
Year, Model & Trim Level
02 F-150 S.C. Lariat FX4
I have a 99 X 5.0 AWD w/Auto - Everything is stock except K&N flat filter.

For the last few months I've noticed that my battery voltage gauge will drop when multiple electronic items are running (i.e. - defroster, heater, stereo, headlights...) and the interior lights will dim. When this happens, it seems as though the truck loses some acceleration power and the idle becomes rough for a little while. I've had the battery checked, which it was determined to be okay.

Would this be caused by a bad alternator or voltage regulator?

What would be some symptoms of an alternator/voltage regulator going bad?

I've had a problem with the X for a while (very poor gas mileage & horrible performance). The X runs like a POS, could this be attributed to a bad alternator/voltage regulator?

I have done a search regarding this, but I am looking for some other symptoms that could be caused by a bad alternator/voltage regulator.

Thanks for the any advice that is offered.

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go to auto zone and have them test the alternator in the cars for free......

Yes all of those things can be a bad voltage regualtor or alternator. BUT they can also be other stuff, like corroded wiring, loose belt, belt tensioner, instead of guessing and throwing $$$ at it, have it tested........should put out a steady 14.4 volts of more at 1500 RPM..

Also have you added a bunch of aftermarket lights, stereo, alarm, amps, hydraulics, etc to the wiring???

In my experience, when an alternator goes it goes fast.
A good quick check is to disconnect your neg. battery terminal with the engine running, it shouldn`t effect the idle, or at least it will be minimal. You can use a meter at your battery terminals to check the charging rate, which should be 13 point something(?) volts.

The single most obvious indicator of a screwed alternator is that your battery is not charging!

Don't alterantors had diodes in them to convert AC to DC
I think an alternator can put out 13 volts but have a weak or blown diode?

I've also heard the easiest way to blow them it to jump start another car.

In a basic most common alternator:

The windings and stator in the alternator will produce AC current.

The diodes (rectifier) are part of the method of converting ac to dc. Basically you will have at least three pairs of diodes. Each pair has one diode which will let positive current through it only, and one to allow negative current through it only.
Then you end up with a positive charge source and a negative current source to give you DC, as opposed to a current which fluctuates from positive to negative (AC)

The regulator switches the magnetic field off and on several times a second, to produce a steady flow of current (14 volts or so, I guess, now that I think of it)

I was told DO NOT PULL THE NEG CABLE WHILE THE CAR IS ON!! You risk damaging the computer. This was from a shop that specializes in alternators and charging systems..something to consider

I got the same problem in mine. I'm running 2 amps for the stereo and when I crank the stereo up you can feel a noticeable difference in how the truck runs- I mean a real noticeable difference the thing starts stumbling and bucking if I hit the gas hard.

I have a bad PCM too, but if I leave off the electrical stuff it runs fine even with the bad PCM.

I already found a solution to the power problem PowerMaster 200 amp alternator.

I just can't put one in until I have Ford change the PCM, otherwise they'll blame the high-amp alternator.

The 200 amp alt cost about $500/with shipping, but it'll power anything you put on it! :D

There's a whole lot of things that'll do in the alternator and the power consumption of the amps I'm running definitely weakened mine.

I can see where boosting somebody could do in an alternator- the quick hard drains when they crank...

Like the other guys said-
Definitely check the belts and tensioner first and alternator output.

Mine just went

First of all I noticed my batt would be dead if I left anythin on for an hour or so.

Then I stuck a screw driver to the metal housing with it running and you sould hear the bearing was shot.

It's like a stethoscope for your car.

Also make sure if you are running a lot fo accessories that you have the bigger, 140 AMP one. not the base, 95 AMP

ID50 and others
I'm with you on your explaination. This may be a little academic but it was my understanding that there is reduntancy in the diode circuitry.

Someone made the comment "when then go.... they go quick." Well, an alternator may be "going " but totally fails when "the last of the diodes" go. (in the absense of other problems)

I may be wrong.
I think there is a diode test to detect a failing alternator but I have not been able to find it. A dvom should be able to test it ...I think

IgotTwo, you're right they'll run as long as the diodes are doing their job...what you'll notice is a reduction in performance.

That's why when I crank up my stereo the truck starts running real rough and bucks when you hit the pedal.

The diodes weaken and the alternator doesn't put out what it should under a load.

That diodes weakening theory sounds right. It would make sense that you could lose a couple diodes, still put out 14 volts at idle, but it wouldn`t be able to handle a large load. From what I understand once your battery is fully charged you basically run off the alternator only (the current kind of runs past the battery in the circuit). If the load overcomes the alt then your battery current will be drawn to try to compensate (?)

Maybe you could damage a computer by disconnecting the battery, but I don`t think it would be immediate. I recently learned that the battery acts as a filter to smooth out any voltage spikes caused by your charging system. Deja vu, I swear I learned that after discussing it here!

As to damaging the computer by disconnecting the neg, i think you're right, the battery helps to level out the energy levels. But its not a thing that might do a little damage each time, basically the computer will get fried or it wont. It doesnt happen real often, but why chance it ya know? PCM costs a few hundred..AutoZone tests for free

call me silly.......but food for thought
Your fix to correct the system and power requirement seems a tuff route.....correcting the weak diodes won't do it for you?

I'm not an electroincs guy ..but before I went nuts rebulding and replacing ...maybe there is a DC surge or choke (my knowledge-less terms) that you can put after the fuse box and before your add ons as in to - smooth any spikes in the "amp draw" that may be straining the system and whacking the alternator.
- without damaging anything

I have to believe that you are no the only one with that problem and there maybe an easier fix. I am always amazed what the electroinic guru have come up with. Often very simple

My favorite one liner:
I can't be the only only one with this problem and some greater mind than mine may have a fix...why reinvent the wheel?

Try a heavy duty search on the web? to places that don't sell stuff hahaha!

I'm sure there is a fix, I know there are a couple of amps that WILL NOT draw the way my amps do, but hey it's the choice I've already made.

I figure it's worth it to get a high amp alternator and put some additional work into getting a second battery w/isolator because of all the auxilliary lighting and other stuff I've got on the truck...air compressor, coupla sets of fogs...and whatever else I throw in.

I'm sure I'd find a way to level the surges and thanks much for the tip (cause to tell you the truth...I hadn't had that particular thought till you mentioned it :confused: ) I was thinking I wanted the extra juice anyway... :D :D

You guys always throw another monkey wrench out there so I spend even more of my loot! :D :D

When I installed a new stereo system with multiple amps in my X, I had problems with operating my stereo at higher volumes and running electronic accessories (stock 1991 alt only puts out 95 amps). Since I was too poor at the time to spend a lot of dough on a nice, high output alt, I simply upgraded the "Magic 3 wires."

I used 2 guage wiring (from lowe's) and some big gold plated wire spades (circuit city) and ran new wires from:

-Neg battery terminal to the body
-Pos battery terminal to the terminal on the alternator (the one with the threaded bolt)
-Engine block to the body

This created a nice big path for current flow and helped to eliminate dimming headlights and dropping alt voltage when my stereo was cranked and I was running other accessories.

You might also try taking off your battery cables, polishing up your battery terminals with some fine grit sandpaper and give them a coating of dielectric grease before putting the cables back on.


my mechanics teacher told me if you want to know of your alternator is going then stick a screw driver or something metal around the middle where you see the wiring inside and if there is a magnetic feild and it repels then it is still good, if it doesnt then its no good. (while the car is on)