hi...the other night i was leaving work and my lights went dimm and the battery light came on and then went off. it did it a couple more times and then one of the times the volt meter on the dash bounced up and then down and then back to normal. does anyone know what could be causing this? any info would be greatly appreciated.

Rudy

 

Alternator brushes worn out?

Tell us a little something about your truck. Year, model, engine, mileage, etc.

Pop

 

I have the same issue. i have a 01 f-350 and the battery light came on. now if i pull the negative battery cables off both batteries should it stay running? i know on gas engines thats an easy way to tell if the altinator is bad.thanks for any help.

 

If you do that you may put a LOT more voltage on your electrical system than the normal 14.4 to 14.7 volts.

You've got electronic stuff in there, sensitive to over-voltages, so beware of lifting your battery grounds.

Pop

 

i understand, but does that theory stand for diesels. my batteries are less than a year old so i'm pretty sure it's the altinator. it has almost 200k on the ol girl.

 

I agree with Pop.. don't pull your cables and run your engine.. you are asking for trouble..

 

That theory stands for modern vehicular electrical systems.

Pop

 

I would venture to say 99% that is it your alternator. I just went through the same thing. Battery light came on ect. Mine was putting out just barely enough voltage when traveling at 60 mph towing the trailer so I kept going the next 2 hours home.

Get your self a multi meter and set to voltage. Take the positive lead on the meter and touch it to the positive stud on the back of the alternator and the negative lead on the meter to the positive post on your battery. Of course all this with the truck running if it will. That should give you the alternator out put.

You can also test the voltage in your batteries by going positive lead to red post and negative lead to negative terminal on the battery. With the truck running my truck was dropping voltage very slowly like in the matter of 2 minutes it went from 12.69 volts in the battery and after 2 minutes it was down to 12.47 and dropping slowly.

Hope this helps a little bit Megawatt if I remember right is the electrical guru around here hopefully he will chime in shortly as his advice is top notch and well explained.

Make sure your battery terminals are clean and tight as well. A loose cable can take out your alternator. I found out one of my ground cables on my battery was not all the way tight so I probably should have made the installer of the batteries pay but I was not in the mood to fight with it as I had to change out the alternator and get down the road to rescue another PSD 5 hours away. BTW changing the alternator is maybe a 15-20 minute job takes longer to go get one and get the tools out to change it. Mine works great now.

Hope this helps shed some light on it.

X3 on DONT pull the cables with it running your asking for more electrical problems.

 

new alternator and all is good. had the old one checked and it was only puting out 12v. thanks guys.

 

I have the same problem with the battery light with the problem with mine is this. After I start the truck after it sets all night once started it runs for 10-20 seconds then the battery light comes on then goes back off. It will repeat this 2-3 times varying in lenght from 10-20 seconds to 20- 30 seconds. The voltage at the alternator is 14.25-14.50 depending on load.
When cranking the voltage drops a little lower than 11.75 fo just a moment till truck starts and GPR shuts off. Then goes to 14.25 volts I have had the Alt. tested at 3 different places and all agree Alt. is ok. But I dont think they ck. for amp. output just voltage. Could the Alt. brushes or diaode be bad or getting bad. Have new optima batteries only thing I have found is what I posted a few day's ago is the positive battery cabel and negetive one's are cracked. Could this cause not enough voltage at the battery to cause the light to turn on. I think the light is controlled by the PCM that is sensing low voltage via a control wire.Any Ideas or help would really be great.

99 ford F-350 7.3L 4x4 Dually....Build date 5/98

 

Cracked insulation on a conductor does not, in itself, have any effect on the conductivity of that conductor.

If your alterrnator is original from 1999, its brushes are likely worn beyond the point they can do their job, getting current to the field windings.

Replacing the brushes/regulator might correct the problems you're seeing, but the commutator rings that the brushes ride on likely have deep grooves worn in them, requiring replacement of the slip ring assembly at the least, and the entire alternator assembly at the most.

Pop

 

Mine alternator small wires were shorting out just below the connector (on top).., with the battery light going off and on.. they were rubbing on the alternator housing.. My voltage was good at 14.4.. Had to use a magnifier glass to see the bear copper wire.. Rearranged the wires and re-insulated them.. been good ever since.
Also read a post where a guy's main wire harness that goes over the top of the driver's side valve cover were also rubbing and shorting out.. causing the battery light to come on..

 

I relize the truck is 10 years old but only has 25,000 original miles I know this for sure as I ordered it new in May 0f (98).I have a new set of Battery cabels to install as the old terminals are really cracked. They hold tight But. As far as the Alternator goes could it test good for voltage and still have bad diodes or bad rectifier. I am ready to go nuts over this issue

 

Have you checked the wiring per my post above.? Having only 25K on the motor seems unlikely your alternator is bad specially with your voltage checking out good... Install the new cables and report back the results..

 

Yes I checked the wire at the Alt. and across the top of valve cover drivers side and found no problems of any kind.