OK I have 14.04 volts at the batteries with the engine running but my battery light is on. I had the alternator checked off the engine and on the engine and it tests good. Shows good batteries and good charging. Is there something internal to the alternator that would make the light come on even though the alternator is charging ok? Or is there something else that would make the light come on. This is on a 1999 F250 Superduty Powerstroke 7.3. I'd appreciate any help. I did the search function on here but am still not clear as to the problem.

George

 

Power is sent from the battery to the key and then into the coil of the alternator. Then 12v is sent through the dash bulb and into the alternator. If the alternator has 12v on the coil and the bulb has 12 going into it, then both sides of the bulb have 12v + and no flow so no light. If the cluster is shorted out then voltage can come the bulb and light it, or it the coil is not energized, voltage can go through the bulb and light it.

 

The coil you are referring to is a coil within the alternator?

 

Yes. There are a couple of coils inside the alternator. I'm not very up on how it works, but here's some idea. An outer coil is charged to create a magnetic field, then the armature is spun and starts pushing electrons around and creates an AC voltage(something like 30vac). That voltage is rectified in 12vdc which is what you want. Without the extra magnetic field you don't get the high voltage AC and I believe it either reduces the amperage or the DC volts depending on how power hungry the rest of the system is.

 

OK thanks, so if I have 12V on one of the small wires and do not have 12v on the other small wire with the engine running, the light is on. If 12v on both wires than I shouldn't have a light. That's how I think I understand it, right?

 

I believe there are three pins in the pigtail. However if it's like the gas alternator, no wires in the middle because it's internally grounded to the alt case. So if you have 12v on both wires, then the alt isn't the cause of the light. Now I think I recall there being a low voltage drop to trigger the light, you must have atleast 5v or 7v to keep the light from coming on.

 

I would suspect the alternator. Inside are 6 diode. The AC current produced by the alternator is run through the diodes and it becomes DC current. If 1 or more of these diodes go out you'll still get some output but not as much current. During episodes of higher electrical demand the current cannpt go up, so the voltage goes down. which is triggering your light.

Have your alternator retested. Hopefully the shop testing it will spin it up and put a load on it. If it still passes take a close look at your battery cables and battery,

 

Do you have a diagram of a six diode rectifier? I've only seen 4 diode bridges.

 

Start the truck.... while it's running pull the postive connection off the battery.

If the truck stays running. the alt. is GOOD.

if NOT replace the alt.

 

The problem with that is it can cause a voltage spike that could damage the computers. It's not terribly likely, but something to be concerned about.

 

Hate to bring this back up but now I am having the same problem. Did you ever get this resolved? And if you did what was the problem? I will go out and test the alt plug right now and when I pass by the auto parts store I will have them bench the alternator.

 

I too am having this problem, then after awhile the light went off. Also 1999 with 210,000. If I do replace the alternator, what is the difference between the high output vs the regular? mIs it worth the difference in price? No amp but pull travel trailer and other water trailer. Appreciate the input. Dave

 

If the standard alternator worked OK for 210k miles, there is no need to upgrade to a higher output alternator.

 

Thank you for your response. It is folks like you that make this forum. I appreciate your responses and know others do too! Regards. Dave

Cleaned cables and all last night and light was off for about 15 miles this morning and then came back on. Hope to make it home.

 

When I replaced my exhaust manifold I had the batt. light come on, then I cleaned up the cable that goes from the truck body to the manifold stud and the light went away.

Just the other day I checked the manifold bolts and made sure they were all tight, and now as of yesterday the batt. light is coming on again.

I have not had a chance to re-check the cable but I am going to and when I do I will let you know the outcome.