I have a 97 f350 and it recently died because it was just running off the battery and I suspected it was the alt and replaced it but it is still not charging unless I touch a wire from my positive batt post to the green wire connecting to my alt. After I touch the wire it will charge to normal (14.2) but it wont stay at that voltage and drops to about 13... I have replaced the alt about 3 times so far and charged the batt before installing each time. I also ran a continuity test all the way to the pcu and took it to the dealership to ge the pcu flashed because I thought it was the computer, but its not...

Thanks in advance and I hope someone can help

 

it sounds like a trigger wire like when you turn the key on and send excitement voltage to the alternator to energize it you need to see if you have power at the alternator when the key is on i believe you have a 1 plug alternator don't quote me but as far as the 1 goes a typical ford alternator i usually see at least 14.5 volts at the alternator engine idle should raise under load or idle crack. so see if you have power at the connector key on engine off should read battery voltage sorry if i say power in stead of volts but if you have battery voltage at the pin in the connector with key then its a good first step also do you have a battery charge light of some sort on the dashboard lite is it a green wire by chance if i remember should be a plug with about 4 wires it is a gas or diesel engine i am assuming a gas. by the way does it die when running or after you shut it off

 

The voltage at the back of the alt is barley reading 11, its a diesel and it does have a batt light on the cluster. Also thank you for the help

 

so you are taking a reading from the positive output of the alternator the stud with i thought was red insulation not to focus on color as battery plus is b+ no matter the color. the wire i was talking about was the plug with 3 to 4 other wires if memory serves and the hot wire is either green or red and green in the connector that plugs in to the voltage regulator in the alternator. have you disconnected the battery with the engine of if so do you get any sparks from the negative battery cable. and do you have any blown fuses haven't looked for a diagram yet but can if it would help you at all

 

yes I am taking a reading from the pos output of the alt and no blown fuses in both fuses panels. I have 4 wires coming off the alt a green wire, white/black and white with it pigtailed to another connection. The white/black is hot and so is the white and its pigtail. The green wire barley has 1 volt coming off it.

 

not charging unless I touch a wire from my positive batt post to the green wire connecting to my alt. After I touch the wire it will charge to normal (14.2)

if you mean the green wire with 1 volt key on then that is most likely the cause you have little to no excitement voltage going to the alternator to energize it so the alternator can charge the other wire should be hot key on or off engine off or running but the green wire should have b+ so the green wire should run back to the ignition switch some way through a series of shared circuits also let me get a diagram to be 100 percent but im certain the green is B+ key on so is this the wire you jumped to make the alternator charge. again i will be looking at a diagram shortly
by the way with the key on engine of does your battery charge light come on it should without the engine running also i should say hot instead of B+ because this is not always the case sorry about that

 

I do not have the batt indicator light on either times

 

I agree 100%..the Light Green/Red wire is the exciter voltage to turn on the alternator. Without it the alternator is a paper weight.

This is a basic diagram for the alternator circuit:

courtesy of oldfuelinjection.com

There is supposed to be a 510 ohm resistor across the bulb, but I have heard it is frequently faulty, so if the bulb is burnt out the exciter voltage will never make it to the alternator.

 

thank you rla2005 for the diagram and for your incite i agree to that the bulb being blown is a possibility to verify its blown i am looking at some connector pins to enable you to manual energize the circuit if the bulb still does not light then you can isolate that its with in the bulb if it lites with this next test i can post shortly after i find out what the direct route to the bulb without taking the cluster apart and it lites then you know the issue is else wear let me get a couple more pieces of info by pins a mean shortest route to the bulb that would allow you to single of that circuit as best you can

you need to do what you did to make the alternator charge look at the cluster to see if the lite comes on and of and do the same with key on engine off i assume you used your meter lead or a pin to back probe the wire in the connector and jump it off the battery

 

Mine was doing the same. Ended up being a faulty instrument cluster. Would of never thought that the instrument cluster would have anything to do with it besides taking a reading.

 

The instrument cluster is part of the exciter circuit, so yeah it's gotta be working right.

How it works is like this...

You get up and realize it's STILL not #*@&! Friday yet... get ready for work, go out to your truck and give it a quick walkaround (I do, anyway) with your flashlight. Get in, get comfortable, and turn the key...

Battery power goes from the ignition switch to one leg of the bulb in the alternator idiot light up on the cluster, and from there it goes out the other side of the bulb over to the alternator, where it connects to the field windings. These are located on the rotor inside the alternator, and the electrical connection is made by brushes and slip rings. Current flowing from the ignition switch, thru the idiot light, into these windings causes the idiot bulb to glow. The battery power energizes the field winding, creating a magnetic field. The alternator is ready to go, the key has made it to the START position by now, and the engine lights off and starts accelerating to idle RPM...

Now the rotor with it's field windings is spinning inside the alternator, and it begins inducing current into the stator windings. This is another set of windings that are located around the rotor, attached to the housing. Current is induced into these by the rotating magnetic field, and they generate 3 phase AC power (the word alternator comes from "alternating current generator") which goes thru a set of diodes to be converted to DC, then from there to the battery and electrical systems around the truck.

Some of that current is diverted off the stator to another set of diodes, the "diode trio", and after passing thru and becoming DC, it goes back to the field windings. Now the alternator is producing it's own field power, so it can self-sustain as long as the engine keeps turning. Because the field coils are now energized by stator current, voltage rises at the field terminal which of course is (was) also where the idiot light was sinking it's current (using as a ground). Because the field terminal has + voltage on it, the idiot bulb now has + voltage on one side from the field terminal, and on the other side from the ignition switch, and so it goes out because it has no ground.

If your idiot light burns out, or your cluster has bad connections in that circuit, your alternator can never get the initial field current to begin charging and of course since it can't begin charging it will never self-sustain. Because your bulb is blown, you won't know it ain't charging unless you have a voltmeter, which our trucks do. Fords have a ~500 ohm resistor in parallel with the bulb to prevent condition this from happening.

By the way, I didn't mention the voltage regulator so as to keep things simple. Field current actually passes thru this on it's way to the field windings, the regulator adjusts the actual voltage that is applied to the field windings, which controls how powerful the magnetic field is around the rotor. Stronger field = more output.

Damn, I used up most of my break typing that lol. I don't mind explaining though, I hope you got some good info from this.

Back to work...

 

you need to see if you have power at the bulb being as it is not coming on sounds to me like its a connection issue since you have 1 volt at the pin in the connector at the alternator so you need to test the wire under a load when its connected to the alternator and the engine running at the green wire in the connector on the alternator and be sure you have a good ground that needs to be right or your testing will be off do this first then worry about the bulb later when you do this test you will probably get the same 1 volt then you can move on to the bulb see if you have power at the bulb or verify it it blown if it is blown then when the circuit is complete the alternator should now be charging if the bulb is blown if not then see were in the circuit back to the switch that you have power up to were you do not and then you should be able to find what is wrong broken wire bad connection or something of the sort could not find a master diagram of the whole exciter circuit but it should be fairly simple and pulling the connection to get at the bulb is the only way or best way i see to go unless something changes that you find an issue before hand as far as that goes one last thing i think you already said the meter on the truck is reading low voltage correct