Battery Drain - What is the “Alternator Field” circuit?
 

I have a:
1999 F250 ext cab
4.6L Automatic
170,000 miles

In the beginning:

About two weeks ago my entire dash gauges zeroed out and my radio (I still had the clock function) went off. I had no lights on the dash either.
I drove to the next exit which was approximately 1 mile and pulled to the side of the road. As I hit the pot-holes on the side of the road, all the gauges went back to normal. I drove another 10 miles to work with no trouble.

For the next several days, the alternator light (battery symbol on the dash) would come on intermittently. Finally the alternator light came on for good.

Because I have 170,000 miles on the battery and alternator, I replaced both. The alternator light is not on any more, but my battery drains the charge.

I felt there was a short somewhere because:
A. When I let my truck sit overnight, battery cables attached, the battery will be dead in the morning. I charge the battery, crank it and drive to work.
B. When I disconnect the battery (either + or -), it does not lose the charge overnight. It cranks right up in the morning.

I searched this board to find how to locate the shorted circuit and think I narrowed it down to the "Alternator Field" circuit which has a 20 amp fuse under the hood on the firewall.

I "think" this is correct because when I pull the fuse to this circuit, the battery does not drain overnight. Also, when I jumped the negative terminal and negative cable with a lamp type circuit tester, the lamp goes out when the fuse is pulled -- exposing the drain on the battery.


Now the questions:
What is the Alternator Field Circuit? It has something to do with the charging system (duh) because when I crank the engine without the fuse in, the alternator light comes on. I have checked the Chilton Repair manual for a schematic and did not see this circuit.

What does this circuit do?
What does it power?
How can I by pass this circuit?
Am I over my head and need to take it to the dealer?
Thanks,


Bo

I'll be watching this one! My truck is doing the same thing... but the wiring is such a cluster**** we're having trouble pinning it down. *sigh* Okay, more searching before posting.

The old style generators that cars used to have used permanent magnets. Spin a coil of wire in the magnetic field, it makes a current. An alternator is the same, except they use an electromagnet instead of permanent magnets. This is your field coil. It should only take juice when the ignition is on. Sometimes the rebuilt ones are bad out of the box, or something else done at the time is incorrect.

-Smoky

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The alternator field circuit is the circuit from your battery through the rotor windings in your alternator to ground.
It creates the electromagnetic field required to induce a voltage in the stator windings supplying the regulator/rectifier with AC which is in turn converted to DC and regulated to 13-14V.

The circuit your talking about only powers the rotor windings.
You can't bypass it, the alternator won't work.

Try disconnecting the alternator and see if the short disappears,
if it does, your alternators bad.

As Smoky said you can get them bad out of the box especially rebuilt one's.
I went through 3 rebuilt alternators before buying a Bosch.
Put the Bosch on, problem solved.

Hope this helps

Steve

That was some good troubleshooting you did to find the problem. If your regulator is built into the alternator, you will have to replace the alternator.

One thing you can check is to make sure none of the smaller wires going to the alternator have power. I think the one you are looking for in particular is a red/green wire if I remember right. This comes from the keyswitch, and when it has voltage, it brings the alternator "online". Of course if this wire had power all the time, the alternator would be online all the time and run the battery down.

If the wiring checks out good, it's probably something wrong in the circuit in the regulator that triggers the alternator field windings to activate.

Dave, it was your post that helped me locate the problem. -- https://www.ford-trucks.com/forums/s...d.php?t=380350 Between me and my 11 year old son, we narrowed it down.

>> . . . make sure none of the smaller wires going to the alternator have power. << I think the problem could be this or the voltage regulator.

Based on advise above, my next steps are:
1. Check wires as indicated above. (Easy rout)
2. Check the voltage Regulator and replace if needed. (I think it has a seperate VR)
3. Check the Alternator and replace if needed. (So much for AutoZone)
4. Make a b-line to a dealership. (I hope they don't rip me a new one)

I will let you know what happens.


Bo