True the lamp is what excites the field.(old fords)
The lamp still does the same job but also has a resistor jumped acrossed the lamp.
This will excite the field in the event of a defective lamp.

Bill

 

540fan think I've solved our problem! Not to flame anyone but some of the info I've found concerning this issue is either wrong or contradictory. So here's what I've discovered and it's working for me. Since my wire colors don't match the wiring diagram I'll give this as you face the motor standing at the bumper. First disconnect the charge cable from the stud on the alternator and carefully place some where it wont contact anything and arc. I slipped with mine and 2000 amps is quite a flash! Start the truck and check the voltage from the negative terminal on the battery to the stud on the alternator. Mine was over 24 volts with no load. If you have 13.5 or above your alternator is working and the problem is likely a burned out fuse-able link just below the ring terminal on the hot side of the starter solenoid. If you have 0 volts as I did next step with the truck off disconnect the 3-wire plug. From right to left, (drivers side to passenger), the first wire (corresponds to the "I" pin on the regulator) should be 0 volts key off, 12.5 (battery voltage) key on and the same w/ truck running. The left wire, ( "A" on the regulator) should be batter voltage ALL THE TIME. This is were my problem was. No voltage at all. I stripped about 1/8" of this wire just back of the plug and used a light gauge (18-22) jumper to the positive post on my battery. BINGO! Battery icon is out, alternator is charging and this jumper is buy far the weakest link so it will go first before damaging anything else. Could not find any info on if there is a fuse-able link between this wire and the battery. Assuming the jumper continues to work I will install inline fuse about 5amps with permanent wire and cut wire at plug to isolate from wiring harness. My voltage at the charge stud dropped to 14.5 under load / jumper installed. Hope this helps.

The "S" pin should be 1/2 half battery voltage with the truck running. This is the center wire that jumpers over to the single pin and induces the field to charge. If you have the correct voltage at the right time in the outer pins but still no charge from alternator it's likely the voltage regulator on the back. This can be replaced but I don't know where to get one. (Direct to a local rebuild shop I expect.) If you do replace it you need a paper clip to insert in the tiny hole of the regulator to keeps the brushes aligned.........or so I was told.

 

Paladin,Great explanation! I am terrible with electrical. This morning a friend at the parts store drew me a diagram and explain it the same way. Help me find a bad wire. The alternator needs power from the battery to activate the field. I'll Try to send rep points.

 

Iallwork,

Thanks and you're welcome. Great if I can help someone in turn. There are a LOT of very knowledgeable people here. But to often they forget the rest of aren't. I've spent as much time looking up or wondering about abbreviations as finding the help and info I came for. Hope the explanation helps you. If you need any further assistance you know where to come.

Chuck

 

Getting back to paladin921 the question is what is the amount of voltage on the green wire KOEO?Should this be 12 v also and if not what would be the primary cause.....voltage regulator?It seems that there would be 12v to the yellow wire all the time and then with ignition switch on 12 v to the green wire.Or also what would be the reason for the voltage to drop on the green wire?

 

MONAVIERONJON,

I can't answer you question without knowing what position the wires are in. My colors don't seem to match others so rather then guess or give you the wrong info tell me from drivers side to passenger what wire you are referring to. What is KOEO? If you re-read my explanation and ignore wire colors, go buy wire position in the 3-wire plug it should answer most of your questions. I'm glad to help if I can. If you don't have power to the drivers side wire with the key on your have a break in the control circuit which includes a fuse and the ignition switch. I still don't know why my passenger side wire in the harness lost battery power but at 225k I don't care. My fix is working and the truck does what I need it to.

Chuck

 

Trying to understand the alt 3 wire layout and function, with the drivers side wire with the key on ,engine off (KEOEO)shouldn't there be 12v to that wire from the alt?A possibility of a bad voltage regulator.wonder the direction of voltage travel on that wire,from alt to ignition lamp/switch as it would seem the lamp is inline between the alt & the ingition switch?Then the wire that is on the A side would have power going to it until the switch opens.is this a reasonable theory?

 

I don't know what function the right (drivers side) wire has. I do know it should have NO voltage with the key off and should have 12v (more accurate-battery voltage) when the key is on or the truck is running. The left wire (passenger side) should have 12v (battery voltage) ALL the time. It's a direct link to the battery. With the truck running the center wire should have 1/2 of whatever battery voltage is. This is coming from the regulator and induces the "field" in the alternator to produce electricity. Therefore if you have no voltage or low voltage in the right wire with the key on or truck running your problem is in that circuit including the ignition switch. If you have no voltage in the passenger side wire it's broken somewhere and the alternator is not receiving the required current to produce a field. If both of these wires have the correct voltage as stated here but you have nothing in the center wire it's your regulator and or the alternator is shot. You can replace just the regulator (from a re-builder). First remove alternator, then regulator and check condition of alternator to see if it's worn out inside. Hope this helps. I'm signing off as 6am comes early.

 

Best wiring diagram I've found so far was in the bottom of an alternator box. I returned the alternator but kept the diagram. Can scan and email it to you if that would help.

 

Must be tired as I missed your main question. The wire on the drivers side turns the regulator on and off.....I believe. It does NOT receive power from the alternator. It should only have power when the key is on / or the truck is running. You wouldn't want the alternator to produce electricity at any time except when the truck is running or bad things could happen.

 

okay,thanks for the info as for this truck the ignition side wire is the concern.Saw the diagram on page 1 of this thread and it has the voltage lamp in between the switch and the alt so it seem as if there wold be current flow from alt to lamp.Noticed there was extensive corrosion on the right side inner fender connector behind relay switch,so cleaned that and soon to test.Was wondering how the wire would have current flow towards alt.most likely pcm.The ignition key has been by passed and using the main switch under the steering column to start.Tested 3 different alts as i have a few and had varied results on that same wire so was guessing that the regulator might be the concern.This same wire on the 3 different alts had 2.7v,2.6v,3.8v so was thinking it might have been the diode conditions.On the passenger side wire there is 12.67v so was thinking that current would flow from one side of switch to the other.thanks for the assistance,Ronjon

 

I recently had the an alternator fail on my '96 PSD with 197K miles. Many of the symptoms listed above in this thread. Removed the old alt. and headed to the parts store. Returned home with the new alt. in hand thinking I would be done in 15 min. After installing the new alt. the truck still was not charging. Logged into the trusty Ford Truck Enthusiasts webpage and found this thread. After trying all the "fixes" listed above the truck still wasn't charging. Now for the real kicker, after 5 hours of struggling to come to the solution everything was pointing back at the new alt. Removed the new alt. and headed back to the parts store and had the boys test the new alt. To their surprise the brand new alt. was bad. Came home with a rebuilt alt. and everything is working fine.

Take my advice.... if you purchase a alt. (new or rebuilt) have it checked before leaving the parts store. If I would have just tested the new alt. with my multi-tester at first I could have saved most of my Saturday and a lot of frustration.

 

Had alt tested @ parts store and installed a new diode,and have same problem.Went as far as into the dash and found a R/gr from ignition that delivers power to the lamp and on the other side of the lamp is the LtGreen/Red that goes to the Alt.it appears as though the measure of amps is exclusively from this wire.Have 2 brand new batteries,cables and solenoid and checked the gpr system.Perhaps the lamp is bad and might try an extra cluster i have laying around or even jump a wire past the gauge to deliver current to alt.Ground is Good ,starter turns although starter isn't receiving enough juice to turn engine over.Traced this LtGreen wire back to the firewall connector and have a good connection so the 2.67 v is consistent.