Wiring 3G Alternator

Ron R.

New Member
#1
Will, I think I found the cause of my charging issues....duh.

Long story short, I had supplied a 1 wire alternator to my engine builder. Because of fitment issues, he switched to a Motorcraft 3G alternator...and I didn't notice when I wired everything up.

Funny how a 3 wire alternator won't generate when you only hook up 1 wire.

So, my question...There are 3 terminals "A, S and I". I assume that the "I" gets power from the ignition switch. Do I just pigtail together the "A" and "S" together, and hook them to the blue "idiot light" wire?

Regulator pictured below, if you're curious.
 

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User Resigned

Lifetime Premier Supporter
#2
In the SPF harness, the black wire goes to switched +12v on the battery, and thus would be the alternator's "sense" wire (how it tells whether it needs to charge or not). The Blue wire grounds the negative side of the dash Alt indicator light in order to turn it on.

At http://www.oldengine.org/unfaq/3ag/3ag.htm I found the following re 3G alternators:

  • BAT terminal
    • the output terminal
  • I terminal
    • runs to the gen light
    • typically green/red
    • 87-91 units incorporate a 500 ohm resistor in parallel with the gen light. Unknown about earlier units. And it is a question wether a resistor needs to be implimented in the I line when swapping into earlier vehicles.
  • S terminal
    • simply jumpers over to the stator plug
    • typically white/black
  • A terminal
    • runs via fuseable link to a 12volt source
    • typically yellow/white

Based on that I would say A goes to SPF Blue and I goes to SPF Black. So goes next door to stator plug in alternator body (see picture in the doc).
 
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Ron R.

New Member
#3
Alan,

Thanks for looking. I've been finding a lot of conflicting results, and believe me, I've looked for several hours.

The main output, ie, to the battery, is from a separate lug (not part of the regulator). And, obviously, the unit is grounded through its case.

I've seen suggestions that the S is for sense, and the I is for ignition. That leaves the A unaccounted for. The problem is, there are only two wires available in the SPF loom (black-alternator, blue-warning light). There are literally dozens of diagrams of how to wire these alternators, many using jumpers to double up connections...unfortunatly, none that I have found use the same terminology that SPF uses.

My understanding of alternators is limited, otherwise this would be easier. I realize that you need an "exciter" to power up the "field" of the electromagnet that generates the electricity. Then, obviously a ground and output. Some regulators have a separate "sense" input, some dont...to determine if the unit needs to be charging, or be idle. Some have an output used to power/ground the "idiot light", some don't (usually the GM 1 wire versions). Obviously, there are ways to double up on some of these functions, or eliminate some. I just don't know how to do it with this car. I'd hate to fry the regulator trying different combinations when it's likely someone with more experience than I has already worked it out.
 

Ron R.

New Member
#5
Alan,

You'd think....

I've done a buch more looking, mostly on old ford forums, and MAY have it worked out. I'm sure I can get it charging now, but I'd like to get the idiot light working. I have to stop by an Autozone for a wiring harness, and check the SPF wiring diagram for a diode, but I think I'll have a solution to post tomorrow.

Of course, if someone KNOWS what will work....
 
#6
I have the factory plug and pigtail for the 3G units but you can also get tham at a Ford dealer. Do not do the knuckle dragger install with some spade terminals crimped on the wires and the spades pushed onto the posts......
 

Ron R.

New Member
#7
I have the factory plug and pigtail for the 3G units but you can also get tham at a Ford dealer. Do not do the knuckle dragger install with some spade terminals crimped on the wires and the spades pushed onto the posts......

Rick,

I was planning on picking one on my way home this evening. Can't stand hack-jobs either.
 

Pat

Silver Supporter
#8
Here's how I did mine, works great and I did buy the plug from Ford as suggested earlier. One strong word of advice, BE SURE TO INSTALL THE FUSE! The vendor where I got the alternator told me not to put the system at risk without it.
 

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User Resigned

Lifetime Premier Supporter
#9
Here's how I did mine, works great and I did buy the plug from Ford as suggested earlier. One strong word of advice, BE SURE TO INSTALL THE FUSE! The vendor where I got the alternator told me not to put the system at risk without it.

Good point. The SPF harness runs the Alt output wire forward all the way from the Alt along the RHS sponson, over to the ammeter and then back through the RHS sponson before it finally gets to an 80A fuse. So a fuse right near (or on) the alternator would definitely be a good idea. To be even safer replace the ammeter with a voltmeter, and eliminate about 12 feet of high-current wire.
 
#10
Ok,

I've got it working. I haven't finalized the installation, but I know what I've done works.

To start, I had found the same diagram that VEEK had posted, and planned on adapting it to the SPF after reviewing the SPF schematics.

If you look, you’ll see that the blue “warning light” wire and the black “ignition” wire are both part of the same circuit, and come on with the turn of the ignition key. I tried using one, the other, and then both as the “exciter”, but settled on just using the blue wire. It gives me power to the regulator when I need it (on with the key), and when the car is in the run position with the engine not running (or when the alternator not working), the warning light glows. The light turns off when the engine starts (if the alternator is functioning). That’s what I wanted.

The downside is two-fold. First, if the dash light burns out, your alternator will not produce power. Second, from what I’ve read, you could potentially run your battery down or kill your regulator if you left the key in the run position for a long time without the engine running. An alternative is to use the black “ignition” wire. This bypasses the dash light, but doesn’t eliminate the other potential problems. VEEK’s diagram suggests using a diode to eliminate these last two problems, and if wired correctly, could be used on either circuit.

The actual install is easy. Just follow the diagram.

1. Spice blue “warning light” wire to wire coming from “I”.
2. Put a female spade terminal on the wire coming from “S”, and slide it on the male spade on the side of the alternator.
3. Put a ring terminal on the wire coming from “A”, and double it up on the lug to which the large battery cable attaches to the alternator.

Note the suggestions re adding a mega fuse, and eliminating the ammeter. I’ve ordered a voltmeter, but they are stuck in customs for now. Something about importing parts that look like bomb-making materials.

I have part numbers for those who want them, but really you’ll just need to match the wire harness to the regulator on your alternator. Cost me $30 for the harness. You could do it on the cheap with spade terminals, but why do that on a 6 figure car?

If anyone has any suggestions, I’d love to hear them. If my car burns to the ground because of this, I’ll be sure to warn you.
 

User Resigned

Lifetime Premier Supporter
#11
Ok,



1. Spice blue “warning light” wire to wire coming from “I”.
2. Put a female spade terminal on the wire coming from “S”, and slide it on the male spade on the side of the alternator.
3. Put a ring terminal on the wire coming from “A”, and double it up on the lug to which the large battery cable attaches to the alternator.
.
Way to go!

The only perfectionist comment I would make is that the A wire is the alternator's sense lead and will do a better job of charging the battery if you move it closer to the battery + terminal. That's why the diagram has it on the starter solenoid and after the fuse. This allows the fuse to present some resistance and still fully charge the battery; the ammeter basically does whatever it has to to get the A terminal to the desired voltage. That also means you (as a car designer) can back down a little on the size of heavy cable between wherever A is and the alternator, since the alternator will make up for any voltage drop in the cable (up to a point). But all this really means is that your alternator will act more like a one-wire (although with an indicator output). More of an abstract point than a practical one, and not something worth expending any concern or effort over.

And as another academic point: I think you didn't have to use the black wire because the jumper on S took care of supplying the stator. Somebody who knows better please correct me.

PS: a bomb that depends on a Smiths gauge? Now that's counter-terrorism!
 
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Steve

Gold Supporter
#14
Ron, I'm resurrecting this thread wondering how your solution has worked. I also have a 3G alternator and, while your approach works, I'm a bit bothered by the limitations. I'm a complete electrical novice so I'm wondering if you've changed your approach at all over time.

Was also wondering if both the blue and black wires could be wired to the "I" wire from the alternator.
 
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