12 si resistor help

[concretekiller]

Im putting (what i think) is a 12 SI into my 65 C-10. Its for a 1984 Trans Am. picked it up from autozone yesterday. im pretty comfortable with how to wire it, just not sure if i need a resistor inline. dont want to get everything installed and going and end up burning up an alternator if i dont need to. i have a voltage gauge, not the dummy light. any help?

[Rich 5150 69]

No resistor...

[concretekiller]

awesome! thanks for the help rich. i greatly appreciate it!

[tincan1966]

The "idiot light" acts as a resistor. A voltage guage will connect to any "key on" power source. An amperage guage wires inline with the alternator output wire.

Rich's diagram is for a charge indicator light. An easy way to do this is to install an idiot light somewhere in the wire going to the exciter-it does not have to be visible. it's an easy resistor, just tuck up under the dash out of the way, zip tie to a wire loom. it'll only be on for a few seconds at start up.

[concretekiller]

Ok so I do need a resistor. Couldn't I cut the excite wire somewhere, and use butt connectors to attach one? I believe I saw something that says I need a 75 ohm resistor. Is that correct?
Posted via Mobile Device

[HEI451]

No resistor needed. If you don't have a charge light, disregard the part in the instructions about the light, it really isn't needed for the SI series of alternators. Simply, wire 2 and 3 together, at the alternator, as shown. When I add an SI alternator to any make/size engine, I run the number 2 wire directly to the number 3 terminal on the alternator, NOT run a separate wire all the way back to the battery positive post.

[concretekiller]

Ok I see what your saying. In the picture, the charge light as used as the resistor.
Posted via Mobile Device

[mud.man.rj]

You can simply just attach #2 wire to batt post of alternator and #1 to an ign (key on key off power sorce). Make sure though you have a good ground either at case or that it is sized correctly from engine to cab because now the apm size is higher on new style alt.

[concretekiller]

Lol trust me, my wire is plenty big.




Posted via Mobile Device

[mud.man.rj]

Wow that wire is plenty big, just make sure the engine is grounded to the chassis or in cab won't be grounded well, prob is after seeing the engine ground.

[concretekiller]

I actually do need to make a ground from engine to frame. Right now it's battery and alt to frame.
Posted via Mobile Device

[HEI451]

I'll make this a bit simpler.

If you have a charge light, use it, wire from the light goes to terminal 1 on the side of the alternator.

If you do not have a charge light, or have an ammeter and/or voltmeter, the wire to terminal 1 IS NOT NEEDED.

With am ammeter and/or voltmeter, only the large BATT wire, and a jumper wire from that BATT terminal, to side terminal 2, are all that is required.

No charge light, no need for a resistor of any kind, to any terminal, to make the alternator work correctly.

[mud.man.rj]

So are you not wiring in the #1 side with an ammeter, so what powers the alt to turn it on, it's not a 1 wire alt. The #1 wire turns it on and off. #2 term is always live anyway.

[HEI451]

Actually, the 12SI alts don't use an "exciter" circuit, terminal 1 is basically a charge light terminal. Connecting 2 to BATT leaves the alternator "on", just the way GM did it for years. I have used the 12SI internally regulated alts on just about everything out there, and without a charge light, large wire from BATT positive, to BATT on the alt, jump BATT to 2, done, works, no issues.

Charge light, connect to term 1, done, light works.

End of drama.

[concretekiller]

Thank you for clearing all of that up HEI451. The info is greatly appreciated.

[HEI451]

A lot of people think the 12SI needs the exciter setup, but, nope, it doesn't. In fact, with that setup, should there be a back drain of voltage when the system is shut down, engine off, it will still be the diode pack, just like when it is a stock 3 wire setup.

I've taken a bunch of those "special single wire" alternators apart, and all they do is bridge BATT to term 2 INSIDE the alternator, no exciter circuit connect, no resistor. I just got tired of rebuilding them for projects, and having to buy their specialty regulator setup, to do what I could do from the outside,m with a single jumper wire.

There is a stock GM "single wire" alternator for industrial things, uses what I heard called a "farm ind implement regulator", does the connection inside the alternator, just like the specialty regulators. I am sure the aftermarket people just used/copied the farm and implement regulators.

[concretekiller]

One more quick question. All of the wiring that goes to the external regulator; can I cut it all out and ditch it or do I need it for anything? Because this thread

http://67-72chevytrucks.com/vboard/s...d.php?t=119379

Is where I got my wiring info for this project.
Posted via Mobile Device

[concretekiller]

One more quick question. All of the wiring that goes to the external regulator; can I cut it all out and ditch it or do I need it for anything? Because this thread

http://67-72chevytrucks.com/vboard/s...d.php?t=119379

Is where I got my wiring info for this project.
Posted via Mobile Device

[HEI451]

I think the wire connected to the regulator at terminal 4 is the charge light wire. If you are going to charge light wire it, use it. I always advise a very quick test, have an assistant sit in the cab, ignition switch ON, and look at the charge light. You would then very quickly ground that wire, and disconnect, and ask if the light came on. If number 4 did (and it should), that's the one to use for the charge light.

Other than that, those wires can be taped/insulated, and the loom simply tucked away, out of sight. Leaves the wiring in place and intact, and, you get the light to work.

I always like to add the charge light to the projects I have, even if they use an ammeter/voltmeter, just that much more to grab your attention, should you be sleep driving!

[HEI451]

"and #1 to an ign (key on key off power source)". NOPE, if you use terminal 1, you MUST either use the resistor, or the light, NOT just directly to a switched 12 volt power source. If there isn't the resistance there when using terminal 1, the engine will not shut down when the ignition switch is turned to stop.

Just another clarification for everyone to consider.