What are the Two Tabs on GM 12SI Alt? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 05-18-2006, 07:07 PM
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What are the Two Tabs on GM 12SI Alt?

Ok, now trying to understand how my alternator is wired. I know the threaded post is the alt output, which in my Kwik Wire kit, goes directly to the fuse panel installed under the dash.

But how are the two tabs marked 1 and 2 used. My #1 tab runs to the fuse panel. My #2 tab is jumpered to the output post on the alternator. Articles I read say one of these is the "remote voltage sensing" wire, and one is for an ON/Off switch, or a warning light.

Can someone explain what the uses of these tabs are, and why is one jumpered to the output post? And are they interchangeable?

Thanks,

John

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Old 05-18-2006, 07:31 PM
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Doc here,

excitation and field..

Here is your schematic:



The jumper is just there so as not to have to run an extra wire to the excitation wire.

Hope this helps..

Doc
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Old 05-18-2006, 09:23 PM
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It isn't clear how the schematic correlates to my tabs marked 1 and 2 (they don't have colored wires on them).

In the schematic, is the BRN wire the excitation wire?

Is the middle red wire the Bat output?

Is the left red wire used for remote sensing?

John
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Old 05-18-2006, 10:30 PM
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Doc here,

Not sure how 1 and 2 relates to your alternator, but the very first wire on the left is alternator output..the next red wire that is jumpered on yours is excitation, and the third brown wire is field ...

The brown wire goes to the lamp ,then to fuse , then to (sometines) a bypass resistor, then to the switch..and monitors the Differential of power between the alternator and the system ..no charge or lower than system voltage, the BATT lamp comes on ..

The red wire (middle) is used for turning the alternator on ..so to speak..or excitation , for the diode trio and regulator. it is jumpered so you don't have to run the separate wire and fuse link on yours..

What is it you are trying to do?

Hopefully not run the fans from the output..in addition to what I already said in the last post, you are bypassing the battery, which is a "buffer" of sorts for the system..and isolates or reduces all the little "nastys" the real world generates..

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Old 05-19-2006, 12:36 PM
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Doc,

Based on the fan/relay thread, I plan to get the distribution bar power from the battery connection at the starter by running an 8 gauge wire and 12 gauge fusible link from there to the common stud on the distribution bar.

My question about the tabs is because I am trying to understand the remote voltage sensing feature of the SI alternators: which tab does this, and does it make a difference where it is connected to the electrical system. Some articles say the voltage sensing lead should be connected to the junction box if that box provides power to all accessories and the main power-up lead.

Articles I have read use the three terms "excitation", "field", and "voltage sensing" interchangeably or in a contradictory manner when referring to these two tabs. So I am confused on which does what.

Looking at the rear of my alternator, the left or CCW tab is labelled 1. On my car there is a white wire running from this tab to the firewall and into a wiring bundle to the back of the fuse panel. The Kwik Kit wiring instructions for my harness say this wire should be connected to the "Alternator Exciter". The CW tab labelled 2 is jumpered to the BAT output stud. Something doesn't seem consistent between my white exciter wire running to the fuse panel and your description of the "exciter" being jumpered to the output stud.

I will email a picture of the back of my alternator to you.



John
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Old 05-19-2006, 02:33 PM
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Doc here,

Based on the photo, The wire / terminal you want is the 1 terminal, or the terminal on the far left as you face it, Or the one closest to the 6 O'clock position.

If It is wired like the drawing , it will be the wire point at the output wire (jumpered)

If you haven't already done so, go to mad electrical and read the tech article on "Remote Voltage Sensing"..

Mad Electrical

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