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Discussion Starter · #1 ·
Am i just blind... I can't find all the connections shown on this page: http://oljeep.com/gw/alt/Alternator_Theory.html#Section_2

it's the stock alternator on a GM setup. it looks like a 12si, though the mounting is different from most of the pictures i've seen.



I have it wired up with just two wires, the only connections i could find. one to the battery, and one to ground. Is this correct? I can't tell if it's working and i am not at the point where where i can drive around and see if the battery loses voltage or not...

I want to eventually go to a dodge alternator setup that will plug into my stock harness.
 

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None of the SI series wires go to ground. The ground is made up through the case. I'll go and take a look at the one I have to see if these are the SI series. From the front it looks more like a CS.
 

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I looked at the one I have and it's one wire to the battery (5 o'clock position viewed from the rear) next to the oil filter and the other goes to ground. Make sure the hot side doesn't touch the horizontal oil filter. I was an alternator rebuilder ten years ago and this is the first one that I encountered. It looks like a 22SI series without the standard wiring connections. Wire it up using a voltmeter for monitoring.

Bob
 

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Discussion Starter · #7 ·
I looked at the one I have and it's one wire to the battery (5 o'clock position viewed from the rear) next to the oil filter and the other goes to ground. Make sure the hot side doesn't touch the horizontal oil filter. I was an alternator rebuilder ten years ago and this is the first one that I encountered. It looks like a 22SI series without the standard wiring connections. Wire it up using a voltmeter for monitoring.

Bob
I wired it up like you said. and the volt meter confused me. I would have thought that it would be grounded through the block, but ground straps aren't really ever a bad thing, so it seemed possible. It had a short black 8ga wire already installed on the alternator when i got the motor, so that also made me think it was a ground.

There are two terminals on the back. the one is bigger than the other, so i was pretty sure that one is hot. It's hard to look at the back of the alternator the way it is installed but i'm pretty sure the big terminal says "bat" next to it and the small one says "gnd" at least that's what i got out of it.

anyways, back to the voltmeter. When i fired the engine up with nothing connected, the bat terminal would read just over 0V to ground, about .44V or so. But it read 14.4V to the + side of the battery. At first i just checked it to ground and i thought it was a bad alternator, but the 14V to the + side of the battery confused me. The resistance across the terminals of the alternator is about 23k, so it isn't an open.

i then looked at all the pictures i could find and went ahead and wired up the alternator with the big terminal to the battery, despite what the voltmeter said. nothing smoked, no sparks, i was ready with the kill lever if something happened... With it wired like this, this is what i got out of the voltmeter. With the engine off, about 14.86V. when starting, it drops to about 12.4 or so, no surprise there. with the engine running it's steady around 14.73. It doesn't seem to go up or down if i raise the rpms, which is how it should be. it returns to about 14.86V when i shut the engine off again.

Does this seem like normal behavior? I am pretty new to alternators, though i know electricity pretty well.
 

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Ground your engine block to you battery using a battery size wire. Ground the end of this to your chassis using the same sized wire to the frame. Then ground your body to you chassis with a heavy braided connection.

Connect your permanently dash mounted voltmeter positive terminal to the "run side" of your old ignition system (The side that controls your injection pump solenoid) using a fused connection using either an inline fuse located on the engine side of the firewall or at the fused side of your fuse box. If you wire the voltmeter wrong it will sometimes bypass the ignition switch and the engine won't shut down.

Your voltage readings sound about right.
 

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Discussion Starter · #9 ·
grounding is like that.

I was a little confused why the voltage went down though when the engine was on. I checked it with the key turned and power to the fuel solenoid and that didn't drop the voltage. Other than looking at current which is somewhat difficult, my clampmeter only reads AC current. i'm not sure how to tell other than if the voltage eventually starts to drop...

I haven't gotten into wiring up the stock voltmeter. I need to read the FSM to figure out how it works exactly. I know the stock alternator is externally regulated by the PCM, so...it could be picky. I don't want to fry the ECU $$$. I will probably use an aftermarket digital voltmeter for the time being until i can figure out the stock stuff.
 

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Alternators are designed to replace the voltage being drawn from the battery. I have a carbon pile tester that loads the battery down and causes the voltage regulator to kick on when it senses the additional load. No load, no voltage increase unless your battery is old and dying and then goes from a storage state to a load state. Old weak batteries are an alternator shops best friend for $$$'s of income. :eek:
 

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This looks exactly like the unit that came on my engine. No numbers or lettering on it anywhere. Just attach the big post to the positive batt. Neg battery cable to bottom alternator bolt and then a good chassis ground from engine to frame. Vehicle should have 1or 2 braided lines from frame to body. I dont have voltmeter installed yet but been running this setup for a year with no problems. I will eventually go to 100+ amp unit with all my gages in dash, but havnt had time to figure the layout yet.
Carl
 

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Discussion Starter · #12 ·
awesome, sounds like i have it setup correctly. I couldn't figure out another way to do it, so that's good, haha. Just wanted to make sure i wasn't crazy. I couldn't find any real marks on it either ot say what it is, though it may be on the back, i can't really read it too well.
 

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If you find any markings let me know. While I had mine off I turned it over and over and over, no markings of any kind.
Carl
 

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mine also has just 1 big hot to the battery works goo for me .
 

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In theory the ground wire is supposed to go to the ground of a idiot no charging light. You give the bulb a charge and the resistance activates the alternator. When the alternator fails, it will turn on the idiot light. The fact it will work through a chassis ground I think is dumb luck..
 

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Discussion Starter · #16 ·
In theory the ground wire is supposed to go to the ground of a idiot no charging light. You give the bulb a charge and the resistance activates the alternator. When the alternator fails, it will turn on the idiot light. The fact it will work through a chassis ground I think is dumb luck..
hmm any reason it would be an 8ga cable? it's only about 18-24" long too, this was the stock cable that was in the bread van. seems kind of heavy for a lightbulb... i thought there was something for an idiot light too.
 

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There are two wires on the harness. Then there is the positive lead that goes to your battery. The fat one connects to the positive also, I usually pig tail it off the super fat wire that charges the battery, right there on the alternator. Then the little one is the "exciter" that goes to the ground of an idiot bulb.. You wire in the + of that bulb with 12V switched, so alternator turns on when truck is running and cannot if the engine isn't in the on position (theft prevention). There might be a slow drain or premature alternator failure issue with wiring to chassis too, I dunno...

Then, start and run truck and alternator should turn on within first couple seconds of running.. All GM 3 wire alternators (si and cs series) are like this and have the same setup. For the cs alternators, they have 4 plugs but you buy an adapter harness and they are plug and play...
 

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On the SI series the idiot light gets its ground through the regulator. The idiot light is actually wired directly across the two terminals of the regulator. When the system is first switched on and the unit is not spinning one of the terminals becomes negative grounded through the regulator. This lets the other side feed the idiot light then it makes up to ground via the regulator. When the unit's rotor is spinning and reaches the threshold voltage the negative terminal changes polarity and becomes positive. Since the idiot light now has positive direct current applied through both wires to the bulb the bulb goes out. This is the reason why the idiot light must always remain insulated above the chassis' negative ground side or it will act as a resistor to ground across the regulator.
 

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Instructions at this link are what I followed:

http://www.rmahc.com/alternator.html

I wired differently and it seemed to be charging when it was not. The above has worked well for me. I think mine's a 12SI, yours may be different, don't know. Idiot lite gets power from ign. sw. and ground from other alternator wire as explained by others. Idiot lite was about $3 at Radio Shack!
 
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