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Discussion Starter · #1 · (Edited)
Hoping someone can point out what I might be missing with this issue because it's slowly driving me insane...

Carburetted L98 in a street/strip 68 Camaro, running an LS1 Mitsubishi alternator (used on ~2001-2006 Commodores in Australia or Pontiac GTOs for you guys in the States) with the two pin connector:

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Purchased a brand new genuine GM alternator when the conversion was first done. Pin A was looped to alternator post, pin B to light in dash (switched 12V). This ran fine for a few initial startups until I received low voltage warnings and determined I fried the alternator. I suspected my wiring was incorrect and I had burnt up the voltage regulator - confirmed this on the test bench.

A bit of research indicated using an arbitrary 470 ohm resistor to drop the voltage seen on the exciter wire with little explanation why 470 ohms specifically and a lot of people seemed to be parroting information, but for the sake of the exercise I decided to try it with another brand new genuine GM alternator. Pin A rewired to the starter post to sense true battery voltage, pin B to light in dash with 470 ohm resistor inline... nothing, zilch, nada. Alternator did not charge and checking voltage at pin B indicates a large voltage drop with the 470 ohm resistor inline, likely to the point the alternator wasn't being excited therefore not charging.

More research indicated the exciter circuit needs to see somewhere between 5-8V to work correctly so a quick calculation using ohms law to determine the correct resistor needed for the desired voltage drop was done, the resistor was soldered inline, voltage at pin B was checked and within the 5-8V range, hence test three began. Success! Or so I thought... Alternator seemed to be charging, voltages where I'd expect over three or four quick test drives, all seemed fine until Saturday just gone... Car had to be moved out of the workshop and subsequently sat for most of the day outside with the battery connected. Went to fire it up in the late afternoon to move back inside and had absolute nothing at the hit, battery was drained. Charged it enough to get it started and moved inside and called it a day.

Next day, charge battery to a healthy capacity and performed a test to determine parasitic draw. Multimeter showed 4.7A with key off.. way too much and little surprise why I had a drained battery. Disconnected cable from BAT+ to alternator post, current draw drops drastically to ~80mA. My instant thought was a diode had gone out within the alternator but after doing a diode test with the multimeter (one probe on alternator post, one on case, and vica versa), the diodes seem to be fine.

So this is where I'm currently at - one fried alternator with a second likely the same and still no answers as to what I'm doing wrong. I've completed many other LS conversions with zero alternator issues so it's throwing me off as to why I have these issues now. Can anyone shed some light on where I'm going wrong here?
 

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Discussion Starter · #4 ·
This is where the different types of LS alternators can get convoluted as this is a different 2 pin connector to the one I'm using. I believe the linked connector is for a LS Delphi alternator whereas I'm using a LS Mitsubishi alternator. Delphi style on left, Mitsubishi style (mine) on right.

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470 ohm resistor should go to ignition switched 12 volts (pink wire), not the dash indicator light.

I have not seen that a 12 volt connection is used on pin A before. Usually only pin B is connected.

Don
My understanding of the alternator warning bulb is that it's wired in series between a switched ignition source and the voltage regulator:

12V switched source -> alternator warning bulb -> resistor (for the LS alternators) -> pin B (exciter wire terminal/positive side of the voltage regulator)

When ignition is switched on, current flows from the battery, through to the alternator warning lamp, through the voltage regulator and to ground, hence the bulb illuminates. When the car is started and the alternator is generating electricity the diodes feed alternator output back to the regulator to serve as the field current supply. This would equalise voltage across the bulb and therefore it no longer glows (there is no longer a path to ground). This means the bulb serves as a warning to let the driver know of a voltage difference across the circuit and a potential alternator issue. The reason I was wiring it this way is that I'd like to retain this OEM style functionality.

12V connection on pin A is a sensing wire. Wiring this to true battery voltage is required so the alternator can make a comparison between true battery voltage and its internal setting and regulate up/down it's output to keep the battery properly charged. Failing to do so will cause the alternator to output in a default mode which can be fine in some situations but in a high demand situation can cause voltage drops at the accessories (fuel pumps, fans, etc) and an alternator that won't ramp up its output to keep the battery charged in this high demand situation. Initially I had this wired to the alternator post but determined after research that it would be sensing the voltage pre-drop and prematurely limiting the alternator's output, hence the move to the starter post.

Sounds simple enough, but something is definitely amiss here...
 

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Discussion Starter · #6 ·
Just an update to close this out for anyone who may come across this in the future. After what I thought was something so simple that was driving me to insanity, I stumbled across a Porsche 944 forum thread where someone was detailing similar issues to mine. It turns out this diagram:
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is WRONG, well, kind of. At first, second and probably third glance, you would think you're being presented with two front views of the connector. You can seemingly even line up the shape of the pin housings which would confirm this belief, but you would be mistaken. The labelled (A/B) view of the connector is a view from the back of the connector inwards, that is to say it's the view of where the wires are going into the connector. As a result of my initial belief, I had the pins in the connector the wrong way around and would've been applying a constant 12V to the voltage regulator, causing it to draw 4.7A with the key off and drain the battery. After rewiring this connection with a spare connector I had, a test of the draw on my battery was showing approx. 40mA, well within the acceptable range. Firing it up saw voltage jump straight to 14.4V meaning the alternator is charging correctly. Problem solved.

The important notes for anyone reading and wondering why your LS1 Mitsubishi style alternator, found on VT/VX/VY/VZ LS1 Commodores and 2004 Pontiac GTOs, is draining the battery, killing voltage regulators, etc (after confirming it's not just a dud alternator):
  • The charge indicator wire, also known as the exciter wire and denoted by the brown wire in the above diagram, goes into the connector in the pin housing closest to the post on the back of the alternator. In the photo below, it should connect with the pin on the bottom
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  • A lot of people mention a 470 ohm resistor being needed on the charge indicator wire as a full 12V could kill the voltage regulator, either straight away or over a short time-span. All of this discussion references the F-body Delphi style alternator. I have a resistor inline on the charge indicator wire but am unsure if this is needed for the Mitsubishi alternator. I may remove this resistor in the future to test without it but I can confidently say my alternator is charging correctly as is.
  • Whilst the sensing wire (white) is optional, it's strongly recommended that you wire this up to 'sense' voltage at a main distribution point in your setup. This will allow the alternator to regulate it's output up/down to compensate for an increase or decrease in load on the electrical system, ideal for high demand situations. Note: looping this to the post on the back of the alternator is not the correct way to go about this.

Hope this helps anyone searching for answers on these very little documented alternators. Onto the next task :)
 
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