Alternator drawing current when car is off
 

I traced the reason to my battery dying to my alternator drawing 3.4amps when the car is off.
I am running a NB alternator on my VVT swap into my 1990 with MS3 basic with alternator control.
Is this probably a bad alternator?

If you unplug the MS, does the dark current stop? If so, probably an internal MS problem (or the MS is staying powered on when you turn off the car.)

If not, unplug the small connector from the alternator. Does that stop it? If so, wiring problem somewhere in the car. If not, bad alternator.

When I unplug the connector from the alternator, it stops. (The big plug, not the spade one). Let me unplug the MS and see what happens.
I did not touch this part of the wiring in the car harness and it ran fine last year with the 1.6 alternator.

No change when unplugging the MS. Still drawing current.
So:
1) Bad alternator
2) Something going on with the wiring, (but I didnt touch it)

How accurate are the autozone alternator testers?

If you mean the one that spins the alternator on a bench while applying a load to its output, those are perfectly suitable for telling you whether the alternator is able to supply X amount of current at Y voltage, but not especially good for telling you things like "your field coil is drawing dark current." It's possible that such a fault would be revealed as an inability to regulate voltage at low loads.

I'm skeptical about the alternator having a leaky diode. What if, in the other hand, the amperage is being drawn through the alternator by the MS3 trying to regulate it's amperage?
I know the na alternators have an internal amp regulator while the nb alternators are controlled by the ecu. Thus, Dimitris made my ms3 specifically to control the nb alternator.
Also, the plug is the exact same on the na vs nb. All I did was to plug it in but I don't know if the plug wires are in the right place on the plug.

This isn't a leaky diode, nor is it related to the MS3 trying to control the alternator. Right now, your ECU isn't even connected to the alternator, because:

For what it's worth, all MS3s can control the NB alternator. This was added to the base code a couple years ago. However, you MUST make some wiring changes if you wish to control an NB alternator in an NA chassis, no matter who built your ECU. Which brings us to...


Well, right off the bat, this will prevent your alternator from working properly, if not destroy it outright. While the plug is the same between the NA and NB, the wiring behind it is massively different.

On your NA, the white / black wire is closed to ground by the alternator to illuminate the alternator warning light on the instrument panel, which is pulled up to +12.

On the NB, the wire in that same position (grey on the later cars) is the signal from the ECU which controls the field coil on the alternator, which is what governs how much power it produces.

So, right off the bat you have created a nonfunctional system by just plugging it in without cutting that wire and connecting it to the appropriate pin of the ECU. (Actually, both ends would be connected to the ECU; the alternator side to a PWM output wire of the MS for voltage control, and the instrument cluster side to a GPO pin of the MS, to control the light on the dashboard.)

I have no first-hand experience with what happens if you connect the NB alternator without modifying the wiring, but from a reading of the schematics. It'll apply a constant +12 to a circuit that's expecting a closed-loop PWM closure to ground. Definitely won't work, but I honestly can't say whether it would cause damage to the alternator.



You need to try unplugging the smaller, two-wire connector from the alternator. It still won't work (it won't produce any output), but if this stops the dark current then at least you know you didn't kill the alternator, and if you re-wire it then it'll probably work properly. My assumption is that with the key *off*, the field control circuit is back-feeding through the instrument cluster to ground, and turning itself on when the field coil is stationary.



For your reference, here's the wiring on a '92:

https://www.miataturbo.net/attachmen...ine=1454126471



And on a 2000:

https://www.miataturbo.net/attachmen...ine=1454126471

Thanks Joe for being patient and explaining stuff like this to idiots like me.
Dimitris also replied my pm's saying that I need to use my original alternator.
I think the simple solution will be to get a 94-97 alternator and deactivate alternator control from the ms. It will be hard to make any wiring changes with the engine in, there is less space in there with the 1.8 and the oil cooler lines.

Either way will work.

Replacing the NB alternator with a '94-'97 model was the standard treatment when doing an NB swap into an NA during the years prior to alternator control being well understood and subsequently implemented into the MS. There's really not a significant advantage to keeping the NB alternator and placing it under ECU control aside from the cost savings and a small benefit to idle quality (by turning down the alternator load when engine RPM dips.)