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Old 05-13-2015, 03:13 PM   #1
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Default light duty alt for 2004 miata

I'm considering swapping out the heavy (18#) and heavy duty (80amp) alternator on my 2004 miata track day car for a lightweight alt.

The problem that am considering, is that the OEM alternator uses a phase/driver circuit (in which half of the regulator resides in, and is controlled by the ecu).

I would like to swap in a self regulated "one wire" alt and merely unplug the ecu connectors; but am unclear if this would merely send a code, or if the car might not run.

Has anyone tried this?
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Old 05-13-2015, 03:17 PM   #2
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Tons of NBs have done this. There's a few dozen threads on the subject here.

https://www.google.com/?gws_rd=ssl#q...miataturbo.net
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Old 05-13-2015, 03:45 PM   #3
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Thank you. I was able to get a decent diagram from a couple of the threads.

However, I'm still wondering what will happen if the ECU alt leads are left unplugged?

In reviewing the search, I haven't seen anyone swap a self regulated alt for a P/D alt and still retain the OEM ECU.
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Old 05-13-2015, 03:56 PM   #4
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unplug your current alternator and find out.
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Old 05-13-2015, 04:48 PM   #5
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Quote:
Originally Posted by Braineack View Post
unplug your current alternator and find out.
It sounds like the easy/obvious test!
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Old 05-14-2015, 01:18 PM   #6
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Why bother keeping the stock ecu? The 8-10lbs you save from the alternator is kind of a drop in the bucket compared to the like 10hp you get from a standalone.
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Old 05-14-2015, 05:13 PM   #7
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Why bother keeping the stock ecu? The 8-10lbs you save from the alternator is kind of a drop in the bucket compared to the like 10hp you get from a standalone.
Are you telling me there are 10hp simply to be gained from a better tune on a stock car? or is my sarcasm meter broken?
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Old 05-15-2015, 10:00 AM   #8
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Quote:
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Are you telling me there are 10hp simply to be gained from a better tune on a stock car? or is my sarcasm meter broken?

No, that's what he's telling you.
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Old 05-15-2015, 10:14 AM   #9
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Quote:
Originally Posted by G3ML1NGZ View Post
Are you telling me there are 10hp simply to be gained from a better tune on a stock car? or is my sarcasm meter broken?
Depends on the year miata, but yes it is possible with removal of the MAF/AFM.
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Old 05-15-2015, 12:58 PM   #10
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Quote:
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Why bother keeping the stock ecu? The 8-10lbs you save from the alternator is kind of a drop in the bucket compared to the like 10hp you get from a standalone.
Good plug for the Megasquirt.

I did just replace the PD alternator with a standard lightweight. I used a small NDIF/nippendenso internal fan and set it up with a standard regulator... set it to 14.7 V. It charges great, but does cause a dash charge indicator light to light up. With the ck engine, airbag, batt and the occasional the abs lighting up the dash kinda looks like a Christmas tree.
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Old 05-15-2015, 01:03 PM   #11
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then you didnt mod the wiring properly.
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Old 05-15-2015, 02:20 PM   #12
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Quote:
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then you didnt mod the wiring properly.
Yeah, I'll mod it by pulling the bulbs..

...Unless, of course, you are being serious?

It looks like the charge indicator light (brn/red) comes off the ecu, and I believe that this is fed by the phase/driver circuit (gry, gry/red). I suppose that I could try to hook up a method to simulate the signals that the P-D portion of the circuit would normally see, rather than just unplugging the connector as I have done. Also, I need to mention that the only new light is the ck batt light. The others have been on because of previous lapses in judgement (gutting airbags, re-chipped ecu and poor braking skills)

For the moment: Leaving for the track tonight... and the car charges/runs!
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Old 05-18-2015, 12:48 PM   #13
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The alternator worked perfectly over the event this weekend.
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Old 05-18-2015, 12:56 PM   #14
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Quote:
Originally Posted by speedj View Post

...Unless, of course, you are being serious?

It looks like the charge indicator light (brn/red) comes off the ecu, and I believe that this is fed by the phase/driver circuit (gry, gry/red). I suppose that I could try to hook up a method to simulate the signals that the P-D portion of the circuit would normally see, rather than just unplugging the connector as I have done.
or just do this:



i.e., mod the wiring correctly.
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Old 05-18-2015, 02:59 PM   #15
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Quote:
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or just do this:



i.e., mod the wiring correctly.
yeah, that looks good!
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