99 alternator going bad, or did I not make my voltage regulator correctly? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 12-31-2014, 11:24 AM   #1
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Default 99 alternator going bad, or did I not make my voltage regulator correctly?

I have reached 130k on my 99 NB. As far as I know, it has the original alternator. I left me stranded last weekend - I had about 30 s notice before the engine died. I drove it home with an extra battery shunted in.

Running a DIYPNP with Cuadra alternator circuit on the proto area.
I cannot remember if I confirmed good voltage at one point with the PNP regulator, but I think that I did.

Now, I am getting 12.24 vdc out of the battery [new].

Running, I am getting 11.53 - with all accessories, it drops to 11.18.

Normally, I would immediately suspect that the alternator was bad, but...
How can I confirm that is not the fault of the alternator circuit?

1) To run it again with the original ECU to check alternator voltage output means MAF and IAT and possibly injector rail swap again. I like to reserve this pain only for my birthdays.

2) Or, I could buy a new alternator, swap it in, and curse when I learn it is the regulator. Or maybe it is the alternator after all.

Is there an easier way of confirming that the alternator is indeed going bad?

Perhaps, you will tell me that it is no question a bad alternator, but I want to spare my self the embarrassment of step 2, if there is a first check of the circuit that I can run.
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Old 12-31-2014, 01:18 PM   #2
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if you're noting making over 12.6v, then you're not charging the battery.

is there voltage on the field out wire? (IIRC it's 2O)
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Old 01-14-2015, 07:34 PM   #3
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Update: I saw only .6V on the field out {1O}. I changed the alternator out for a NEW one, and.......same situation. Less than 12V when running. Near-battery voltage at the alternator when not running. Everything on pulls down to near 11 V.

So - I rebuilt the alternator circuit, since I probably messed that up. Ordered some new PN2907A and waited for them to come in.


And - STILL at less than 12V. Field output only shows 0.33V.

Perhaps I have some component shorted [next time - I will use all the proto space].


BUT - shouldn't the new alternator put out a rough approximation of 14V at the main battery connection? Or is it completely dependent on the alternator circuit for this?


What are the chances that my NEW alternator is just as shot as the old one [which I think ran on the stock ECU]?

I think my next move is to plug the stock ecu back in, plug in the IAT and the MAF, and see if I can get voltage out of the alternator.

If I do, then I guess it means that something is up exclusively at the alternator circuit on the proto area. Maybe I have an open resistor or transistor?

Also - I verified the 12V feed to the 3 branches of the circuit.

Thanks for the help
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Old 01-14-2015, 09:06 PM   #4
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field out will be above 5v when the alternator should be charging, if not, the circuit is bad.
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Old 01-15-2015, 09:55 AM   #5
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I guess this means that if there is an open or a short, then the alternator will not charge whatsoever.

Is there a way to bench test this circuit [beyond checking continuity and resistance]?

I would love to have an indication that it will work before I install it again....
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Old 01-15-2015, 09:57 AM   #6
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if you have a power supply you can test just the cudro circuit, yeah.

supply less than 14.5v and the field out should be above 5v, supply greater than 14.5v and the field out should drop to like ~3v or so.
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