99 NB1, about 120k miles. E street car, no modifications.
Three year old battery replaced a few weeks ago under warranty. Car sits for a few weeks at at a time, so not sure if the premature death of the battery is significant.
First time out, drove 30 minutes to Daytona for an autocross. Did five runs, the car sat for about 3 hours before leaving for home.
Alternator light on when I started home, showing 11.8 volts on the Scangauge II. Battery connections were tight, alternator belt intact.
Made it home with 11.4 volts showing. Weird thing is that when I stopped the car, the idle would hang at about 1400rpm for several seconds, then drop.
Where to begin?
Make sure you have battery voltage at the heavy cable at the alternator. You should also have battery voltage at the one or two terminals at the connector with the engine running (unplug to test this) but if it's a computer controlled alternator, it might not? Would need to look into diagrams to be sure.
It's probably just an alternator. There's a slim chance that repeatedly trying to charge a discharged battery weakened it, or it could be just 25 years old. I wouldn't worry about it unless it becomes a pattern failure.
If you could drive home with no charging system, the battery isn't doing too badly, after all. That's actually pretty good for a tiny Miata battery!
In reply to Pete. (l33t FS) :
Voltage regulation is done by the ECU on the NB, FYI. I'm guessing dying alternator as well.
In reply to Keith Tanner :
I suspected that the ECU might be doing something, due to the fast idle.
Frustrating, won my class today, was hoping to back it up with another win tomorrow.
I'd guess alternator, too. I gave a good one for a '96 Miata to my brother in law in Ormond Beach a few months ago, and I think it's still sitting on his shelf. If it will fit your car, I can see if he still has it tonight.
Floating Doc (Forum Supporter) said:
Frustrating, won my class today, was hoping to back it up with another win tomorrow.
How far away is it? Close enough that you can charge the battery tonight and just drive it on that?
Grab the battery from your riding mower and civic. Charge them all. Runn total loss with redundancy to get by. Fix next week. Take extension cord and charger to track to topp off to cover your ass
Alternator is a pretty safe bet, NAs and NBs eat stock alternators like potato chips.
I went with the safest option for a hassle free day.
H Street
Not looking forward to swapping 8 tires on Monday. Probably won't get them all done on the same day.
Tom Suddard said:
I'd guess alternator, too. I gave a good one for a '96 Miata to my brother in law in Ormond Beach a few months ago, and I think it's still sitting on his shelf. If it will fit your car, I can see if he still has it tonight.
Thanks for the offer!
I guess I'll do some part number checking, unless Keith happens by again. He's likely to know.
Part numbers on RA don't match. Thanks again for the offer.
Now, new or have it rebuilt at this place
I probably have to renew my Mazda Motorsports discount. I am reluctant to buy anything but OEM.
You should look into aftermarket heavy-duty options that will last longer than stock-spec, here are some I found, although they're not cheap:
https://www.powerbastards.com/proddetail.asp?prod=13895-160-HD1-1
https://drifthq.com/en-ca/products/dc-power-180-amp-hp-high-output-alternator-mazda-miata-1-6l-na
You can also fit an FD RX7 alternator with some mods:
Floating Doc (Forum Supporter) said:
Tom Suddard said:
I'd guess alternator, too. I gave a good one for a '96 Miata to my brother in law in Ormond Beach a few months ago, and I think it's still sitting on his shelf. If it will fit your car, I can see if he still has it tonight.
Thanks for the offer!
I guess I'll do some part number checking, unless Keith happens by again. He's likely to know.
NA alternators will bolt up to an NB, but you will need to make some modifications to the wiring. NA alternators have a built-in voltage regulator, so they need a voltage reference line from the battery. NB alternators are ECU-controlled, so they need a control line from the ECU.
Beware high output alternators. They often have less output at low speeds.
I knew this, but bought a 140a RX-7 alternator anyway, because I can be pretty dumb sometimes, and how bad could it be?
No appreciable output below 2200rpm. Like, not even above 13v.
Back in the box it goes...
I appreciate the tips. I don't need a high output alternator for this car. A 99 sport is the stripped down version, not much drain on the electrical system.
