New to "forums", with electrical questions, in the PNW
 

Hi. I have an '04 (NB3) MSM and had the battery and alternator go out around the same time 2 years ago. Was given a replacement battery 1st and it was an AGM, but the stock was not...only to find that the alternator was bad, so was given a replacement for that, but the lower alternator bolt was different (small thread size) and the new alternator did not come with a new bolt...so that was the start of the issues..

​​​​​​Got a (non automotive) bolt that would work, but had to slide the bolt in "backwards" to make it work... Just not adding up...
Battery failed again, and got a replacement, but it was still an AGM...30 miles down the road and the "new" alternator blew...this has been the story for 2 more alternator replacements along with 2 more battery replacements... When the car is running, I'm loosing voltage slowly...but can't figure out why... I feel like the alternator is not "big enough" to keep the battery charged... Can anyone help.?

I'm in the Seattle, Washington area and have owned this car for ~10 years. We participate in parades and Miata group get together a, yet no one that I know is able to understand my issue... There are VERY few MSMs around me so not many ppl to help...

Thanks in advance.!!

You are 100% certain you have a NB alternator and not a NA alternator?

I think NA alts won't charge without the dash charge light in the picture and NB do not have the charge light wired in this way, so even though NA alts are internally regulating it will not without the charge light.

Long shot, but worth checking.

You don't tell us about your car. Is is stock electrically - stock ecu, stock wiring?

NBs use the ecu to control the alternator. That means you can't throw any old alternator at them and expect them to work. My aftermarket ecu works the alternator just fine, so that is not a necessarily a reason to switch to an internally or externally regulated alternator, but if you do (stock or aftermarket ecu), there is a small wiring adjustment that is required probably the one deezums mentioned above. I did it on one of my earlier NBs, but the details are too hazy to be useful to you.

The AGM is not part of the problem though, I use them and haven't made any changes to accommodate them beyond the physical mounting.

I have had terrible luck with non-OEM NB style alternators. Finally, broke down and ponied up for a genuine Hitachi (IIRC) and it has been clear sailing since.

Used OEM also no-good.