I have a turbo + ms3propnp, 2002 miata with a built 2000 motor. From time to time, if I do a tuning session or have extended idling, or similar, randomly and without warning, the car will act terrible. My digital gauge shows everything normal from a boost and AFR perspective, the only thing of note is the battery voltage DROPS. The only 'fix' to this is that I park, let everything cool down for a few hours, and then when I return, everything is normal again. Normally when I drive the car, my digital dash shows 14.0 for the battery. When this is happening, voltage drops to below 12 and slowly trickles down as I'm driving. I have no idea why this is happening and curious if anyone else has thoughts.

 

How are you regulating the alternator - ecu, alternator inbuilt or external? It sounds like the regulator has failed, the alternator is not charging and so the car is running down the battery. That would be the first thing I checked out. If that checks out it's probably the alternator.

If you are going to change the alternator some people replace it with one that uses an external regulator, but generally I have been happy with the internally regulated ones. ECUs vary in their ability to do this job, that may have changed these days, or you may have one that does fine.

 

Thats a great question. I have no idea. the alternator in it came with the car and I just hooked up a MS3, added basemap and then had someone tune it (this is over the course of a couple years) so I've never even thought about alternator regulation. I'll do some research and come back. The system seems fine 99.9% of the time, its just these weird flukes that seem to happen internmittently.

 

If it's a PnP MS3 for the NB chassis and using the stock NB alternator (externally regulated) I believe the MS3 is controlling the alternator and defaults to that in the 02 basemap. Should be able to confirm in the alternator control tab in tunerstudio.