Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
First of welcome to the forum I will copy your thread into the Repairs Questions section where it will get more responses.
The Miata/MX5 specifications have changed considerably over generations and are different on models sold to different markets so we can help you better if you give use more information.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Like Andy said, we need more info. What's the model year of your Miata? What's the specific alternator issue you're trying to fix? Not charging? What testing and repairs have you done previously?
Oh, I just introduced myself here. Thought that Miata DIY would be most appropriate description of myself.
Just saying hello
I just posted a separate topic on my particular issue, more informative.
Vlad