I haved installed a start button on my miata, ( NC) with a SPDT relay, using the IG and ST wire from ignition harness. When I push the button the starter's bendix engage but don't start de starter. Is it possible that my ground connection is not good enough ? How can I check it ?
Where are you getting a holding circuit for the rest of the ignition switch for acc,and ign. You can power the relay to provide power to the start circuit but you still need acc and ign.
Keep the original key, this will supply ACC and ignition, then feed off the ignition 1 circuit (that is, if you have more than 1 wire going to 12volts+ on ignition not the ACC, it's the one that stays "live" whilst key is in "start position).
Feed that wire to your push switch then the other side to 86 on the relay.
Ground to 85.
Constant live fused at 20 amps to 87.
Feed to starter wire at ignition from 30.
Don't use 87a.
-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.
I really have a problem with using a NC switch and using the NC contacts of a relay to engage the starter. If your switch fails or loses power, the starter will engage and run until you kill the ignition switch. At speed this failure could go undetected. But your starter motor will know that there is a problem.