The main relay powers your ECU, fuel pump and your ignition system, amongst other things. And when they fail, you're stuck. It's a mechanical relay, and after a couple of decades they just get tired. Higher power fuel pumps will also draw more current and will hasten the failure.
Typical failure is immediate. The car will be working fine, then it simply won't start. This will strand the car wherever it sits - at a restaurant, at the beach, in your garage. We've even seen them fail mid-drive, which just shut the car down. Once the relays fail, they almost never come back.
Luckily, it's an easy fix. Swap out your relay as preventative maintenance or toss a new one in the glovebox. When you do remove the original, take a good look at the condition of the socket in the fusebox. If it's corroded, we recommend cleaning it to prevent a recurrence.
This particular relay is an 80A unit that's able to handle higher current than the stock relay in most Miatas. It's the stock part in 1990-95 models, replaced by a downgraded version in later cars. This makes it a good choice for cars with upgraded fuel pumps. It's more expensive than our lower amperage unit but it costs less to buy a better part than it does to burn out an underspecified one over and over! For cars with really high fuel pump draws or that cannot take the chance of a failure, look at our Trackspeed fuel pump rewire kit.
Mazda part JE16-18-811. Fits all NA and NB Miatas.