After the engine starts, and the ignition key is returned from the start position to the run position, the alternator then takes over supplying the power requirements of the car. A defective alternator can cause damage to the battery and may leave you stranded on the side of the road.The alternator on the 1990 Mazda Miata is located on the left (passenger-side) front of the engine, below the intake manifold duct. Replacing the alternator is a repair that the average home mechanic can perform in an afternoon using basic hand tools.
Disconnect the negative (black) battery terminal using a 10-mm wrench. Remove the air duct that connects the throttle body on the left side of the engine to the air flow meter on the right side of the engine. The duct runs across the front of the engine and is removed by loosening the two hose clamps with a screwdriver and twisting as you pull it off the throttle body and air flow meter.
Remove the upper alternator tensioner and bracket using a 14-mm socket and socket wrench. Rotate the alternator slightly clockwise to loosen the belt and remove the belt from the alternator pulley.
Remove the lower alternator pivot bolt using a 14-mm socket and socket wrench. Pull the top of the alternator forward slightly to collapse the spacer located in the lower mounting boss of the alternator, and slip the alternator forward into the engine compartment. Remove the battery cable attached to the back of alternator using a 10-mm wrench, and unplug the wiring harness from the back of the alternator. Remove the alternator from the engine compartment.
Slip the new alternator into the engine compartment and plug the wiring harness into the back of the alternator. Bolt the battery cable onto the alternator and slip it into place on the lower mounting bracket. Install the lower pivot bolt and tighten it snugly. Bolt the upper tensioner bracket to the engine and the alternator. Slip the alternator belt onto the pulley. Tighten the 12-mm tensioner bolt until the alternator belt is tight. Tighten the upper and lower alternator bolts tightly.
Reinstall the air duct, and reattach the negative battery cable. Start the engine, and check for proper alternator output. With the engine running, voltage at the battery should be more than 13.5 volts.
