A Dummy's Guide to '94 MX5 ignition

How it works

1. The cam sensor sends signals to the engine computer unit (ECU) that tell it when each cylinder is ready for fuel and spark.
2. The ECU sends a drive pulse to both coils when needed. It also sends out signals to control the fuel injectors..
3. The tachometer reads the coil outputs for RPM. The engine looks at the output and knows the coils are working.

Uh Oh. Waveforms

There are two cam angle sensor signals. Here we see the one that controls spark. When the pulse is high, a cylinder is ready to be fired. The engine computer will send a firing pulse on alternate strokes to coil A and coil B. These are shown above as blue and red pulses. When the pulse goes back to zero, each coil fires and we see a spark. On my 1994, the signals that go to each coil is a 2 volt pulse. You can't see it with a volmeter.

The engine computer will calculate when to send the coil pulses, based on the engine speed, which it figures out by measuring the time between the cam sensor pulses. As the engine speed goes up, the interval between the pulses shortens. The time between the cam sensor pulse edge and the coil firing determines the amount of advance. When you set your timing, you set the advance at low engine RPM. By the way, the speed goes up, the computer adds additional advance to ignition timing.

To save money, and also because it works fine, the each coil fires two cylinders simultaneously. Coil A runs cylinders 1 & 4, while coil B runs cylinders 2 & 3. When cylinder 1 is on its compression stroke, cylinder 4 is on its intake stroke. This is same for cylinders 2 and 3. Spark is only used on the compression stroke, so the miata system is called a "waste spark" system, because only one of the cylinders actually uses the spark that it receives

There are four fuel injectors, so each has its own control voltage. This allows for sequential fuel injection, where each injector is fired exactly when a cylinder is ready for fuel. . The length of each injector pulse controls how much fuel is squirted into the cylinders.

Finally, the tachometer voltage is a combined signal from both coils. Your tachometer is really a voltmeter that measures the average value of the tachometer voltage. As the engine speed goes up, you see more pulses and this means a higher average voltage. The computer will also read the tachometer voltage, but I believe it uses the cam sensor signal to determine the actual engine speed. If the computer cannot read the tachometer signal, it shuts down the fuel injectors and the engine dies.

Here is what the signals look like on an oscilloscope.

Hey, what about me, I don't have a 94 miata

Beats me what you got then. I've only looked at my two cars, which are a 1990 and a 1994 miata. Here's some comments though.

Coil Signals I believe the trigger signals for the coils on a 1990 are also 3 volt pulses, but they go to a separate component called an igniter which is mounted on the passenger fender. The igniter module then powers the 1990 ignition coils and also generates the tachometer signal. On the 1994, the igniter circuit is combined into one package in the coils.

Fuel Injection: The 1990 is similar, except that there are only two fuel injector signals, not four like on my 1994. On a 1990, cylinders 1&3 and cylinders 2&4 have the same fuel injector timing. That means it's not possible to time the injectors to squirt on the intake stroke of each cylinder. One cylinder would have to wait til the intake stroke to suck in the fuel. When you think about the old fashioned carburators, that was always the case for all the cylinders, so this is not a big thing. It must be a big thing for efficiency though, as 93 miatas for California have their injectors wired for sequential injection like the 94.

Tach Signal The 1990 tach signal is a positive going pulse, starting a 0 and going toward +12 volts while the 1994 tach signal is a negative going pulse, starting at +12 and going toward 0. In 1996, the tach signal was removed from the coil pack. On those cars, the computer provides the tach signal.



I'm neither a certifed auto mechanic nor a trained Mazda technician. The information on this page is my own interpretation and may be flat-ass wrong. Use at your own discretion.


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Rev 1.0 November 19, 1998