how it broke my wallet
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The Erratic
Tach and how it broke my wallet |
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Look for the wire harness above your alternator on the left side of the motor. Bet it's loose. Take a 10 mm socket and tighten it. This only works for the 94-97 miata. On the 90-93 miata, the tach problem has a different cause and I believe it is just a nuisance. On the 94-97, it can damage your coils, so you probably want to fix it..
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If the ground wire gets loose, the ignition system starts seeing electrical noise. When the noise level gets too high, the coils can interpret it as a signal from the engine computer to fire the spark plug. The electrical trace in the figure below captures one of these extra sparks.
Explanation for geeks
like me:
1) The top waveform is the tachometer signal. It is normally around 10 volts
and drops to 0 volts whenever one of the coils fires a spark.
2) The lower waveform is the trigger signal to one of the coil packs. This input
comes from the engine computer and is at 0 volts until it is time to fire a
spark. The signal is a pulse that goes to 3 volts. When it is shut off and goes
back to 0 volts, the coil pack will spark.
The top signal shows a sequential spark to cylinders 1&4 and cylinders 2&3. Remember that the cylinders fire in pairs. Now see the third spark in between these two events. This is the actual misfire! Note how the coil trigger signal bounces around at the time of the misfire. That bounce is activating the coil and causing the misfire.
Why it should be fixed:
The misfire can ruin the ignition coils because they will overheat from the
misfire. That happened to me.
What else did I try:
Perhaps I should have found a dealer to work on my car. However, my car has
a supercharger and I knew from past experiences that my local dealer wouldn't
touch it. So I ended up learning the hard way. I swapped virtually every electrical
component in the ignition system without success. That included the engine computer,
cam angle sensor, tachometer, spark plug wires, plugs, and ignition coils. I
borrowed the tachometer and computer from a friend and bought junkyard replacements
for the CAS and coils. Nothing helped and along the way I burned up the original
and the replacement coils. That's when I hooked up an oscilloscope and discovered
the tach twitch was actually a spark misfire. I realized it was a ground problem,
but could not figure out why the grounds were bad. I added extra ground wires
and cleaned all the grounds at the back of the engine, not knowing it was the
ground at the front of the engine.
I did not discover this fix. After about three weeks of frustration, I was referred by a friend to an article in the summer 1998 Miata Club of America Magazine. It described the same problem and detailed the loose nut. I hooked up my scope, tightened the nut and confirmed that the electrical noise went away. So if I had read my magazine and done my homework, the world would not have these fancy oscilloscope waveforms and I would have saved $300.
By the way, I did not have to buy a third set of coils. Since the coils come in pairs, I had only burned up one coil in each set, so I was able to make a third set out of the remaining parts.
I also have a old 90 miata, and its tachometer started to twitch for a short time about ten minutes after a cold start. This seemed to be temperature dependent and went away as the weather got colder. I was able to get a scope on the tachometer signal while it was twitching, and I saw no extra pulses. The signal was steady, so I suspect that on the 90-93 miata, a twitching tach is due to a bad ground on the tachometer. There, it only seems to be a nuisance and I'm not worried about it harming anything. I plan to take apart my 90's dash when it gets warmer and see if I can clean any ground wires under the dash. I've already cleaned all the grounds under the hood.
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Edited July 11, 2002