96 Miata tach jumping

Does it once in a while on the highway. What should I check?

Does the car hiccough at the same time, or is it just the tach?

just the tach. No signs of ignition coil troubles.... yet.

Check to be sure that the #4 spark plug wire (the short one closest to the firewall) has not pulled loose at either end. Since you're in Massachusetts and it's starting to get cold at night, the insulation may have contracted and pulled it back a bit. The shortest wire is most susceptible.

From what I recall, the tach isn't driven off the coils, but from the ECU and shares the same signal as the coils. Since the car drives okay, I'd concentrate on either the tach signal to the gauges or the grounds for the gauges. All of the others are damped, so only the tach would show a skip from a momentary interruption.

I was having a similar issue with my temperature gauge in my 90 NA. Under throttle, it would jump around. I added a second ground strap from the head to the firewall, and it has never happened again.

I have a similar thing occur with my speedometer needle at speeds below 30 to 35 mph. Not sure what is causing it, but its not really interfeering with any performance of the car. I wouldnt mind getting a more accurate readout of how fast I am going though. It only does it under acceleration. Any ideas?

The speedo on a 1990-97 Miata is mechanical. That's the sign of a cable that's getting old.

ok. How do I go about getting that fixed?

Good to know. My '91 does the shifting tach on the highway too. Exact same symptoms, been doing it for a couple years now. The guys at miata.net were not helpful when I looked for answers. Keith, you're the man.

miataman86 wrote: ok. How do I go about getting that fixed?

Speedometer cable: buy one from Mazda. Unscrew the old one from the transmission--visible underneath; I think it's on the passenger side at the rear of the trans, near the prop shaft. Unscrew it from the back of the instrument cluster. Put the new one in place. Screw it back in to both spots. Enjoy.

It's an easy job, and if you're 1. determined and 2. have narrow arms/hands, you might be able to do it without taking out the instrument cluster. I vaguely recall leaving the cluster in when I did mine, though I think I scratched my arms some and was in a very uncomfortable position reaching up under the dash like that.

I'll look into doing this. Doesnt sound hard to do at all.

In reply to Alan Cesar:

I think the 91's tach takes the signal from the coil packs. It changed later to ECU driven later on.

You're right, on the 1.6 the signal seems to come off the igniter. I haven't been as deep into those cars of late...he says, procrastinating so he doesn't have to go down into the garage and finish the Torsen swap on his 1990 so he can move the car to a different part of the garage and yank the motor this afternoon for the 1.8 conversion...

I'd still be looking for problems for the gauge grounds or possibly the signal from the igniter to the tach if the engine runs smoothly, though. The tach signal for a 1.6 is yellow with a blue wire, and memory tells me the 1.8 is black with a white stripe - but you'll want to confirm that. The ground for the tach on a 1.6 is under the dash just in front of the doors. That'll be near the fusebox on the driver's side, but there's one on each side.

The 1.6 runs like a champ and I dont have any problems from the ignition system. The tach moves with no problems. I'll have to look into testing the ground just to make sure. The speedo is the one that has the needle bounce a bit under acceleration.

miataman86 wrote: ok. How do I go about getting that fixed?

before you remove the old cable, attach a wire or string to it. You can use it to pull the new cable back into position.

miataman, the ground discussion isn't for your car

.....got a new slave cylinder coming in tomorrow and I'm gonna put my stainless steel clutch line in. I should notice a difference in the feel of the clutch pedal, right?