The car in question is a 93 Miata, the tachometer needle fluctuates very randomly at random times (did it while idling and while I was driving). But the engine load does not change, it is the gauge it self that is reading incorrectly. What could make the gauge fluctuate like that? It seems like it gets a surge electricity, bouncing up and down very randomly
You're getting an intermittent signal to the tach from the ECU or the tach sensor. If the tach comes from the ECU (which it seems to after a bit of Googling, but I don't know Miatas well enough to be sure I'm reading what I find accurately), then the sensor side is probably good since the ECU will be using the sensor for engine control and it sounds like that is working.
So that leaves the delivery of the signal to the tach and the tach itself as the possible problem areas. The easy (easier) thing to do will probably be to check the wiring to the tach for any sign of damage of corroded connections and repair or clean up as needed.
It may also help to see if you can find a pattern to when the tach drops. If, for example, it happens on acceleration it might be that the engine movement is causing an intermittent fault in the wiring. Or if it happens when going over bumps it will likely be vibration related – could be a crack in the circuit board in the instrument cluster or a cable fault.
There are likely to be some typical problems that you might be able to sort out by reading some of the Miata specific groups.
Your car is pre obd2 which means there probably is no way to reliably look up what the computer is reading and what it is putting out to the gauge.
So this can be a few things:
- It could be a loose wire. best way to isolate this is drive over a sharp small speed bump or 2x4 quickly and see if it reliably causes your gauge to fluctuate.
- it could be the RPM sensor, usually the crank position sensor, that is dying and coming back.
- you can usually isolate this by revving in neutral so there isn't any road vibrations or bumps. Fluctuate the RPMs up and down slowly and keep it steady at different spots. Listen for odd blips pops or slight engine differences when the gauge does go crazy. If there is a difference than that usually means the issue is somewhere from the sensor to the computer but not the tac. At Least it will give you a place to start looking.
- it could be the computer glitching out but this is not likely.
- it could be the gauge glitching out which is also not likely.