A less common problem is a poor connection in the RPM signal wire. In this case, the needle will usually bounce around below the actual RPM reading because it will miss some of the voltage spikes. Look for corrosion, a broken wire, or a loose connection anywhere between the coil terminal and the tachometer.

What causes RPM gauge to bounce?

Oftentimes, when your RPM fluctuates while accelerating, it's going to be because of an issue with the idle air control valve, or IAC, in your car. The IAC is in charge of controlling the idle speed of your engine, and it could be the reason why your tachometer is out of control right now.

What causes tachometer to fluctuate?

A blown fuse is a potential problem with the tachometer. ... Bad wiring can also be the cause of a malfunctioning tachometer. If you are getting erratic readings, this can be a sign of bad wiring. Another potential problem is calibration issues.