Low cost Tachometer Filter

If you're tach is troubled by a bouncy needle, or the needle "floats" when you get on the gas, this little filter will solve the problem.

Why the tach bounces: A tachometer "counts pulses" If the pulses from your ignition are not smooth, the tach may see more or less than there really are, and this can cause erratic needle movement. With most aftermarket high performance tach's, this may not be a problem, but if you have a factory tach, and add an HEI or other high powered electronic ignition, you may get a bouncy needle. This device will smooth out the tach needle on any engine.

What the filter does: This is a simple "low pass" filter. It contains a resister and a capacitor. Low frequencies are passed through to the tachometer, but high frequencies are sent to ground. In this filter, the cutoff is about 10,000 rpm with a V8 engine.

Will it affect my ignition: No, it can't. It is a completely passive device. The resister isolates the ignition system from any effect.

Can it hurt my tach: Again, no it cannot. In fact, it's ability to free your tach from unwanted hi frequency, high voltage spikes may help extend the life of your tach!

What do I need to build it: If you can solder, and you have a Radio Shack nearby, no problem. You'll need a 270 ohm, 1/2 watt resister, a 1 ufd electrolytic capacitor, and some wire, terminals, and shrink tubing. The ring terminal on the right is soldered to one end of the resister. It connects to the - coil leadThe black wire with the ring terminal is your ground, and it is soldered to the "loose end" of the capacitor.In the middle, the resister, capacitor, and a wire are soldered together. This wire goes to your tach.This pic shows the almost finished product. The resister and capacitor and all the connections are securely covered in shrink tubing. For my install, I will put an insulated quick connect on the blue wire which goes to the tach.Here's my installation. The ring terminal from the resister (blue) connects directly to the - coil stud. The fuzzy red thing in the foreground is pin type quick connect and that blue wire goes to the tach. The ground is attached to the coil bracket.Note that this is just one possible way to install this. Others have put them under the dash or elsewhere.