Tach Bounce Options

[914Bryan]

What can I do about a bouncing tach? When I shift, the needle wildly bounces around until the rpm settles in a gear. Then it either climbs or drops depending on acceleration or decelaration. Any ideas? (IMG:style_emoticons/default/confused24.gif)

74 2.0, stock fi and dizzy/coil

[Spoke]

There's 2 basic reasons the tach bounces:

1) Points opening and closing causes transients causing bouncing almost anytime the engine is running even a constant RPM. This happens when you have points-issues.

2) The early model tach needles do not have sufficient damping to prevent under shoot and over shoot when the RPM changes. Change the RPM and the needle oscillates back and forth and finally settles to the correct RPM.

See the general step responses below (step being from one RPM level to another).

If the tach is not damped enough, it oscillates.



To damp the 914 tach, simply connect a 2200uF capacitor across the needle winding as shown below. This will transform the operation of the tach to be like tachs in today's cars which never bounce or oscillate.

[Valy]

My dear friend,
you solved a symptom, not the root cause. I'm sure the German engineer that designed the tach knew how to attenuate the bouncing but didn't do so cause there was no need.

The capacitor on the distributor is supposed to smooth that bouncing. If the capacitor is dead, the bouncing will be there.
If you changed the capacitor and the bouncing is still there, I bet the coil has a short between the winding on the high voltage side (almost dead coil).