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).

[Spoke]

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).

Keep in mind we're talking about 2 different scenarios:

1) Issues with points/condenser/ignition.

Issues with the points/condenser will cause the tach to bounce or jump/drop without warning as the car is running. Electronic points and properly adjusted and functioning points/condenser should cure this.

2) Design of the tachometer itself.

I could imagine that back in the 50s or 60s when the tach was designed, a faster response may have been desired over a tach that does not overshoot. Note that a fast response will get the needle faster to the correct value but will overshoot and oscillate at a frequency dependent on the design of the needle electronics and mechanicals.

In this case calling the tach phenomena as bouncing is not as correct as calling it a fundamental oscillation of the needle response to a step input.

So the capacitor across the tach windings settles the tach needle from fundamental oscillation but does nothing about issues with the points/condenser.

I put the capacitor on the tach in my 74 and it solved all oscillations and made the tach respond like in my 86 930 and all the other new cars I have. It responded beautifully. BTW, my car has has Petronix electronic points.