Tach Bounce Options

[larss]

I had about the same symptoms on my tach, the needle was jumping, especially when accelerating. The reason showed to be the advance plate in the dizzy being loose (i have original dizzy with points). The small "bracket" on top of the ball (which is supposed to press the ball and then the plate steady down) was simply not tightened properly. After tightening - no more bouncing.

/Lars S

[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).

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.

The problem is that the bouncing fools the tach to see a double RPM value. You need to filter that.
Just adding a capacitor on the tach winding will force the tach to average the input information so the needle will move with delay.
The best thing is to solve the rot cause. This will also improve the spark.

[Spoke]

The problem is that the bouncing fools the tach to see a double RPM value. You need to filter that.
Just adding a capacitor on the tach winding will force the tach to average the input information so the needle will move with delay.
The best thing is to solve the rot cause. This will also improve the spark.

I think you're talking about points issues and I agree 100% with you.

On my 71, with Pertronix electronic ignition, I get no contact or points bouncing.

The tach design allows for significant overshoot with fast RPM changes.

I get harmonic oscillation when I downshift or upshift. I've observed at most a 1000 RPM overshoot when downshifting. I would estimate the oscillation to have a time constant of about 1/4 second.

When downshifting to a higher RPM, the overshoot goes as much as 1000 RPM over, then undershoots once and settles at the correct RPM.