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

[Dave_Darling]

If your car still has ignition points, check them or replace them and the condensor.

If not, then rebuilding your tach will likely be needed.

--DD

[Valy]

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

Check for bad connection.
Replace the points + condensor.

[914Bryan]

I was thinking points too..Side note, I was thinking of replacing the dizzy and coil anyway. If you were to recommend one, what would it be?

[VaccaRabite]

If you are running Djet, you need to keep the stock parts.

If you are running carbs, talk to Chris Foley for one of his modified Mallory Unilite units.

Zach

[avidfanjpl]

Get a pertronix for that car. I only keep points and a condenser in a kit in the trunk if the pertronix dies on me.

The bounce may go away, but points are not in my life any more.

My car has NEVER run better.

Just my opinion.

John

[Valy]

Just for reference, this is what your bad points/condenser is doing. This was supposed to be a clean square waveform.
The tach interprets this as twice the RPM.

[914Bryan]

Awesome info guys! Thanks!

[Tom]

Valy,
That is some bad switch bounce! Then again, what are points but a switch.
That is one of the reasons electronics are better.
Tom

[michael7810]

My tach does the same thing. I have a new distributor with pertronix and MSD ignition. I took the tach to the local speedo/tach shop when I had my odometer fixed and the guy checked it out and said the bounce was normal. He said there is little to no damping to smooth out the bouncing and there was nothing he could do to fix it. Maybe I need to send it elsewhere if you guys are saying your tach does not bounce when shifting.

[914Bryan]

Sooo maybe an Autometer monster tach instead of the VDO? Just saying...lol

[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.

[VaccaRabite]

I don't suppose you could point out the cap that you added for those of us that are electronically challenged?

Zach

[Bartlett 914]

I don't suppose you could point out the cap that you added for those of us that are electronically challenged?

Zach

My Tach jumps also. I don't remember which cap can be changed. I did change mine (forgot which one) it didn't help. I think the tach needs mechanical dampening. Probably something like a special grease at the pivot point. Here is how I fixed my tach problem.

[Spoke]

I don't suppose you could point out the cap that you added for those of us that are electronically challenged?

Zach

The capacitor is an electrolytic type and is value 2200uF.

In the pic, the capacitor is shown at the bottom of the can between the turn signal tubes. The little green wires in the middle of the tach circuitry are the needle armature wires. It is these wires that the capacitor is connected across. The capacitor wires are the red (positive) and black (negative) wires leading to the cap.

Here's 2 places to find a 2200uF cap:

At Mouser.com

Radio Shack

[Spoke]

My Tach jumps also. I don't remember which cap can be changed. I did change mine (forgot which one) it didn't help. I think the tach needs mechanical dampening. Probably something like a special grease at the pivot point. Here is how I fixed my tach problem.

The capacitor placed directly across the winding of the needle will settle the tach. I used 3300uF and it was too much. My guess is you used too little capacitance or didn't put it directly across the armature of the needle.

Once you put a 2200uF cap across the winding, you will see new-age tach operation without the bouncing of the 1970's era tachometer.

[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.

[IronHillRestorations]

Sometimes just putting a clean piece of thin cardboard between the points and pulling it out (to clean them) will help.

[mgphoto]

My tach does the same thing. I have a new distributor with pertronix and MSD ignition. I took the tach to the local speedo/tach shop when I had my odometer fixed and the guy checked it out and said the bounce was normal. He said there is little to no damping to smooth out the bouncing and there was nothing he could do to fix it. Maybe I need to send it elsewhere if you guys are saying your tach does not bounce when shifting.

North Hollywood Speedometer, they rebuilt my tach using newer technology parts!

A Pertronix with the Bosch Blue coil, the best spark I've ever got!