Tachometer behavior

[Norbert Drager]

My 64C has developed a strange phenomenon, having worked normally for years. When I start the engine after the car stood in the garage for a few days, the tachometer starts indicating very high revs, even when it is idling. With a slight throttle it will go up to 5000 revs. It will then jump aroud erratically. After driving the car for about 5 minutes the revs settle down to normal and staying like that for the whole trip. Could this problem be temperature related? Has anybody else observed it and what might be the cure?
Greetings from a cold South Africa.
Norbert

[Gary Carriveau]

This sounds similar to a problem that I had with my '64 C Coupe.
I carefully disconnected the cable from the back of the tach, slowly snaked the cable out into a large plastic tub so it would not get kinked (and controlled the oily mess), lubed the cable while in the tub, replaced the cable (the tub held it while I fed it into the housing), twisted the cable to make sure that it was connected at the engine end, and reconnected to the tach.
The erratic bouncing and high rev indications disappeared and have not come back.

Gary
'64 C Coupe

[Doug McDonnell]

Early 64 with mechanical tach or late 64 with electric tach?

[Norbert Drager]

Sorry, I did not mention that my C has an electric tach!

[Doug McDonnell]

Then check and renew the ground cable that goes from transmission to body.

[Dave Wildrick]

The electric tach receives the distributor signal from a wire connected to the post on the right side of the ignition coil (where the distributor wire also connects).
Make sure that the connectors to this post all have a tight fit.

[Peter Burger]

I had a similar problem with my 64 C. After a lot of trial (and error) I replaced the condenser on the distributor - that fixed the issue.
Regards
Peter

[John Brooks]

The electric tach is showing the average voltage of the points coil. Ie. points open and close, the faster they cycle the higher the voltage, and the higher the indication. Going to 5k is almost battery voltage, I would look for a short behind the tach to one of the indicator light bulbs.

It's pretty tight back there, a wire with voltage is getting to the male spade on the back of the tach, also look at the insulation for cracks. No voltage on the tach wire, the internal watch spring pushes the needle to zero. The voltage on the distributor side of the coil to the tach moves the needle up based on the amount of time the points are open. Look for a loose wire spade or a bulb on the back of the tach. It's jumping to max. That's max voltage, probably not seeing that from the coil lead.

[Norbert Drager]

Thanks guys,
I followed your suggestions. Ground cable and condenser replaced and all connections on coil, tach and lightbulbs are tight with no cracks or shorts in wires. It still jumps after starting, but works perfectly after a few minutes. Still wonder if it's temperature related. I expect I have to live with it.
Regards
Norbert

[Mike Wilson]

I wonder if it's hanging up until the rpm's are strong enough to pop it loose.

[WilliamVaughan]

My experience with erratic behavior of the electric tachometer in '69 912 was: that is the breaker points signaling you that they need some attention. Used to happen after approximately 9 K miles on a set of points. Usually happened either on a busy narrow toll bridge or near Zanesville Ohio.

The electric tachometer turns the voltage pulse from the opening points into a constant width current pulse to the tachometer meter movement coil.

I think what may be happening is: points are closing and bouncing open again (late) thereby giving an erroneous indication of twice the actual RPM. Try lubricating the pivot for the points swing arm. Also, a used set of points does not make great contact. The rough pits and peaks that have developed due to metal transfer in the arc may result in intermittent contact. That could produce multiple contact opening events that will be misinterpreted by the tachometer pulse generator circuit as indication of high RPM.