Jumping tacho

[prebordao]

My tacho jumps around when reving hard or when changing gears. It looks like the needle is "loose" going up and down before stabilising.
Could be some bad spring inside or an electrical thing ?
My car is a 74, I'm not sure if the tacho is mechanical or electronic...

[matt demaria]

Check your points.

[prebordao]

currently no points. Powerspark (similar to pertronix), but the problem existed also with points.

[p911dad]

Prebordao, fwiw, when the tach on our 84' Carrera started jumping, it turned out to be a bad alternator (according to the dealer). The dealer changed out the alterator and no more jumping tach. I am not absolutely sure if your tach is electr. or mech., but I believe that era Porsche had an electronic tach.

[Wiese]

74 is electronic tach. Check your connections for the sending unit and the ones on the gauge itself. Sometimes it can be the gauge itself as the damping unit wears/fails.

[prebordao]

Where's the sending unit ?

[Tidybuoy]

I have a '74 with the exact same symptoms. I'm anxious to find out where the sending unit is as well.

[prebordao]

Just remembered. I have a 70' tacho at home and it's electric / electronic so the 74 has to be too.
But where is the signal coming from ?

[prebordao]

The Cdi box ?

[kwikt 911]

Mine does the same. Not the points, alternator, connections,cdi, I'm positive it's the tach but i just live with it.

[Jim Sims]

In a '74 911 the tachometer signal comes from the distributor; the wire is color coded black and purple. In addition to the connection on the back of the tachometer, this wire passes through two connectors on the driver's side of the engine compartment at the regulator panel (electrical chassis) - one connector is at the front of the panel and the other is at the rear of the panel. The connection at the distributor also mounts a white wire going to the CDI ignition box. Given the symptoms you report and my experience with my '76 911 (same arrangement as the '74) I suspect neither the point dwell or a loose connection or an alternator issue are the cause. Addressing these possibilities did not solve my jumping and sticking tachometer problem. I concluded that the 36-year old electronics (especially any capacitors) and mechanism in the tachometer were in need of replacement or repair. I pulled the tachometer and sent it to Hollywood Speedometer in California for refurbishment - problem solved. The tachometer is now steady and accurate. Good luck.

[prebordao]

Yes the capacitors could be to blame.

[prebordao]

As i'm quite far from hollywood i might replace them myself.

[prebordao]

In a '74 911 the tachometer signal comes from the distributor; the wire is color coded black and purple. In addition to the connection on the back of the tachometer, this wire passes through two connectors on the driver's side of the engine compartment at the regulator panel (electrical chassis) - one connector is at the front of the panel and the other is at the rear of the panel. The connection at the distributor also mounts a white wire going to the CDI ignition box. Given the symptoms you report and my experience with my '76 911 (same arrangement as the '74) I suspect neither the point dwell or a loose connection or an alternator issue are the cause. Addressing these possibilities did not solve my jumping and sticking tachometer problem. I concluded that the 36-year old electronics (especially any capacitors) and mechanism in the tachometer were in need of replacement or repair. I pulled the tachometer and sent it to Hollywood Speedometer in California for refurbishment - problem solved. The tachometer is now steady and accurate. Good luck.

Jim, do you know what kind of signal goes to the tach ?
I mean, 12v square pulses ? 1 for each spark ?

[Jim Sims]

~11 volt square wave each time the points open/spark is produced. You will need an oscilloscope to properly detect the square wave.