Tachometer goes wacky, not working
 

My tach has been driving me bonkers. It has never really worked since I bought the car. I actually purchased another tach thinking it was broken, installed it, and it didn't work right. So it is not the tachometer. Most of the time, it will not work at all, stuck at zero. Other times, it bounces around a little, sometimes alot at higher revs.

Does anyone have any idea where I can start to figure this thing out and get it working properly? It is a 78 911....Thanks

Sometimes tach problems, such as a bouncy needle, can be attributed to dirty ignition points. Check the connections on the coil and clean your points.

Put a volt meter on your battery when the tach starts to act up...could be an alternator (V-reg) going bad. And/or have a helper rev the engine to 3-4000 with a meter on the battery...confirm voltage.

You can tap into the rear power panel in back too if you don't have a helper.

Edit: The 78 does not have points, but cleaning contacts is a good start...start with the 14 pin connector in the back...also clean the connector contacts to the CDI box. Make sure it the contacts make good...contact.

Thanks for the info. I will give it a look and cleaning.

Cleaning contacts is indeed a good start, and checking your system voltage while the engine is running is also a good idea. Voltage should be about 14.5 volts, maybe 14.7, maybe 13.8, but it should not be more than high 14's and it should be steady.

You also want to fairly thoroughly take a look at your ignition system, since I think this is where the tach gets its information. What I'm saying here is that your tach may be warning you about an ignition problem. Ignition problems can be spendy and they can leave you stranded.

Check the CD box contacts, connections, and your grounds. Check the trigger wire contact from the distributor. Points can cause this, but there will be no points on a '78.

Bouncing around and not registering is caused by dwell being incorrect. Reset it and that should take care of everything.

Dwell? On a 78 SC?