Intermitent tach

[Melman]

1977 911 I think I read a repair for this problem but i cant seem to find it. My tach is usually at zero then suddenly works perfect. If i lightly tap the glass when the tach is working the needle drops like a stone back to zero....I'm thinking its more of a mechanical issue than electrical....any help would be greatly appreciated.

[Bob Kontak]

I have that issue as well. New V-reg a few years back fixed the bounciness in the needle but occasionally it will just die and a hard tap on the glass with my finger brings it back to life (half the time).

Most reading I have done suggests pushing tach out (into cabin) from inside the front trunk - I think you can snake your hand in by the brake booster. Clean and/or re-seat the connections.

The transmission signal generator unit is a puck type thing on the side of the trans. Not sure if that would be the issue (or even a culprit) since you (we) can tap on the tach and make things change.

Here is a link about cold solder joints inside the tach itself.

http://forums.pelicanparts.com/porsche-911-technical-forum/366603-tach-dies-when-hot-why.html

[Paulporsche]

Aside from the above mechanical and electrical items make sure your points are OK. Points that are old and not opening wide enough can cause the tach to go carazy.