Tach strange and it's not the alt. problem

[DeFreest]

1979 911SC with a tach that seems to have a needle thats seems as though it not dampened or to springy. Hard to describe but when I blip the throttle quickly say to 4000 rpm by ear the needle goes to 6 or 7K on the dial with out the engine actually going to that rpm. Same with the other way when I let off the quick throttle blip the needle bounces down to 0 then settles in to actual idle speed. This symptom is not to be confused with a bad alt issue. I know it's not the alt issue as it will show a steady rpm and will stay there say like when I'm cruising at a set speed. No needle bounce. It only happens when I do a quick throttle blip. The needle just launches and shows a higher than actual rpm. The needle is just springy. Anyone have this issue and have a fix for it?

[DeFreest]

No ideas? Anyone?

[Targalid]

Had something similar and changed spark plug wires. Fixed the bouncing tach. You might do the dark run test: check running engine compartment in total darkness to see if there is arcing around the ignition components. When I did this it looked like Nicola Tesla's lab in Colorado Springs: lightning bolts everywhere. New wires fixed it.

[joe payne]

My 78 sc does the same thing. I just thought it was 70's technology.

[DeFreest]

Originally Posted by Targalid

Had something similar and changed spark plug wires. Fixed the bouncing tach. You might do the dark run test: check running engine compartment in total darkness to see if there is arcing around the ignition components. When I did this it looked like Nicola Tesla's lab in Colorado Springs: lightning bolts everywhere. New wires fixed it.

I installed new wires cap and rotor. I'm wondering if it's something in the tach thats just not quite right.
I wonder if I could borrow the tach from my 87 930? Are they compatible enough to try it with out screwing up either one?

[timmy2]

Dies your later tach have the up shift feature?
May not work.

[DeFreest]

Originally Posted by timmy2

Dies your later tach have the up shift feature?
May not work.

The little anoying light? If thats it then no it dosent have it.

[wayner]

Originally Posted by DeFreest

No ideas? Anyone?

Common problem when they age.

Even listed here on the fifth bullet down

Porsche 911 Service and Restorations

[DaveA]

I've got about three tacks that the damping has gone bad, same problem. The cure is as wayner indicates, restoration.

R/Dave

[DeFreest]

Originally Posted by wayner

Common problem when they age.

Even listed here on the fifth bullet down

Porsche 911 Service and Restorations

Ok thanks. I was afraid it was in need of repair. Anyone know the cost to fix the tach?

[dipso]

If you have points, check them. My tach was acting funny for about a day, right before the points wore down to a complete close, and the car died on the freeway.
Take you 5 minutes, check the gap. If you have them.