tach is normally dead but sometimes comes to life

[ryan911]

The tach in my 77S works quite well, seems to display the right RPM with no bouncing or erratic behavior. Unfortunately it only does this once in a while, and the rest of the time the needle sits right at the bottom.

Everything else on the tach instrument works (high beam indicator, signal indicators, etc), and the power and ground going to the tach are good.

I don't have an oscilloscope but when I measure the tach signal wire with a digital multimeter while the car is at idle, I get readings around 7V. There are no other symptoms, so I have no reason to suspect the rest of the ignition system, the car runs quite nicely in all driving conditions.

Has anyone run into this sort of tach behavior before? Any suggestions on what to check next?

Thanks,
Ryan

[ryan911]

Oh ya I wanted to add that I tried moving the wires around in a few places (dist, coil, signal wire connector, CDI wires, wires behind the tach) while the car was running, and this didn't have any effect on the tach. I've also checked all the terminals that I could see and tighenened up the spade connectors that slide off easily, etc. I've done the basics, so I'm thinking the problem is probably inside the tach?

[PorscheGAL]

May need a rebuild. I have used North Hollywood speedometer for a similar issue. They are super easy to work with.