1973 911E, intermittent tachometer operation

[carl888]

I have a tachometer than works occasionally. No rhyme nor reason to engine rpm nor time running. When it works, it could be for 2 minutes or 30. And when it does so, the operation of the needle is satisfactory, no bouncing or any other maladies.

Since the tachometer does work at times, it is probably not the tachometer at fault.

I have performed the following:

- checked the connections on the back of the tachometer.
- checked the connections on the underside of the ignition amplifier.
- exchanged the ignition amplifier for another working unit.
- cleaned the earth connections.

All the above yielded no change.

Question, with regards to the mainly useless factory wiring diagram attached (That doesn't even show the tachometer connection to the tachometer, nor labelling connections like A, B, C and D on the ignition amplifier) can anyone tell me from the attached diagrams:

1. Where wiring harness 84 and 85 is located?
2. Does anyone have a spare tachometer I could try?
3. Anything else I should check?

My next step was to connect a scope to the ignition amplifier and then the connection at the tachometer but I lost motivation. I've had so many electrical issues with this car, I've nick named it the ****osauraus. So thought I'd have a whinge here instead!

Thanks for any information.

CJ

[Andrew Vidler]

Mate.
I have a couple of spare tachos you can try