Tachometer and Speedometer not working after reinstall – 1977 Porsche 911 3.0L, carbs

[patipatina]

Hi everyone,
I recently bought a 1977 911 with a 3.0L engine and carburetors — quite a patinated car. I had all the instruments (which worked fine before!) professionally redone. I took detailed photos of the wiring beforehand and reconnected everything correctly.

However, neither the tachometer nor the speedometer (including the odometer) are working now. I even sent both instruments back for inspection, along with the CDI ignition unit — everything tested OK.

Still, no life in either gauge. I’m out of ideas. Has anyone here experienced something similar or could point me in the right direction?

Thanks in advance!

[Dave Colangelo]

Check the fuses...

Could be the sensors, if you have access to some electrical diagnostic equipment you can sort it out. I dont know the reading off hand but the sensors should have a nominal internal resistance that you can check with an ohmmeter. If you have access to an oscilloscope (lots of cheap options out there these days) you can confirm, if the engine runs, that they are outputting appropriate square waves. You should see a smaller pulse width as the RPM's get higher.

From the description it sounds like they are either not getting appropriate power or signal (or both).

Side note, the ODO is driven by a small motor inside the speedo unit, since neither are working it sounds like your not getting signal to the whole unit. In some cases the ODO can stop working when the small plastic drive gear shreds but the speedo will continue working.

[patipatina]

Thank you Dave ... Indeed, there was an issue with the 14 pin connector in the engine bay. one pin had a bad connection, therefore the signal didn't get trough to the tach and since that signal goes through to the speedo, none of these did show anything ...

Had some help measuring ... And he pointed me in the right direction and we've solved the issue a few minutes ago.. Thanks again for pointing us in the right direction!