Need Assist Troubleshooting Tach - 69T

[carsandrealestate]

Finished my long list of to do's, which included getting my gauges redone by Pal Alto. While I was at it I installed a new dash pad & trim, painted the cluster, installed new headliner, carpet, SSI's, oil cooler, harnesses, yada yada. So while it looks the way I want now - and no more leaks. Still one problem. The tach is not working. Assuming it is a user error, and hoping it is in the engine bay and not behind dash.

I replaced the harnesses while I had the engine out, and have since traced things and now unsure of where the black w/purple trace coming from the engine harness socket connects. I though that went to the coil but in looking at diagram I am not sure. Can someone please confirm? I really dont want to have to pull apart things again. Its been laid up for a year now - just want to drive.

[greens]

Finished my long list of to do's, which included getting my gauges redone by Pal Alto. While I was at it I installed a new dash pad & trim, painted the cluster, installed new headliner, carpet, SSI's, oil cooler, harnesses, yada yada. So while it looks the way I want now - and no more leaks. Still one problem. The tach is not working. Assuming it is a user error, and hoping it is in the engine bay and not behind dash.

I replaced the harnesses while I had the engine out, and have since traced things and now unsure of where the black w/purple trace coming from the engine harness socket connects. I though that went to the coil but in looking at diagram I am not sure. Can someone please confirm? I really dont want to have to pull apart things again. Its been laid up for a year now - just want to drive.

According to my wire diagram, The black/purple wire goes to the three prong ballast resistor in the engine compartment on the terminal marked "C". The red/white off that same component terminal B goes to the coil through the plug connector near near the rear bulkhead. A continuity checker is helpful to trace where the leads are connected.

[SFE]

The question is : do you still have your ballast unit (item 2) in your car ?

[frederik]

Some tachs need the ballast unit, some don't. It's possible you have a later tach. Pull it and the white connector on the back will have a "C" on it if it's a later one. (If you don't want to remove the steering wheel, you can also pull the combined temp/oil press gauge, move the tach out halfway, and try to look at the back of the tach with a mirror or a phone camera.)

So the CDI outputs a lower voltage pulse than the early non-CDI cars, and the ballast unit converts the low voltage to the high voltage expected by an early tachometer. Later on, Porsche changed to the "C" tachometer which doesn't need the ballast unit. See post #5 here:
https://www.early911sregistry.org/fo...ing-Tachometer

See also post #21 in that thread which has an original Bosch instruction mentioning the markings on the back of the tach. I remember there being a C but here it says the mark is "15" for a later tach.

[SFE]

I don't have the ballast and I do have a later tach on my 69E.
This is my wiring and it works just fine :