Tachometer Problem
 

I am wondering if I could solicit some ideas from the group concerning my tach problem.
I was driving my '80 911SC yesterday when out of the blue, the tach needle goes from 3500 RPM to zero. For the rest of the ride home it stayed at zero without any movement. The turn signals and high beams indicators all worked, and I didn't have a alternator charge light. I checked the battery and grounding straps for tightness, and also pulled the tach out (as far as I was able to move it) to confirm the leads on the back of the gauge were attached. I looked at the wiring diagram for my MY, and the wires going from the back of the tach are illustrated, but I can't find where they attach in the engine bay to generate the RPM indications on the gauge.
Any thoughts or recommendations?

On my 85 there's a plug in the frunk I think the wires are black with a violet tracer. I had one of the connections come loose and lost my tach. I think the SC has the same connector.

If I remember correctly signal comes from the coil. I had a thread on this issue a few years ago. Correction, not the coil, the CDI. Uwanna covers this nicely in my thread. Pic of the connector at post 33
https://forums.pelicanparts.com/porsche-911-technical-forum/871214-tach-not-working-after-engine-drop-install-2.html

Cabmando,

Thanks for the recommendation and I'll look through the tread for any ideas. I appreciate your help.

Bret

Could be your alternator. When my alternator failed the tachometer needle did like yours. A voltage surge fried my clock and high beam lights. I would check your alternator.

cabmando: I don't think my '80 911SC has that plug in the frunk, or at least I can't find it.

martidj: Should I be seeing an alternator light if my alternator has issues?

Tach signal wire on an SC comes from the CDI and runs down through the engine harness to the 14 pin connector. From there it is a direct run to the Tach.
Check the voltage output of the car at different RPM’s and report back.
If you see anything much above 15 vdc, your voltage regulator is suspect.
The remote VR on an 80 is “Normally” mounted beside the CDI but if alternator has been replaced it is likely internal.

timmy2:

I checked the continuity of the wire going to the CDI (TD) and it checked good. As far as voltage, my lighter voltmeter indicates 14.2 - 14.3 volts at various RPMs.

Is it unusual to have a Tachometer just suddenly die? I talk with the folks at NSI, and they offered to bench test the gauge, and if it it could be rebuilt, they quoted me a cost around $235.

Sounds like the best road to take.

yep, there is a $3 part that goes bad inside the tach. Getting to it cleanly takes some skill.

Where does the CDI obtain information (electrical pulses) to transmit to the Tach for RPMs? I’m just trying to eliminate other potential causes before pulling the tach out of the car.

The CDI gets the signal from the distributor (green wire).

lsf: what specific item (rotor?) generates the signal that travels through the wire to the CDI and then onward to the tach? Is it an item that could fail and be isolated with the tach thereby allowing the engine to run with no apparent issues?

If signal from Distributor fails, CDI will not fire. Car will not run.
You could use an oscilloscope to see the Tach out signal .

The TD signal out of the CDI requires a pull-up current. This is provided by the tach itself. So make sure the tach is connected if you try to measure the TD signal otherwise you won't see much. Normally, there is a square wave signal that alternates between around 1V up to 8V. Weird that you actually caught it in the act of suddenly dying!

Thank you for all of the inputs and recommendations. I plan on pulling the gauge and sending it to NSI to see what their bench indicates.