Did you buy a dwell meter? Seems to me if the points are able to drive a dwell meter correctly when connected at the distributer and when connected behind the tach then your wiring and speed switch is OK. If the tach works fine for a minute or so then its likely a capacitor failing inside the tach. Its a cheap fix to replace the capacitors. Inside my 1972 tach there is a single electrolytic capacitor and what look like 2 x polyester capacitors on the copper side of the circuit board. Last of all, does an aftermarket tach work in the 911? Its not so hard to swap out the internals of the 911 tach for another and keep the original appearance. I did this and was even able to use the original 911 needle.

Thanks for a good explaination. I ordered the dwell yesterday. It's a multimeter with dwell and tacho funktion. I hope i understand how to hook it up.

I was waiting for my new refurbished tach and CDI (from a pro in germany), to be sure those things were okay.

I suspect my distributor to make some kind of fault. It's an old Marelli, but it feels good, and without much (i dont know the correct word) moving/wiggeling.

The speed sensor is not so important, because the fuel Cut system is missing on my car.

Watson, stop buying things.

What are the labels on the back of your tach? +12| | | | what is in the blanks?

What is the production date of your car? It is on the door post

What ignition system? Stock?

I wrote the threads referenced above and am happy to help you figure it out.

Hi 304065,
The label in the door post is gone, but due to Porsche, it is produced in June 1971.

My tach has been working for the past 7 years I have had the car.
It has the number 911 641 301 01 (0017)

12V/24000lmp
230/41/10

(and the connection has the (C) not (1))

The tach is stamped in the metal wit 3.71, but the blue ink stamp says 5.71.

It's stock ignition. Bosch CDI and Marelli distributor.

Ahh so.

Therefore your car was manufactured after the October, 1970 cutoff for use of the "intermediate unit." (It is properly called the "intermediate unit" and says "Einbau Senkrecht Klemmen Unten" on the side.

Therefore you have the correct tachometer with C, and your Bosch CDI coil should be an 001 coil also labeled with a "C."

Have you tried disconnecting the black/purple wire from the speed switch? You probably have. As you say correctly it is connected to the points and then runs to the speed switch and then to the back of the tach to the "C" connection.

If you are seeing square wave pulses at the black/purple where it connects to the tach with your meter (or oscilloscope) then of course the tach is your problem. It can certainly be low system voltage that is causing the fault as well-- you start car, battery runs down, CDI will continue to function with low voltage but the tach needs an 11V square wave. If you are showing around 14V with the engine running above 2000 RPM, then I think you can confidently send your tach to NH Speedo without worrying that you are wasting money.

More tools is always good, the dwell meter is a useful addition to any garage that has a points operated engine. Same with a multimeter. The dwell meter is easy to connect, positive and negative battery and the sensor clamp on the Black and Purple wire. I think if you had a distributor problem the engine would stop with the tach. At worst a worn distributor would cause an erratic tach reading. I wonder how Google translate will handle "erratic tach". If you are not using Google translate then your Engelsk er fremragende.

304065 and Porboynz, thanks for your replys (and a little danish phrase :) )

When I meassure the wolt on the batteries with the engine on, it's showing 14,1 volt. The batteries is under a year old, and the alternator is professionaly refurbushed last year, so theese things shouldnt be a problem.

Well... I finaly got my new "Automotive Meter". I bought this one:
=http://www.ebay.co.uk/itm/US-PRO-Full-Size-Specialty-Automotive-Multimeter-Dwell-Tacho-Extra-Large-Display-/321279364231?pt=UK_Diagnostic_Tools_Equipment&hash =item4acdbe0c87]US PRO Full Size Specialty Automotive Multimeter Dwell Tacho Extra Large Display | eBay

For tacho measuring, the manual says:
Connect the black test lead to negative pole.
Connect the red test lead to the connecting pole for the low voltage of the distributor or the terminal "-" of the firing coil.

Do I understand it correct, that I have to put the red test lead on the connection with the black/purple wire on the distributor?