QUOTE(Peashooter @ Apr 7 2019, 08:29 AM)
Thanks for all the replies. I'll check the simple stuff first, like grounds. If that is all OK, I'll look around for a 911 tach I suppose. Wonder if I put a jumper wire in place if the tach might work again?
No, it won't work on a 70. The 1970 and older tachometers were designed for a Kettering ignition(points, condenser and coil). When Porsche found they were fouling plugs on the early 911s, they put a CD ignition on the cars. The problem was, the tach on a Kettering ignition is wired to see high voltage spikes from the coil negative terminal. The CD ignition won't deliver the high voltage spike, and the tach won't work. They had a lot of tachs already built by VDO in the pipeline. So they created the tach amplifier. It is basically a minature coil triggered by the CD box tach signal, that ups the output voltage to a level that will drive the tach designed for the Kettering ignition.
For the 71 model year, they finally ran out of the tachs they had in stock, and started installing the tach that would actually trigger from the low voltage signal from the CD box ignition. They then put jumper wires in place of the tach amplifier cans.
If you look on the back of the tach for a 70 and earlier factory six, the tach connection will be marked with a "-" (negative symbol). On a 71, it will be marked with a "1". The tach marked with a "-" will not work if directly connected to the tach output from a CD box.
I know for a fact you can use any tach up through a 911 SC tach. All used a variation of the CD ignition and will work fine in a 914-6. But I took the tach out of my 70 /6, and put the guts from an SC tach inside it. I wanted it to look bone stock on the outside.