Printable Version of Topic

Posted by: Peashooter Apr 4 2019, 01:16 PM

Hi All,
I was out driving the six yesterday and the tach quit working. It came back on for a bit, then quit again. Before I tear into things, I thought I would ask here on the board to see if there were some quick checks to make. When I turn the key off, the tach needle jumps up, but that is all it is doing. Granted, the car is going on 50 and the tach has never been worked on, so it may just need to be repaired. I did check the relay board to see that nothing was loose there, but haven't pulled the tach out yet.
Bill

Posted by: ConeDodger Apr 4 2019, 01:17 PM

Ground...

Posted by: sb914 Apr 4 2019, 01:28 PM

From your title ,I thought someone died. Glad it’s only you’re tac!

Posted by: 914Sixer Apr 4 2019, 02:50 PM

Ballast resistor for tach is on the relay board in engine compartment. It may have went South. Resistor is in tach on later 911 models. I think it is called engine speed control in diagram. Tach can be modified to run with out relay or use later 911 tach with 914-6 face. Not sure about modification details, check with expert.


Attached image(s)

Posted by: ClayPerrine Apr 5 2019, 04:21 PM

It is probably the tach adapter on the relay board. It's the round can (primary control unit in the picture). Find a 71 914-6 or 911 tach. They don't use the tach adapter, just a jumper wire. I took the guts out of an 911 SC tach and put it in the 914-6 housing, and opened up the can that went on the relay board to put the jumper wire inside it. Now it looks stock for a 70, and the tach adapter can't fail any more.

BTW.. the tach adapter is NLA, and the odds of finding one are slim. They are the same as a 65 though 70 911, and the concours guys will pay big money for them in working condition.


Clay

Posted by: Peashooter Apr 7 2019, 07:29 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?

Posted by: ClayPerrine Apr 7 2019, 08:10 AM

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.

Posted by: Peashooter Jul 12 2019, 09:18 AM

Old thread, but I wonder if anyone knows the value of the resistor? Mine has overheated and is darkened to the point where you can't read the bands. I can measure it, but doubt if the value is still the same at this point.