tach question

[gregmankin]

I'm rebuilding a 1973 911T Pro Rally car for a buddy. I'm a novice 911 mechanic, but I've wired many a car. The car was converted to an '88 3.2 drivetrain complete with Motronic DME before we bought it. When we bought the car it had an aftermarket Mallory tach in the dash. It was not accurate. At an actual 6200 on the engine the Mallory tach read almost 8000. Using the 'K.I.S.S.' principal, we decided to install a stock '73 tach that came in a box of parts with the car. I can't get the stock tach to work.

The 911 tach has 12.51v and a good ground. When I turn on the key, I don't get any initial movement or 'bump' of the needle. Is this normal with 911 tachs?

All the wiring diagrams that I can find for a 1973 911 show a BLACK/PURPLE wire going to the tach, but the wire path is not complete on any of the diagrams, it just 'ends' on the diagram and shows circuit path "43". Did this wire originally attach to the negative side of the coil? If not, where did it attach? Given the switch to the Motronic engine controls, should it now attach to some other circuit in order to function properly?

Thanks in advance for all your help.

Greg Mankin

[304065]

Greg,

A '73 has CDI ignition and the tach is driven from the points. There are two wires connected to the side of the distributor: one goes to the speed switch and the tach, the other goes to the points input on the CDI box. The reason you don't see the whole thing is because there are two diagrams: you have to trace the black/purple to the other diagram to see where it enes up.

That being said, if you have converted to a 3,2 with Motronic, then you need a tach from a 1984-1988 911 designed to work with motronic. The signal requirements of a '73 tach and an 84-88 are very different. Chris Bennett posted the Motronic signal here: Tachometer pulses per Rev??

In any case, you should be able to sell the '73 tach and buy an '84, or if you prefer, send the tachometer to North Hollywood Speedometer or Palo Alto Speedometer, they can convert it to work with Motronic.

[Early_S_Man]

Greg,

The tach is on part I of the '73 schematic, and circuit #43 is continued on part II of the schematic. The '71 - '77 tachs are driven by the input signal to the CDI unit from the points.

If you feed the TD signal from the Motronic DME ECU to the tach input and put a 100 Ohm 5 Watt pullup resistor to +12 Volts at the tach input signal connector ... I believe the tach will work, assuming nothing internally has been harmed by connecting it to the coil + terminal in your previous work effort to get it running.

[Early_S_Man]

Greg,

You may want to build this simple tachometer calibrator to check & test the '73 tach before trying to install it. At one time you could get all of the parts from Radio Shack for about $18 ... or so one Pelican told me. I designed and built my first one 39 years ago from junk-box parts!

[gregmankin]

I'd like to thank all of you for your replies. I've learned alot.
Greg Mankin