Tachometer won't work Now with Pictures
 

I searched the forum and found some pretty helpful stuff, but nothing directly helpful in my situation.

I got the 74 911 running with the help of several members of these forums, Thanks All! But for some reason the tach does not work. I traced the trigger wire dirctly from the TD input on the tach to the points on the distributor, but the remainder of the wiring has me a bit confused. Ihave the 3 pin CD box on my ignition system, I believe that is the correct unit.

The wiring at the coil and dizzy are as follows:

Points contact outside of dizzy (distributor) is connected to the tach wire, engine block, and CDI Unit.

One side of the coil goes straigh to ground, the other to the CDI unit.

I have 12 at the tack 12v input and a good ground on the tach "-" input.

The thing I'm not so sure about is the points terminal on the dizzy going to ground.

I also get 6.2v at the tach drive wire directly at the dizzy and at the tach contact, doesn't seem to be enough to trigger the tach. are the points supposed to go straight to ground?

Thanks

Lee

First off, a tach with a "TD" terminal is not a 2,7, it's for a 911SC. TD is intended for six pin CD boxes, not three pin. A three pin should have a "C" or "1" in the tacho drive terminal.

The tachometer expects an 11 volt square wave, you can only measure this with an oscilloscope. If you use a DC meter you probably get around 6v, it's a 63% "mark" duty cycle wave.

I don't understand "points terminal on the dizzy going to ground." Are you saying that you have a ground wire from the engine case to the distributor and to the CDI case? That is good. Even better if it's a braided wire, that means it is original. And then you should have another wire connected to the distributor 1/4" faston connection that runs forward to the tachometer.

A photo would help.

Here is the distibutor. Where the 2 wires come off the black and purple is the tach and the brown goes straigt to the block and tees off to the Ignition Box

http://forums.pelicanparts.com/uploa...1214340571.jpg


Here is the Coil: Side closest to me goes straight to ground, the other goes to the Ignition Box:

http://forums.pelicanparts.com/uploa...1214340634.jpg

Here is the Ignition Box 3 wire:


http://forums.pelicanparts.com/uploa...1214340682.jpg


Here is the tach, it has a TD 12v and - input. The tach wire (td) goes straight to the dizzy

http://forums.pelicanparts.com/uploa...1214340750.jpg


Thanks for responding I hope these pictures are helpful

OK, you don't have a Bosch CDI, you have a permatune, but the pins are the same.

There is no reason why a ground wire should be connected to the points. Are you SURE it goes to the engine case?

The way it's supposed to work is, one wire goes to the tach, the other goes to the trigger input for the Perma-tune.

Can you trace the wires coming off the distributor? One black/purple, that's good, where does it go EXACTLY? The other one, where does it go?

Diagnosis is much harder because someone has hacked the wiring over the years, all those extra wires and whatnot. But if you have patience it can be sorted out.

Thanks John,

I cut the wire from the dist to the block and it didn't effect the running of the car. But it didn't help the tach either. The black and purple wire dissapears into the original wire loom and re-appears at the thach out of the original loom. The other wire on the dist goes to the ignition box. Does the tach look right for the car as far as the connections go? I tried tapping it and the needle will move when I bumpthe tach but it doesn't respond to rpms. it just lays there.

Any other thoughts?

Thanks

Pretty sure I figured it out with your help. Wrong Tach. I had a friend with a 73 Targa come by and hooked it to his tach and it worked fine. So guess i'm looking for a good early tach.

Thanks again