Okay, Ive gotten somewhere.
With the black/white loop, one end is in the ignition barrel, and the other is on the positive (I think) of the ceramic (resistor?) stuck to the side of the coil.
The engine now runs with the tacho wired up, and works to show idle speed and a little above it, but when revved more than about 1800rpm the needle starts to flicker and wont go any higher, when engine is going flat out the needle just flaps between 0rpm and 2000rpm.
Any ideas?

The + side is constant, put a test light on and run the engine. you will see that the light will stay on. - side it will flash. The only way it would work, that I can see, is if the coil is wired in back to front. The points should always be wired to the - side too otherwise the you can expect a smaller than usual spark KV.

That's what I thought as well. But after looking at the wiring diagram and everyone's comments (including some confirming that it does work) ... been thinking. Note that even on the + side the wire will alternate between current flow and no current flow as the points open and close, so there is a signal.

Our resident auto-electrician phunkdoctaspock says it works off induction, which is why it's a direct wire (only a loop of wire goes into the tacho).

Now, without trying to offend anyone, I believe that the wiring Diagram is incorrect, you wont get a pulse signal (which is required for a Tacho to work) from the + side of the coil. All tachos are wired on the negative side. Hence the White/Black wire. the tacho is wired in series on the 1200, if you wire it in parallel the car wont start, because that will just earth the points (effectively how a kill switch works) Hope this helps.

Here's the diagram for stock 1200 tacho:



Tacho wiring diagram

The two signal wires are connected in series to the coil ballast resistor + side (not directly to the coil) , at least according to my wiring diagrams. These wires are both black/white. One goes to IGN fuse, the other to the ballast resistor + side. In other words, the tachometer is between the IGN switch and the resistor + side. Don't wire it in between the START wire and coil + or it won't start.

The other two wires should be for the light:
black - ground.
red/yellow - light power

green/white loop - connect to the black/white IGN wires.

My 1975 B210 wiring diagram also shows the B210 tach connected to a 'black/white' wire.

On the other hand, most tachs are not connected to the coil + wire. The "signal" is on the coil - side -- it signals both the coil and the tachometer. My Toyota Celica tach is wired this way to the A12, and the B310 elec. ignition even has a special tach wire wired into the coil '-' wire harness. The aftermarket tachometers I've used are also this way.

there should be one trigger wire, with two ends, this loops through the RPM counter, disconnect the negative of the coil and connect wire to one end of trigger wire, the remaining end goes to the negative on the coil.

Thanks for the advice, the wiring diagram was helpful, although it says to connect the wire to the positive terminal on the coil, but it only worked on the negative terminal.
Now I have another problem, when the tacho is plugged into the coil, there is not enough current to make the engine fire. Is there a way to get around this?
In response to having a round tach in a square hole, I am planning to remove the clock and mount it behind the hole, and only a small area will be obscured.

Tyrie

I cant speak about the lat model 120Y's but most of them had square instruments, while 1200's had round ones, so switching a 1200 Tach into a 120Y doesn't sound like a practical deal.

My 120Y owners manual clearly shows that a tach was an option & it fits to the left of the speedo. There may be a clock in this position.
The wiring diagram in this owners manual also shows how it all hooks up.
Try finding a 120Y coupe in a wrecker & buy the instrument cluster. From there, you can either swap the whole thing, or fit the tach. This way, you end up with two Datsuns with factory Tach's

do a search of 'fitting coupe tacho into ute' and a diagram and some threads should come up.
Should be able to work it out from there

Henry

G'day,
I'm new to this site and i own a '76 120Y and a '71 1200 coupe.
The coupe isn't running at the moment and i want to put the tacho into my 120Y
Can anyone tell me where to hook up all the wires.
There's a red/yellow one and a black one that were plugged into the back of the speedo, and a green/white one looped so its got two ends.