240Z Tachometer Hook up

[24OZ]

It's my understanding that the B/W wire running along the radiator subframe is what provides the + side of the coil with a 12V key on Positive power and also gives the stock tachometer the signal it needs to operate.

 

I am hooking up the wires to the relay board and following this diagram: http://www.diyautotune.com/images/cartech_articles/280zx/280zxt_relay_wiring.gif

 

Now the diagram doesn't take into account of the stock tachometer and it suggests that the Coil + should go to a fuse and then spliced into the fuel pump wire which leads to the fuel pump (Terminal 5) of the relay board, which I think must be a +12V?

 

Bit stuck as to how I should proceed now. Would it be ok to wire up the coil as per the diagram and also connect the B/W 12v+ to the + terminal of the coil also? Thereby regaining my tach signal? Or would this be wrong?

 

Thanks all.

[e_racer1999]

from what i've heard, if you are using an electronic ignition you need to use a 280Z tach. i gave up and just used an autometer 5"

[24OZ]

Thanks for the response. I forgot to mention that I'm using the VB921 to drive the coil.

 

Hope to get my stock tach working if possible!

[X64v]

To use the stock tach, don't worry about powering the coil from the relay board.

 

You should have four wires up there, a black, a black/white, a green/yellow, and a black yellow. The black wire used to connect your coil negative to the points in the dizzy, so you can ignore it completely. Take the black/white and green/yellow wires, and connect them together (these were previously connected to either side of the ballast resistor). Now take the black/yellow and connect it to coil positive. Bing, you're done, and your tach works.

 

Basically, you're not changing anything, just eliminating the ballast resistor. I actually kept mine in place while running the original 240 coil, and so I just never touched the wiring, and it worked fine.

 

Beware, I have a '73 240, so these are my wiring colors. I've found wiring diagrams that show other colors, presumably for other years.

 

IF YOU COLORS ARE DIFFERENT THAN MINE

 

With all the wires clipped (not connected to anything):

 

One has power while the ignition key is on, but no power in start. This is my black/white.

 

One has power while the ignition key is in start, but no power in on. This is my green/yellow.

 

One will not have power. This is my black/yellow.

 

Connecting two that have power will complete the loop through the tach, giving that last one power during on and start.

[z-ya]

It's my understanding that the B/W wire running along the radiator subframe is what provides the + side of the coil with a 12V key on Positive power and also gives the stock tachometer the signal it needs to operate.

 

I am hooking up the wires to the relay board and following this diagram: http://www.diyautotune.com/images/cartech_articles/280zx/280zxt_relay_wiring.gif

 

Now the diagram doesn't take into account of the stock tachometer and it suggests that the Coil + should go to a fuse and then spliced into the fuel pump wire which leads to the fuel pump (Terminal 5) of the relay board, which I think must be a +12V?

 

Bit stuck as to how I should proceed now. Would it be ok to wire up the coil as per the diagram and also connect the B/W 12v+ to the + terminal of the coil also? Thereby regaining my tach signal? Or would this be wrong?

 

Thanks all.

 

What year is the tach in your 240Z from? 70-72 are current sensing. 73' and later models are voltage sensing. If on the back of the tach there is a loop of wire going through a plastic fixture held on by one screw, then it is a current sensing tach.

[24OZ]

Thank you guys!

 

X64v -I don't have some of the wires you describe, so thanks for letting me know how to identify them, will get my volt meter out and check. Just to clarify the black/white and green/yellow go to Coil Negative?

 

Also, you advice not to use the relay board for wiring the coil but when looking at the diagram for the relay board, the wire coming from the Coil Positive goes to the fuel pump via a 10 amp fuse. So I guess without the relay board I should just connect the fuel pump positive wire directly to the coil positive wire? And where shall the 10 amp fuse go?

 

 

Z-ya- it's a UK 1973 240Z, would I have to take the rev counter out to check? And what difference does it make as to where the wires should go?

 

Thanks again!

[X64v]

Just to clarify the black/white and green/yellow go to Coil Negative?

 

No. My black/white and green/yellow are soldered together, but not connected to anything else. The only thing that will be connected to the coil negative is pin 36 (terminal 11 on the relay board), so that megasquirt can drive your coil.

 

And for the fuel pump wiring, the relay board diagram shows that terminal #5 is really for the fuel pump, and you can just power the coil off of it if you wish. Since you won't be doing that, just run terminal #5 to the fuel pump positive terminal, and you're good to go.

 

I did not know about the differences in tachs, but it sounds like it has to make a loop to and from the tach either way, so the wiring setup should work with both tachs.

[24OZ]

Thanks very much!

 

I haven't had time to check over this but I will do.

[whitechocolatelove]

so 70-72 240 tachs wont work with switching those wires?

[TooSlowToCare]

I tried to follow your guide to checking the wires since mine on my 73 240 wires are different, but i've been having strange results. I have 2 B/W wires (one is loger with sheilding around it, and has power with the key in "on" position as well as when cranking), and the other one only has power when cranking. Then I have a G/W that has power only when cranking, and i have a B wire that has no power.

 

On that same harness, but extended from it, where it goes to the temp sensor in water neck, i have a Y wire that has pulsing power with the key on, a B/Y that has power when cranking, a B with no power, and a G with no power.

 

I can't seem to match up the pulsing yellow with anything in the wiring diagram in my haynes, and i'm unclear how to connect the wire to go to my coil and give me tach signal. Can anyone help shed some light?

[Nwar]

Black/white wire from the positive terminal on your coil is the wire to your tach. If That wire is a different color it shouldnt matter. This wire is the signal it uses to determine rpm. The second wire on your tach is used to supply power when key is turn on to you coil, ballast resistor, dizzy.

[TooSlowToCare]

Black/white wire from the positive terminal on your coil is the wire to your tach. If That wire is a different color it shouldnt matter. This wire is the signal it uses to determine rpm. The second wire on your tach is used to supply power when key is turn on to you coil, ballast resistor, dizzy.

 

 

So the B/W with constant power should go from coil + to tach, and the B/W with power only when cranking and the G/W with power only when cranking are the two that should just be soldered together?

[softopz]

when you switch to aftermarket tach do your signals still work>? mine arent I guess cause the circuit goes through the tach gauge and thats where it lights up.

[ukcats07]

I'm trying to hook up my tachmatch from Brian Laine and I have the same wiring situation as TooSlowToCare.  

 

So the B/W with constant power should go from coil + to tach, and the B/W with power only when cranking and the G/W with power only when cranking are the two that should just be soldered together?

 

TooSlowToCare did this work for you?