Tachometer and wiring

[Dcreech0]

Let me start off by thanking this community for the help they have provided so far. Every post I have made, people have provided helpful insight and solutions to all my questions….. so again thanks!!!

The car is a late 71 production date 240z. I'm next trying to tackle getting my tachometer to work. I have searched and understand that a BW wire feeds a signal from the positive side of the coil to the tach. I think I have that correct but still no tach movement. The speedo, and three small dash gauges al work as they should. Where should I look next to see if I can get my tach to work? I took a couple pictures I will attach of how I have things hooked up. There is also a long branch of two wires that splits off right under the ignition coil, and I have no clue where they go as well. The colors are solid green, and B/Ystripe. Might also be worth mentioning when I got the car, the previous owner left a cut solid blue wire with a female spade connection laying in the engine bay above the coil. 
this car has had a rough life and hasn’t been driven for 6 years or more. Just trying to get her back to a good running state as the previous owner didn’t do to many favors to wiring. Lol

[Dcreech0]

Just an update on this to see if I can get any insight….. I pulled the tach and checked around with a multi meter. I verified with the continuity setting that I have the correct b/w wire going from the tach to the negative terminal on the coil. I verified the green/white wire in the plug behind the tach goes to the ignition switch like it should. The other two wire should be 12v and ground correct? When I switch the ignition on I don’t see any voltage getting to the green (power) wire behind the tach. Anybody have any knowledge why? Maybe an odd fuse issue?

[SteveJ]

No black/white wire goes to the negative on the coil.

[Zed Head]

[Zed Head]

Nissan's 240Z ignition wiring is kind of confusing.  If the diagram is correct it's an odd parallel circuit with two power wires to the tach.  One through the resistor and one direct.  One must be Start and one must be Run. 

Anyway, they both end up as Black/white stripe at the coil positive.  Green/white stripe at the tach gets powered either through the resistor or directly.  The diagram is confusing because Run is Black/white stripe at the switch, then converts to Green/white stripe after the resistor, then is back to Black/white stripe after the tach.  So, at the tach Green/white stripe should have power during Start and Run.  If you test both and only one works you might find that the junction of the two is no longer joined.

I think the above is correct I rewrote it a few times...

[Jem2749]

Here's a really terrible pic I took of my coil with a working Tach before I stripped the engine bay. I believe this illustrates/confirms what @Zed Headwrote above. 

[SteveJ]

I'll see about shooting a short video after I'm done with work today.

[SteveJ]

I hope this clears up some confusion about which wire goes where.

 

[cgsheen1]

Good explanation of testing Steve. My usual method is the second you illustrated. With the  two B/W and the G/W wires disconnected, I turn the ignition switch to the ON position then check the two B/W wires for battery voltage. In stock config the B/W wire with voltage would attach to one side of the ballast, the G/W to the other side of the ballast, and the remaining B/W to the "+" side of the coil.

An aside to understand the circuit: The early tachometer is "current sensing" - it calculates RPM by how much current is flowing through the coil to provide spark at the plugs. (If there were no ballast resistor there would be no need for the double wire and return the tach/coil system uses.)

Power for the coil comes from the ignition switch in both the ON and START positions. Nissan (and everyone else in that era) used a ballast resistor to increase longevity of the distributor points - SO, they want the current flow to the coil to run through the resistor. Wait, we also need a tach signal so we have to take that wiring to a loop it at the back of the tach - then bring it back to the coil. So, They use the B/W (battery voltage at IGN ON) to run to the ballast resistor, the G/W brings it back under the dash to the tachometer, and the second B/W comes out of the tach loop and all the way back to the coil "+" to make sparks happen.

IGN SW B/W -> ballast -> G/W -> tachometer connector -> B/W ->  coil "+"

Edited August 18, 2021 by cgsheen1

[Dcreech0]

16 hours ago, cgsheen1 said:

Good explanation of testing Steve. My usual method is the second you illustrated. With the  two B/W and the G/W wires disconnected, I turn the ignition switch to the ON position then check the two B/W wires for battery voltage. In stock config the B/W wire with voltage would attach to one side of the ballast, the G/W to the other side of the ballast, and the remaining B/W to the "+" side of the coil.

An aside to understand the circuit: The early tachometer is "current sensing" - it calculates RPM by how much current is flowing through the coil to provide spark at the plugs. (If there were no ballast resistor there would be no need for the double wire and return the tach/coil system uses.)

Power for the coil comes from the ignition switch in both the ON and START positions. Nissan (and everyone else in that era) used a ballast resistor to increase longevity of the distributor points - SO, they want the current flow to the coil to run through the resistor. Wait, we also need a tach signal so we have to take that wiring to a loop it at the back of the tach - then bring it back to the coil. So, They use the B/W (battery voltage at IGN ON) to run to the ballast resistor, the G/W brings it back under the dash to the tachometer, and the second B/W comes out of the tach loop and all the way back to the coil "+" to make sparks happen.

IGN SW B/W -> ballast -> G/W -> tachometer connector -> B/W ->  coil "+"

Please watch and give some insight. 

[Dcreech0]

Both my black and white wires get voltage with key in the on position

[Dcreech0]

And this is just for Steve j on the video he posted. Shows that the wire I had hooked to the positive side of the coil has continuity with the green and white wire