I am working on installing a set of Speed Hut gauges in my 240z (my "track car"). Manufactured in 12/70, the wiring for the tach doesn't match the wiring diagrams I see online. Other than power and ground and illumination wires, the Speed Hut tachometer has a single yellow wire that is supposed to be hooked up to the "coil -".

It seems to me, after looking at the stock wiring, I have to not only figure out which wire is "coil -", but I think I may have to connect some of the other stock wires together in order to "bypass" the stock tach.

Color legend is:
blue = L
yellow = Y
Black = B
Red = R
White = W
Green = G

Ignoring the wires for gauge illumination, I am looking specifically at a 4 prong connector for the stock tach. Here is what I am looking at... on the left are four wires from the harness that are in the 4 prong connector, and on the right are the four wires coming from the tach that are in the 4 prong connector:

Harness wiring	Description	Connection	Tachometer wiring	Description
RL	red with blue stripe	----	YR	yellow with red stripe
GW	green with white stripe	----	W	white (with black band on it)
B	black	----	B	black
BW	black with white stripe	----	W	white (with red band on it)

Additionally, I am running a Crane HI6 (multiple spark discharge) ignition which also has tachometer adapter hooked into the circuit. The ignition and adapter were previously in use and worked perfectly with the stock tach. The wiring for the ignition and adapter has been untouched since the car ran.

What I need help with:

1. Which wire is the negative from the coil?

2. Which, if any, wires do I have to jumper together with the stock tachometer no longer in place?

My guesses are:

1. Black - connect the Speed Hut yellow wire to the black wire in the 4 prong connector

2. Connect (jump) the green with white stripe wire and the black with white strip wire (to bypass the stock tachometer)

But, I am not sure. And don't know what to do with the last of the four wires (red with blue stripe at the four prong connector).

Edited Sunday at 04:58 PM2 days by inline6

Give me a call

15 hours ago, inline6 said:

I am working on installing a set of Speed Hut gauges in my 240z (my "track car"). Manufactured in 12/70, the wiring for the tach doesn't match the wiring diagrams I see online. Other than power and ground and illumination wires, the Speed Hut tachometer has a single yellow wire that is supposed to be hooked up to the "coil -".

It seems to me, after looking at the stock wiring, I have to not only figure out which wire is "coil -", but I think I may have to connect some of the other stock wires together in order to "bypass" the stock tach.
...
Ignoring the wires for gauge illumination, I am looking specifically at a 4 prong connector for the stock tach. Here is what I am looking at... on the left are four wires from the harness that are in the 4 prong connector, and on the right are the four wires coming from the tach that are in the 4 prong connector:

Harness wiring	Description	Connection	Tachometer wiring	Description
RL	red with blue stripe	----	YR	yellow with red stripe
GW	green with white stripe	----	W	white (with black band on it)
B	black	----	B	black
BW	black with white stripe	----	W	white (with red band on it)

Additionally, I am running a Crane HI6 (multiple spark discharge) ignition which also has tachometer adapter hooked into the circuit. The ignition and adapter were previously in use and worked perfectly with the stock tach. The wiring for the ignition and adapter has been untouched since the car ran.

What I need help with:

1. Which wire is the negative from the coil?

2. Which, if any, wires do I have to jumper together with the stock tachometer no longer in place?

My guesses are:

1. Black - connect the Speed Hut yellow wire to the black wire in the 4 prong connector

2. Connect (jump) the green with white stripe wire and the black with white strip wire (to bypass the stock tachometer)

But, I am not sure. And don't know what to do with the last of the four wires (red with blue stripe at the four prong connector

(this is for anyone else interested in a little information about some specifics that you raise:)

1. NONE of the above are "Coil - ". (Stock config = Coil "-" is connected to the points output of the distributor. So, new wire from the Coil "-" to your new gauge...(it will be a 2nd wire connection as the Coil "-" needs a "signal to fire" from points or an electronic ignition unit as well - your new gauge is just tapping into that signal))

2. You don't necessarily need to jumper any of the wires in that connector, but you need to understand the coil wiring to decide whether to JUMP or just ABANDON the G/W and B/W in the connector pictured above. The stock circuit goes like this: IGN SWITCH -> B/W -> Ballast Resistor -> G/W -> Tach connector -> Loop on back of Tach -> Tach connector -> B/W -> Coil "+". The coil gets power at IGN ON through this and the Tach "senses" the flow of electricity to the coil. If you're replacing the stock Tach, the re-route of the B/W back to the Tach is not necessary - the B/W and G/W to that connector can simply be ignored (abandoned). SO - the B/W that goes to the Ballast can be connected to the Coil "+" - either through the Ballast (by removing the G/W and running a short wire from there to the Coil "+") OR bypassing the Ballast and connecting the "ballast's" B/W directly to the Coil "+" instead - IF you no longer need a Ballast Resistor in the circuit.

note: I think it's better to abandon and get all that extra wire (and added resistance) OUT of the power circuit to the coil...

BTW, the other two wires in that connector: Black - is a Battery Ground. R/L is gauge lighting - it's listed in the wiring schematic as "IL". So Black is the power ground for the Tach - the Tach gets power (battery voltage) from a Green in another connector.

Edited Sunday at 05:39 PM2 days by cgsheen1

Thanks for the information.

On 12/7/2025 at 12:31 PM, cgsheen1 said:

The stock circuit goes like this: IGN SWITCH -> B/W -> Ballast Resistor -> G/W -> Tach connector -> Loop on back of Tach -> Tach connector -> B/W -> Coil "+". The coil gets power at IGN ON through this and the Tach "senses" the flow of electricity to the coil. If you're replacing the stock Tach, the re-route of the B/W back to the Tach is not necessary - the B/W and G/W to that connector can simply be ignored (abandoned). SO - the B/W that goes to the Ballast can be connected to the Coil "+" - either through the Ballast (by removing the G/W and running a short wire from there to the Coil "+") OR bypassing the Ballast and connecting the "ballast's" B/W directly to the Coil "+" instead - IF you no longer need a Ballast Resistor in the circuit.

So, let me know if my rewording here is accurate: there is a B/W wire going from the ignition switch (routed through the factory wiring harness) to the Ballast Resistor. On the other side of the Ballast Resistor, there is a G/W wire (which again is routed through the factory wiring harness) to the 4 prong tach connector in the dashboard. This is the "re-route" to the back of the Tach (not the B/W back to the Tach). At that connector, the G/W wire connects to one side of the loop in the Tach (white wire with black band which connects in some way with the white wire with the red band - thus the loop). Continuing... in the 4 prong connector, the white wire with the red band connects to another B/W wire. This B/W is again, routed through the factory wiring harness and is connected to Coil "+".

So... I am not using the ballast resistor any longer. I have a CraneHi6 aftermarket ignition and a high voltage coil (for mulitiple spart discharge requirements of the ignition).

So, if I take the B/W wire that was connected to the ballast resistor, and connect it directly to the Coil "+", I am simply moving the B/W wire which is coming from the ignition and moving it to the coil "+"... directly. Doing so frees up both the B/W and the G/W wires (which already are routed through the factory harness).

Question: could I not just attach either of those freed up wires to coil "-"... and then connect whichever of those I choose to the single yellow wire for the Speed Hut gauge? At least that way, I don't have to route a stand alone wire... and I can repurpose one of the now "extra" wires already present.

On 12/7/2025 at 12:31 PM, cgsheen1 said:

BTW, the other two wires in that connector: Black - is a Battery Ground. R/L is gauge lighting - it's listed in the wiring schematic as "IL". So Black is the power ground for the Tach - the Tach gets power (battery voltage) from a Green in another connector.

My 12/70 Tach connector (4 prong) definitely has a RL wire in it (I edited my post and added a picture of the connector). That wire is "supposed to be" green according to the wiring diagrams, but it isn't. No doubt, this is a difference when comparing "earlier" 240z's to later ones. I know the gauge lighting (IL = illumination?) is RL as well (both in the wiring diagram and in my car). So, yeah... that certainly added to my confusion.

Edited yesterday at 02:44 AM1 day by inline6

To combine what we talked about on the phone with the guidance from @cgsheen1, here is the stock wiring.

This is essentially what the modified wiring would be.

On 12/6/2025 at 5:15 PM, inline6 said:

the Speed Hut tachometer has a single yellow wire that is supposed to be hooked up to the "coil -".

It's a voltage triggered tachometer. Like those in the 280Z's. The yellow wire can, and should be, completely separate from any of your existing wiring. To run the Speedhut tach.

Seems like your real question might be "what do I do with the wires from the old tachometer" to keep things working correctly. If you're replacing the stock tach.

If you're keeping the stock tach, you could just leave it untouched, and wire in the new tach using the instructions. It won't affect the ignition system, since it just senses the voltage changes at the coil negative terminal.

Just offering a reinterpretation of the original question.

https://speedhut.com/content/instructions/2_and_2_58_tach.pdf

7 hours ago, Zed Head said:

The yellow wire can, and should be, completely separate from any of your existing wiring to run the Speedhut tach.

Well, I certainly could run a new, and completely separate wire from the coil "-" through the engine bay, through the firewall, and to the existing yellow wire to run the Speed Hut tach.  But, the existing B/W and G/W wires basically already routed in the stock wiring harness from, and to where I need to go, are made completely redundant by doing so.  Why not repurpose one of them and save the time and effort to run another wire?

Looking at @SteveJ's wiring diagram that shows after modifications, I see that he chose the B/W that originally went from the coil "+" to the stock tach 4 prong connector as the wire to repurpose for use with the Speed Hut gauge.  Of the two original wires (B/W and G/W), only the G/W wire has no purpose after this modification.

I've got some other wiring that I need to figure out how to route as well.  For example, I need to wire in a coolant temperature sensor and harness for dual electric fans, and the wiring for oil pressure, oil temperature and coolant temperature senders.  So, I will be thinking about how best to route those through the engine compartment and where necessary, to the gauges in the dashboard.  

I am set for information I needed to wire up the tach - thanks to all who responded!

3 minutes ago, inline6 said:

Well, I certainly could run a new, and completely separate wire from the coil "-" through the engine bay, through the firewall, and to the existing yellow wire to run the Speed Hut tach.  But, the existing B/W and G/W wires basically already routed in the stock wiring harness from, and to where I need to go, are made completely redundant by doing so.  Why not repurpose one of them and save the time and effort to run another wire?

Very good idea with one caveat - the stock wiring is "50 how many" years old and could be pretty oxidized (engine bay). My OCD would be "new gauge... OLD wire? Naw...". Your mileage may vary.

13 minutes ago, inline6 said:

Looking at @SteveJ's wiring diagram that shows after modifications, I see that he chose the B/W that originally went from the coil "+" to the stock tach 4 prong connector as the wire to repurpose for use with the Speed Hut gauge.  Of the two original wires (B/W and G/W), only the G/W wire has no purpose after this modification.

But if you look at the diagram, the G/W gets Battery Voltage in the START position of the IGN Switch. (The circuit provides a bypass of the Ballast Resistor when cranking the engine for start - once started the juice goes through the Ballast Resistor for normal running operation) SO - don't leave the G/W dangling where it could cause a short...

could have done that in one post - but I'm old...

Edited 1 hour ago1 hr by cgsheen1

I was describing its function, not the actual material wire. The trigger wire for the new tach is its own independent circuit.

I was also trying to tease out if you're replacing the stock tach or adding another tach. Are you sticking the new tach in the hole where the old one is?

Your other basic question might be "how do I keep my ignition system functioning if I remove my factory tach?". Since they are tied together in the 240Z's. Just breaking the "how do I connect my new tach?" question in to its elements. Seems like you're on top of it. Good luck.