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Your stock Tach wire was blue and attached to the coil (-) terminal.  It's in the harness that crosses the front of the radiator (goes to all the lights and etc.) and comes back into the engine bay.  Hook it up to the L28ET coil the same way.  You'll need to disconnect your stock electronic ignition module under the dash.  (At the very least, disconnect the blue wire from the module and tape it off...)

 

Otherwise, your Tach signal will come from the Ignitor output to the coil, through the resistor under the glove box (that black square-ish thing in your second picture), then to the Tach.  Just follow the wiring diagram. 

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Your stock Tach wire was blue and attached to the coil (-) terminal.  It's in the harness that crosses the front of the radiator (goes to all the lights and etc.) and comes back into the engine bay.  Hook it up to the L28ET coil the same way.  You'll need to disconnect your stock electronic ignition module under the dash.  (At the very least, disconnect the blue wire from the module and tape it off...)

 

Otherwise, your Tach signal will come from the Ignitor output to the coil, through the resistor under the glove box (that black square-ish thing in your second picture), then to the Tach.  Just follow the wiring diagram.

 

So if I'm reading this correct, connect the old coil (-) terminal along with the new one I put on? Then just disconnect the ignition module that's under the passenger side glove box?
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In the interest of keeping things tidy I'm running into this same issue. I disconnected the entire ignition module and still nothing. Kind of worried I accidentally hooked the tach (-) signal to positive on the coil or something since I ended up pulling all my wiring off, checking it, and redoing it. Not sure if that would fry it...

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I've found that different ignition systems can affect how the tach behaves.  I have a GM HEI module and my tach wouldn't work right until I put a capacitor (condenser) on the blue wire.  I'm not saying that will fix your problem, since apparently neither one of you has confirmed that the blue tach wire is actually connected to anything.  No offense intended, but that's step #1.

 

You can't do this kind of work effectively without using a meter and taking some measurements.  Since you know where the blue wire is at the module, the simplest thing to do is to measure voltage at the wire end with the engine running.  Or even with just the key on and engine stopped.  If your meter has frequency measuring capability you might even try that to see if there's something to count - but that's complicating things, so don't even think abut that yet.

 

You're both doing too much hoping.  Just get a meter and figure out what's happening.

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Else do this:  Grab a hunk of wire and run a new Tach signal wire from the (-) post of the coil to the resistor under the dash. (this can be temporary, just as a test, so you can do it "quick and dirty"...)  

 

The Tach signal MUST go through the resistor.  The resistor is always outside the dash harness under (back behind) the glove box.  It's a small square-ish thing with two wires.  (it's in one of the pictures above)  

 

CUT the blue wire (coming from the original coil and ignition module) a few inches from the resistor and splice in your new wire - you're replacing the original wiring to the resistor with your new wire.  

 

From the wiring diagrams, it looks like the '75 and '76 have two blue wires - the '77 and '78 have a blue from the coil and a Blue/White out to the Tach.  If you have a '75 or '76, you'll have to verify which of the two blue wires at the resistor goes to the Tach and what one comes from the coil/ignition module...  (In all cases, the Blue wire also goes to the stock ECU, so make sure to abandon everything but the wire going from the resistor to the Tach)

 

See if the Tach works now.

 

No?  Check the wiring from the resistor to the Tach.  (you can check the entire length of the wiring: connect you new wire to the resistor but not to the coil yet, unplug the Tach.  Use your electrical meter to check continuity from the unconnected end of your new wire to the signal wire pin on the Tach connector.)

 

You've already made sure the Tach actually has power and ground, right?  (You won't find a RED for power on the Tach connector. On the '75 - '76 it's Green and is common with the water temp / oil pressure gauge. On the '77-'78 looks like it's Blue (signal wire is Blue/White) and common with all the other gauges.  Black for Ground in all cases.  The three wires to the gauge itself are together on one side of the connector, the four pins on the other side of the connector are for lights.)

 

Still NO?

Edited by cgsheen
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  • 2 years later...

I guess I should have done a better search instead of starting a new thread. but here's a cut and paste from the thread i started. Maybe someone here can assist?

 

1978 280Z tach issue with SU carbs and GM HEI
My tach isn't working. I have confirmed that the solid blue wire is hooked up to the negative terminal on the coil and that in the passenger floorboard area the resister is hooked up to the solid blue and the blue/white wire. I currently have the hazard switch unplugged and also have the stock ignition box un hooked. How should I confirm that the tach is getting signal and power? Which colored wires that the tach plugs into the factory harness should I check to see if the tach is getting power?

Thanks

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  • 1 year later...
On 5/11/2015 at 1:53 PM, cgsheen said:

Else do this:  Grab a hunk of wire and run a new Tach signal wire from the (-) post of the coil to the resistor under the dash. (this can be temporary, just as a test, so you can do it "quick and dirty"...)  

 

The Tach signal MUST go through the resistor.  The resistor is always outside the dash harness under (back behind) the glove box.  It's a small square-ish thing with two wires.  (it's in one of the pictures above)  

 

CUT the blue wire (coming from the original coil and ignition module) a few inches from the resistor and splice in your new wire - you're replacing the original wiring to the resistor with your new wire.  

 

From the wiring diagrams, it looks like the '75 and '76 have two blue wires - the '77 and '78 have a blue from the coil and a Blue/White out to the Tach.  If you have a '75 or '76, you'll have to verify which of the two blue wires at the resistor goes to the Tach and what one comes from the coil/ignition module...  (In all cases, the Blue wire also goes to the stock ECU, so make sure to abandon everything but the wire going from the resistor to the Tach)

 

See if the Tach works now.

............................................................

This is an old thread and it saved me much heartache.  Thank you cgsheen for posting this way back in 2015.  My 78 has a solid blue wire that used to be hooked to the negative post on the coil, (I'm installing Megasquirt) so I ran a new wire as discribed above, works like a charm. 

Dave

 

Edited by D9inger
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