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Modifying 280Z Tach To Work With 5V Square Wave Input


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#1 Neuby

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Posted 19 March 2017 - 02:11 PM

Hi all.

 

I wanted to return my tacho from my autometer 5” to a stock functionality with the least fuss.  This mod should work for any Nissan/Datsun direct input style (NOT induction) tachometer reading from a 5v square wave tach input eg. aftermarket EFI, CDI ignition module.

 

In the context of this write up I am using 280z tach internals in a 240z housing and the signal is generated from a MSD 6AL.  I haven’t included dissembly/assembly of the housing in this write up, just mods to the PCB.

 

In the case of the 280z PCB, the input is a 12v feed and typically reduced to around 5V peak internally by resistors.  Putting a 5V source through this will reduce the voltage even further (Ohm's law) and will not be recognised.  All you have to do is locate this internal resistor and remove/bridge it out of the input signal line.  In my case I tinned up from 8g wire and bridged out the resistor.  Make sure there are no other forms of resistance between the tach peg and signal source and you should be right to roll.

 

There was a potentiometer in my tacho which was set to half way and I did not need to adjust.

 

If someone has covered this before please delete.  I had a quick search through the forums and couldn't spot anything previous.

 

Resistor.PNG

Bridge.PNG

Verification.PNG

 

 


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#2 Six_Shooter

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Posted 20 March 2017 - 12:54 PM

Yep, this has been found before, though maybe not published on this site, or maybe just not for a long time.

I have however found one issue with using the OEM 280Z tach with a couple different aftermarket, or more specifically non coil connected sources. One of those being a (USDM) GM V6 DIS ignition module, another being the tacho output function of a Megasquirt.

The problem is that once RPM rises to around 5000 RPM, the tach will either peg briefly and fall to zero, or just fall to zero, and will remain there for as long as the engine RPM is above approximately 5000 RPM.

The issue is that the tach outputs of many aftermarket or other more modern devices is a 50% DC signal, and at lower RPM this is fine, because the high time is short relative to the entire pulse width. What I had to do was make an intermediate circuit based on a 555 timer that kept the high time below about 30% DC all the way up to much higher than teh tach will actually read. To be more accurate it's not limiting the DC, but the time that the signal is high. To remain below 30% DC at higher RPM, the signal had to be very short, only a few milliseconds IIRC, and that's what I set the output high time for. The timer gets re-triggered every time the pulse happens, in other words, every time there is an ignition event. Now my tach will work to well above 5000 RPM without issue, in fact it will work fine well above the 8000 RPM max read of the 280Z tach.

The potentiometer is to adjust the tach to read correctly.


Edited by Six_Shooter, 20 March 2017 - 12:56 PM.


#3 Neuby

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Posted 20 March 2017 - 09:36 PM

That is interesting to know RE breakdown over 5k rpm. 

 

I will have to perform a stress test on the weekend when I get a chance to get some heat into the motor  ;D .



#4 Neuby

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Posted 23 March 2017 - 11:00 PM

I can confirm the tacho with this mod performs accurately up to my redline of 6.5k when used with the MSD 6AL.    ;)


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#5 Howdy

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Posted 24 March 2017 - 08:10 AM

Good info, thanks Guys I wish I knew that years ago.

Just out of interest do you know how to make a tach adapter to drive an aftermarket tacho like Autometer from dual coils? I need to free up an output on my ECU for boost control and I use Mitsubishi VR4 dual coils (waste spark) on my FJ20 (4 cyl) so if I connect the tach input to one coil it will read half the RPM and I can't find a 2 cyl tacho that suits. I'm currently using the ECU to drive the tacho.



#6 Six_Shooter

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Posted 24 March 2017 - 11:23 AM

What ECU?

Also Sometimes using a pair of diodes to grab the signal from both coils works, sometimes it doesn't.

Interesting that your tach works up to 6500 RPM, I have a couple here that haven't with a 50% DC tach signal. *shrug*



#7 Howdy

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Posted 24 March 2017 - 09:13 PM

It's a Motec M4 Pro, I'm using one output for the tach. Do you think this layout would work?



#8 gav240z

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Posted 24 March 2017 - 10:39 PM

I can confirm the tacho with this mod performs accurately up to my redline of 6.5k when used with the MSD 6AL.    ;)


What is the rest of your ignition system running? Do you have a CAS or trigger wheel? Are you able to adjust the calibration of the tach - say for example you changed the design of the tach itself (spacing between numbers / delimiters etc.)?

Apologies if it's a little off topic but I've got an idea and just trying to suss out how it may work.

#9 Neuby

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Posted 25 March 2017 - 12:12 AM

What is the rest of your ignition system running? Do you have a CAS or trigger wheel? Are you able to adjust the calibration of the tach - say for example you changed the design of the tach itself (spacing between numbers / delimiters etc.)?

Apologies if it's a little off topic but I've got an idea and just trying to suss out how it may work.

 

The MSD gets its trigger pertronix unit in the dizzy.

 

I'm not familiar with calibrating the 280z unit to a different scale.  I had an EA Falcon fitted and calibrated to that tacho face.  If you look in the verification photo you can see the holes drilled to fix the face to the unit closer to the needle.  No how much the tech tried he could not get it to align with the scale 100%.  It ended up costing the same amount as the Autometer guage I had in the dash prior to refitting the 280Z unit.

 

 

Interesting that your tach works up to 6500 RPM, I have a couple here that haven't with a 50% DC tach signal. *shrug*

 

 

I think the signal the MSD unit puts out is pretty clean and consistent 5V square wave.






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