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Tach housings and connections?

This is a discussion on Tach housings and connections? within the Electrical (S30) forums, part of the 1st Generation Z (S30) category; I have read that the housings can be swapped between a 240 tach and a 280 tach so that the ...


  1. #1
    Zup
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    Default Tach housings and connections?

    I have read that the housings can be swapped between a 240 tach and a 280 tach so that the 280 guts will fit properly in the dash of a 240Z.
    I have noticed that the round silver tach needle assembly on the 280 tach is larger than the one on the 240.
    Can these be swapped as well so that it will appear the same as the one on the 240 speedo?
    Seems someone suggested using a dinner fork and some delicate persuasion.

    Also---I have attached pics of the rear of the tachs. To connect the 280 tach to the harness can I cut into the black and red tagged loop harness of the 240 tach shown and attach a female bullet connector so that it will clip onto the single pin of the 280 tach and still retain the white connector for the 240z dash harness? Would this work properly?


    Finally---is the 240 tach a three wire or a four wire and are both of these tachs negative trip type?

    While I searched and read to get this far, I am still uncertain as to what exactly will work and what I am working with.
    Many thanks for your assistance!
    Attached Thumbnails Attached Thumbnails Tach housings and connections?-240-tach.jpg   Tach housings and connections?-280-tach.jpg  

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    Registered User EScanlon's Avatar
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    The tach with the wiring still attached is a "4 wire" tach. Note the white wire that "LOOPS" around the screw on the right hand side of the case. This white wire has two connections in the connector visible there. These connections are to the Black/White and Green/White wires that go to the coil. It is through these that the tach senses the discharge at the coil.

    It is these two wires when NOT connected that cause problems with the ignition, and give rise to the myth that the car will not run without the tach IN the circuitry.

    The other two wires are the ones that provide the power for the tach circuitry.

    The second tach looks identical to the first but with the wiring removed.

    2
    Enrique

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    Registered User 72SLOZ's Avatar
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    Zup,

    Any resolution on this?

    I would like to swap in a newer tach into my 240z as I have converted to a MSD 6a ignition system.

    Somewhere else I have read that all one has to do is 'remove' the inductive loop on the 240 tach and connect directly to the MSD tach output signal...presumably to the stud that the inductive loop is fixed to.

    any insight?
    72SLOZ
    HLS3051628

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    Semi-retired admin Arne's Avatar
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    May I ask why you want to change? Many people have used a 240Z tach with the MSD 6a.
    Arne - Former owner, HLS30-37705, 7/71, 905 Red
    Car blogs - 240Z - Porsche 911

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    Actually I would prefer not to swap...

    Some of the solutions posted refer to using a chrysler ballast acroos the coil...while others refer to an adapter (8910) from MSD. Also I am not sure which solutions apply to '3 wire' tachs and which apply to '4 wire tachs' (like mine. 3-wire tachs originally hooked up to the (-) term on the coil; whereas 4 wire connected to the (+) terminal.

    Options:

    1) figure out how to make my 4-wire tach work
    2) swap out for a newer 3-wire model
    3) convert to a Autometer tach (least desired)
    72SLOZ
    HLS3051628

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    Looking at the tach with out the wiring is identical to a three wire tach I have from a 74 260. Wiring according to 74 FSM ,top screw post (green) to 10 amp fuse, bottom screw (black)to ground.The stud connection to coil negative.There is a resister in this line between tach and coil .Looking at a MSD wire connection diagram use coil positive to MSD red wire,MSD orange to coil positive,MSD black to coil negative. The MSD white connects to dizzy Your 4 wire tach should be ok as in the ignition feed to coil positive

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    Registered User 72SLOZ's Avatar
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    Beezee,

    Thanks for your post. My tach is like the picture with the wiring harness attached. The picture on the right is apparantly a 280z tach and is NOT the same tach as the one on the left. They look similar though...

    Question is: Are the 240Z tach internals the same once you remove the white wire inductive loop that is looped around the stud on right?

    Also: Is the resistor you refer to the original ballast and if not/what value is it? hopefully the FSM indicates this.

    Thanks for your help.
    72SLOZ
    HLS3051628

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    Registered User beezee's Avatar
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    Looking at a wiring diagram in the Haynes manual, key switch terminal 5 green-blue early S-30,s and green-white, later 240,s from the ignition switch to the tach induction loop ,then black-white to the coil + Terminal 5 is powered only in key start position.Terminal 2 black-white powers fuse box ignition in key run . Across wires 2 BW and 5 GB or GW is shown a resister,providing reduced coil voltage such as a ballast resister,when the key is on. Looking at the tach operating on the coil positive induction circuit I would guess the MSD + tach should work with out any modifications to tach wiring.I have a 74 260 with MSD ,the tach operates on the coil negative. It has a ballast resister at the coil and MSD #8910 Tach adapter for points dizzy There is an in line resister in the main harness between tach and coil negative located where the harness passes through the firewall.Ichecked resistance at just under 2.2 k ohms.My tach looks like the 280 tach . Hope this helps Rick

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    My 4/73 tach worked fine with the simple addition of a 280ZX dizzy. It wasn't until I added the MSD unit that I needed the 8910 Tach adapter. It may just have been the way I wired it, but I tried doing without it a few different ways. In the end I gave in a wired the adapter in. Tach works fine now.

    Here's a write up I did awhile back.
    What the heck!!!!

    And another with an image I used as a general reference. See my notes on on this image in the link above.
    MSD 6A Installation with 280zx distributor and 240Z tach
    =Enigma=
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    4/73 - HLS30-156236

    My name is Adam and I'm a Zeeoholic!

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    Registered User 72SLOZ's Avatar
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    I was able to resolve my tach connection by going the Chrysler dual ballast route across the trigger and switched power from the tach. With a 280ZX distributor setup to fire the MSD, I was able to bypass the stock ballast and remove the condensor (no points in this setup!).

    My setup:
    280ZX disty with E12-80 ignition box
    MSD 6A
    MSD Blaster 2 Coil
    Stock 1972 Tachometer (Current Driven)
    Removed condensor
    Removed stock Ballast
    Chrysler dual ballast (5 Ohm side used)

    Short across original stock ballast resistor (optional, but this ballast resistor is not needed for the 280ZX disty setup) This means connecting the green/white wire and black and white wire at the ballast resistor together. Note the black and white wire at the ballast is NOT the same as the black and white wire originally hooked up to the coil positive...)

    280ZX disty wired to MSD:
    Brown wire from E12-80 to MSD 6a red, one side of Chrysler 5 Ohm ballast resistor, original black/white wire from tach
    Blue wire from E12-80 to MSD 6a white trigger and other side of 5 Ohm ballast resistor

    MSD 6A Black wire to coil (-) and MSD 6A Orange wire to (+) of coil. These should be the ONLY wires connected to the coil, per the MSD manual.

    You're good to go...assuming that you have also hooked up the power and ground to the MSD box per the instructions. I found mounting it along the wheel wheel under the coil to be the most convenient, which meant extending the power lead (use a larger guage wire to extend the power lead...). I also mounted the chrysler dual ballast right above the MSD in that location. This makes doing all of the hookup at the coil and disty much cleaner at the tradeoff of have a bit longer power lead...but I found no issues with the performance of the unit this way.

    I will post a schematic of the above to help.
    72SLOZ
    HLS3051628

  11. #11
    Registered User 72SLOZ's Avatar
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    Schematic for hookup:
    Attached Files Attached Files
    72SLOZ
    HLS3051628

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