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Thread: Another Tachometer Thread

  1. #1
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    Default Another Tachometer Thread

    I will start off with the following disclaimers:

    I know next to nothing about how tachometers are supposed to work or be wired up.
    I have read a few threads on here about tachometers. I have learned a few things, but also reinforced the awareness that I don't know much.
    I have not yet found answers to my questions...

    The basics: Bought a 1971 Z from a couple of dopes who knew nothing about the car. I am going through it trying to learn about it and fix things as I go. The car still has points in it, although it also has a pertonix flamethrower coil.

    The tachometer needle was off the pin, sitting in the bottom of the gauge. For this reason I never knew if the tach was working or not. Yesterday I pulled the tach out of the car and was able to re-install the needle. I plugged in the 4-pin connector and turned on the ignition... and the needle went all the way to the redline stop and stayed pegged there. No change when the car was running. When the key is turned off, the needle goes back to zero.

    It is an early, 4-wire tach. I do not know where the wires on the other side of the connector are supposed to go, although I understand some or all of them go the the coil and/or distributor. I have two later-style tachometers sitting around, with the multi-pin connector that handles the tach as well as the lights (I guess those are called 3-wire tachometers).

    Does the "jump-to-redline" behavior of the tach suggest a broken tach? Or bad wiring?
    Is there an easy way to hook up the tach in the engine bay with jumper wires to see if it's working properly?
    Should I, alternatively, try to use one of the later-style tachs I have sitting around? (I have a small preference for NOT doing this, as the style of mounting the gauge is different, as I am sure you all know)

    This stuff has probably been covered before, but I couldn't find it. I am not offended by links to other threads with relevant info. Thanks in advance for any help!
    Cheers,
    Christian

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    Default

    I've been studying the wiring diagram, and it seems significant that my car no longer has a ballast resistor. I've noticed that one of the wires goes there, supposedly. So, any advice on wiring the tach properly without the resistor would be most appreciated.
    Thanks!
    Christian

  3. #3
    Nova Scotia,Canada,Earth Blue's Avatar
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    Default

    Early wiring for 240z



    If you ever upgrade, here is a useful walk through
    http://www.classiczcars.com/forums/t...1-housing.html
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


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    Thanks for your reply, Blue.

    The diagram you included above is certainly the clearest, easiest to read drawing of this wiring section I have yet seen.
    However I am still a bit in the dark about how to handle the lack of a ballast resistor on my car. I'm thinking that the resistor must have been deleted on lots of other older Z's, so this question is probably not a new one, and it's probably been successfully answered before...

    Can I just bypass the resistor and connect the GW wire to the red wire?

    I should probably just start playing with the wiring on my particular car, with the aid of some jumper wires, and see if I can get something going...

    Also thanks for doing (and linking) such a good writeup on the 260/280 tach upgrade. I might end up doing that if my original one proves to be toast.

    Christian

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    Christian, check your flamethower coil, if it is a 1.5 ohm you will need a ballast resistor if you have points.
    If the coil is 3ohms then the PO should have joined the G/W wire and B/W wire that are on either side of the ballast resistor together. BUT if the coil is 3 ohms with no resistor you are going to burn through points quickly.
    You might want to search the archives for the dozens of threads on the Petronix ignition module, dead simple and most people are pleased with the reliabillity.
    1970 240Z HLS30 01955 March/70

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    Well I spent a little time playing around with the wiring and with different configurations, both with points AND with a Pertronix module. I got the car running in a few different configurations, with and without a ballast resistor. However, in all cases the tachometer went immediately to redline limit as soon as the key-switch came ON and stayed pegged there until the key went OFF.

    I suppose my next step will be to transplant the guts of one of these 260/280 tachometers into the body of the 240 tach, as described in Blue's excellent write up. I'm going to study the write up to see about "hot-wiring" the individual tachs with jumper wires to verify that at least one of them works first. Do I understand correctly from Blue's write up that the white wires on the 4-wire 240 plug are just wrapped up and not worried about?

    Thanks!
    Christian

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    Oh, and also... my flamethrower coil is 3 ohm.
    Currently set up with Pertronix (I changed it this morning) and NO resistor.
    I think that's the right way to go....

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    I "hot-wired" both of the later tachs I have sitting around, and found that one (and only one!) of them works. I looked at the date stamps and figured that one (not working) is from 1974 (probably 260Z) and the other (the one that works) is from 1978 (280Z).

    Anyway I have already swapped the internals from the working tach into the body of the 240 tach and tested it in hot-wire mode, and it works! Thanks again Blue for that good writeup. It's really a very easy operation.

    In the process of wiring up the new tach I learned that the wire which is supposed to be switched +12VDC (red/black IIRC) was not hot, no matter what I did with the wires emerging from the harness up near the coil. So who knows, that might have been the problem all along. I decided to skip the tach harness connector plug altogether and just run a 3-wire cord from the back of the tach up to the coil and do the appropriate connections there. I have not yet done that (ran out of time to work on the car for now) but I am confident it will work.

    I will update to let everyone know how it goes...

    Thanks again!
    Last edited by robox; 01-02-2014 at 02:48 PM.

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    The tach going to redline all the time is due to bad parts in the tach, not from a missing ballast or the wrong coil. They can be repaired.

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    heres what i did. i had a tach that didnt work. so i installed a blaster 2 coil. the 280zx dizzy. i forgot if it was the turbo kinda or non. i believe non. then installed the 280zx tach with the 240z face. and what do you know it started to work. Now my car is the 6/73 so it had a three wire system. i assume you had the 4 wire system. beermanpete helped me out too! great help.

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    beermanpete, do you know what components go bad in the tach? I cant find a schematic for the internals.

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    I figured mine out today it was a loose resistor inside the tach. This would make sense that some people say that temp. effects the tach. As it expands and contracts with temp change, the connections open , close or change value.

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    The parts I changed are the transistors (2 pcs, germanium, same type, NTE102A works IIRC). Notes I found on the 'net suggest C2 failure is a common problem. The Z tach is a copy of a Smith design used in some of the British cars.

    http://www.classictiger.com/techtips/motach.html

    Click image for larger version. 

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    Perfect. I will gather all the parts so that I'm ready to rebuild mine.
    Thank You

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