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Thread: 71 911 Tach not working

  1. 04-29-2016, 07:00 AM #1
    Rusty
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    71 911 Tach not working

    Getting closer to a test drive, but have run into a few issues. This one is the tach is not working. I have checked voltage to the tach, checked ground and signal to the tach (black/purple wire). Assumed it must be the tach, had a spare from my 72S, nothing. Will get another tach to test today, but is there a way to diagnose the tach or does it just need to go to one of the companies in CA? Any help appreciated.
    Rusty
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  2. 04-29-2016, 07:08 AM #2
    304065
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    Depends on the date on the back of your 71 tach, if it's earlier than 10.70 then it needs a 24 volt square wave, otherwise it's an 11 volt square wave.

    To create those pulses you would need a function generator. If you had a function generator you would know you would need one, so you must not have one. And if you don't have one, you probably don't want to buy one, unless you are an electronics freak (-Gordon Gekko, 1986) like me.

    If you fall short of that description and want to try your hand at fabricating something, our departed friend Warren Hall (RIP) posted this simple circuit.



    It's reproduced more fully here: http://forums.pelicanparts.com/porsc...ting-tach.html

    You can check the voltage to the tach, should be (no surprise) +12v on the +12 pin. How did you check the signal to the tach? With a multimeter with a tach feature? With an oscillocope?

    Here's the thing, if your tach was made before 10/70, then testing it with a 72 tach won't work.

    Post up a photo of your engine electrical console and we can sort it out. Good luck!
    1966 911 #304065 Irischgruen
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  3. 04-30-2016, 05:51 AM #3
    Rusty
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    A friend brought an S tach to try, 73 vintage, works great. Will send mine out for refurbishing.
    Thanks,
    Rusty
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