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Thread: Tach challenge

  1. #1

    Tach challenge

    Looking for some help. I have a 1970 911 that I installed a 3.0, now the tach doesn't work. The shop told me to install a SC tach. I don't want to install it wrong.

    The original tach went from left to right blue wire yellow stripe, black and purple and the brown was a ground. Do I hook it up the same way with the SC tach?

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  2. #2
    Just look at the symbols on the spade connectors. Blue/yellow is power and goes to the + terminal (or it may be marked 12V) Black/purple goes to terminal TD, and brown is always ground and goes to terminal marked with a -
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  3. #3
    Yes Ed power on blue yellow. Connected it that way turned key on tach went to 6 grand with out running.

  4. #4
    The problem is not the tach but, rather, the tach signal. The early ignition (3-pin CDI) is a TD signal. The later SC ignition (6-pin box) is a different tach signal.

    Have your original tach converted or use the SC tach.
    SReg. #778 OGrp: #8 RGrp: #---
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  5. #5
    Put in a SC tach because the original tach did not move

  6. #6
    But the problem is still the signal to the tach. Contact Bob Ashlock at Ashlock tech. He makes an adapter that should work for you.
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  7. #7
    Thanks Ed, I called Bob nice guy, I need to check engine harness to CDI box to see if TD wire is connected. I ordered the adapter, rather use original tach anyway.

  8. #8
    Watch yourself on that blue/yellow wire. 350V DC on it!

  9. #9
    BLUE/YELLOW is battery voltage. I received tach adapt from Bob, installed today with original tach, it works fine. Tapped in at RPM ballister on relay panel. 14 pin from engine harness TD is on #12, harness to tach is on #8. Where should the pin be located on the engine harness side? SC tach is bad, the original tach is working fine. I would like to put the TD in the original 14 pin harness.

  10. #10
    Pin 8 of the 14 pin connector on an SC engine harness is unused on the female side.
    If the tach signal on the 14 pin male side for the 70 is pin 8, (I forget right now, it is late, but trust what you are saying) move the SC engine harness tach wire to pin 8.

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