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Thread: Tach does not work with MSD in my 67 911

  1. #11
    Thank you 304065, that is the other type of "work around" I was seeking in my original post.

    I am currently expecting delivery of Bob Ashlock's "Tach Adapt" which should take care of my issue.
    Paul Abbott
    Early S Member #18
    Weber service specialist
    www.PerformanceOriented.com
    info@PerformanceOriented.com
    530.520.5816

  2. #12
    Quote Originally Posted by 304065 View Post
    Hi Paul! At last I can return the favor of technical advice.

    Here's the story with the early tachs from 1964 to November, 1970. We'll take the Kettering-ignition cars first (before there was BHKZ!)

    A 67 911 has a simple ignition circuit consisting of switched +12V to the coil 15 terminal (positive), then a wire from the coil 1 terminal (negative) to the points. The points are grounded through the distributor and from there through the transmission ground strap to the body.

    When the points are closed, the coil charges to the first time constant. When the points open, the magnetic field in the coil primary breaks down, inducing a current in the coil secondary which comes out of the coil 4 terminal (HV) to the distributor, through the rotor to the plug and to ground.

    Now, the tach in a 67 911 has a /1 terminal on the back that is connected through a black wire with violet stripe to the /1 terminal of the coil. Practically speaking, when the points open, WHAM! the tach input sees about a 200 volt dirty RF pulse on that terminal. This is enough to make it through the input circuit resistor on the tach board and turn OFF the first transistor Q1 which starts the Capacitor C2 charging and the needle starts moving. Successive pulses pin the needle in proportion to the arrival time and there you have your tach indication.

    Ok, long story longer. The MSD box outputs a 12 volt square wave with a 50% duty cycle, if my memory serves. That's not enough to trigger the early tach.

    You could do as I did, and have North Hollywood convert the tach to run on MSD (mine also runs on Kettering ignition as well because that's what I use these days, having given up exotic ignitions in favor of something I can diagnose without an oscilloscope) or use Ashlock's box which is an excellent solution.

    Or. . . you could go down to the local ACE hardware and buy yourself a doorbell transformer, and connect one end to the MSD output and to the points, and one end to ground. This would energize the transformer's coil when the MSD signal was HIGH and when it breaks down, would give you a nice dirty pulse that would probably fire the tach just fine. This is, in fact, what is hiding inside of the expensive plastic box pictured above.

    This also is a solution for 69- cars using BHKZ (CDI, for those who like that term) - the Factory and VDO didn't modify the tach to run off an 11 volt square wave until 10/70, so tachs before that use an "intermediate unit" -- the one that says "Einbau Senkrecht Klemmen Unten" on the side (great advice in more than one area of life, too). Inside those little unobtanium aluminum cans there's a coil of wire that charges up just like a . . . coil of wire. . . and fires off a signal to the tach in the same fashion.

    My apologies for the simplicity and duration of this explanation.
    Great stuff.

  3. #13
    One correction to the above, you connect one terminal of the transformer to +12v and to the black/violet to the tach, and the other to the tach output of the MSD. This will charge the doorbell coil when the MSD is LOW. Then when it goes HIGH the doorbell voltage will break down and send the pulse up the tach wire.

    What is the point of being an electrical guru if you give the wrong advice, I ask myself!

    The nice folks at MSD have actually shown this, sorta, in their diagram.
    Attached Images Attached Images  
    1966 911 #304065 Irischgruen

  4. #14
    Installed Tach-Adapt & all is OK now. Easy install...yippee!!!!!!!!!!
    Paul Abbott
    Early S Member #18
    Weber service specialist
    www.PerformanceOriented.com
    info@PerformanceOriented.com
    530.520.5816

  5. #15
    Senior Member
    Join Date
    Aug 2020
    Location
    SF Bay Area
    Posts
    234
    I too have the Tach-Adapt and it's worked well on my 69 with a 3.2. It was installed by a PO near to the tach, behind the dash.

    However, I was under the impression it could go in the engine bay, using the tach wire to send the "corrected" signal to the tach. So I moved it to the engine bay. I could not get it to work -- quite sure I wired it correctly in the engine bay.. I had to move it back to to the front, where it went back to working as before.

    Is there a reason this cannot work -- does the signal from the Tach-Adapt degrade over that distance?

    Thanks in advance...
    - 1969 911T Ossi Blue #3981

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