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Tach

Discussion in 'Technical Q&A' started by LUCH, Aug 4, 2004.

  1. LUCH

    LUCH Rookie

    Nov 26, 2003
    30
    Massachusetts
    Full Name:
    Karl
    Hi all,

    I haven't been on chat for quite some time now, but it's always the first place I come to for advice because of the vast array of knowledge here. Here's my question...I have an 86 328 with @ 30k mi. on it and just last night the tach started reading half the rpms. I assume there is a sensor on each distributor and is combined and displayed by the tach. I'm thinking maybe a pick-up is dirty or bad. Is anyone familiar with or have seen this before? The car runs great otherwise. Any help would be greatly appreciated.

    thanks,
    LUCH
     
  2. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    2,655
    Boulder, CO
    Full Name:
    Mike
  3. wmnewland

    wmnewland Karting

    Aug 3, 2001
    77
    Manhattan Beach, Cal
    Full Name:
    Newland, William
    I have an 1986 328 GTS and after replacing the sensor and rechecking all the connections the Tach read about half the RPM's and the speedometer read about twice the MPH. Since I knew my car wasn't that hot I sent both instruments off to Palo Alto Speedometer.

    PA Speedometer did a great job, for a very fair price. Turn around time was less than a week, and now both instruments are dead on. They said it was a chip that goes bad on a lot of those instruments.
     
  4. DGS

    DGS Three Time F1 World Champ
    Rossa Subscribed

    May 27, 2003
    39,247
    MidTN
    Full Name:
    DGS
    The 328s have two sensors on the flywheel: one for RPM, one for TDC (2 pulse per rev). These are fed to the Marelli Multiplex ignition computer, which sends the tach an RPM signal. A separate sensor, driven by the gearbox oil pump (on the output shaft) sends a speed pulse train to the speedo.

    If the sensors (or computer) is bad, the engine wouldn't be running "great". It might be a driver chip in the computer, but it's more likely the electronics in the tach instrument itself, as described in the other messages, above.
     
  5. LUCH

    LUCH Rookie

    Nov 26, 2003
    30
    Massachusetts
    Full Name:
    Karl
    Thanks for all the replys. I'll check it all out.
     
  6. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    17,058
    USA
    Full Name:
    Hugh G. Rection
    Yup, remove the tach and send it off to a speedometer repair shop, of your choice. Palo Alto Speedometer has a lot of experience with this. For some reason, ALL the 3.2 motor tachometers fail....apparently there is some component(s?) that go bad...Anyway, mine was fixed by PAS about 1.5 years ago....still working fine.

    Dave
     

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