Tacho-reads low after 30 minutes! 308GTBQV Euro | FerrariChat
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Tacho-reads low after 30 minutes! 308GTBQV Euro

Discussion in '308/328' started by Michael DB, Apr 22, 2024.

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  1. Michael DB

    Michael DB Karting

    Dec 22, 2023
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    Michael D Beswick
    #1 Michael DB, Apr 22, 2024
    My Tacho starts to read low after about half an hours driving. It moves (sluggishly) but has no correlation to the engine speed.
    The wiring diagram and parts diagram shows a sensor: where is it? Can it be simply tested?
     
  2. johnk...

    johnk... F1 World Champ
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    #2 johnk..., Apr 22, 2024
    Looking from left wheel well. Remove wheel line for access. But the wire wrap around the bell housing and the connector buried under hoses and such, at least on my car, and a ***** to get to.

    Not saying it's the problem, but....

    Sensor is a SEN8D. Don't buy from Ferrari. You can get then for around $30, Google search. Mounting holes(for studs) need to be elongated on new sensor with a small round filet though. A chain saw sharpening file work well.

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  3. Rifledriver

    Rifledriver Three Time F1 World Champ

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    #3 Rifledriver, Apr 22, 2024
    Those sensors control the ignition system. If one is malfunctioning motor will not run or run very badly. If there are no performance issues it is the wrong place to look.


    The problem is in the instrument. Tach failures are not terribly uncommon.
     
  4. johnk...

    johnk... F1 World Champ
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    #4 johnk..., Apr 22, 2024
    Yes, that is why I said it's of necessarily the problem.


    He could also try and swaping the connectors to the Digplex to see if the one driving the tach isn't putting out a good signal, couldn't he?
     
  5. Rifledriver

    Rifledriver Three Time F1 World Champ

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    #5 Rifledriver, Apr 22, 2024
    Yes, thats done all the time. Still think its the instrument.
     
  6. Michael DB

    Michael DB Karting

    Dec 22, 2023
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    #6 Michael DB, Apr 22, 2024
    Thanks. It runs fine so I’ll investigate tacho repair here in the UK.
     
  7. Rifledriver

    Rifledriver Three Time F1 World Champ

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    #7 Rifledriver, Apr 22, 2024
    Id go ahead and reverse digiplexs. It cant hurt to try.
     
  8. Michael DB

    Michael DB Karting

    Dec 22, 2023
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    #8 Michael DB, Apr 23, 2024
    My newly purchased workshop manual turned out to be the "Addenda" but I was kindly supplied with a Mondial version by a fellow F-chatter
    It looks like the tacho is supplied by pin 10. I'll investigate!
     
  9. Freddie328

    Freddie328 Formula Junior

    Jul 29, 2013
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    #9 Freddie328, Apr 23, 2024
    Last edited: Apr 23, 2024
    What your experiencing could be caused by a potentiometer within the instrument. It's track gets dirty and starts creating erroneous voltage signals. This has happened with others, including mine.
    Take the instrument apart, mark very carefully the position of the potentiometer arm to the track, then spray the track with some contact cleaner and with a small screwdriver rotate the arm around the track a few times to clean it. Put the arm back to where it was in relation to the track and replace instrument - job done. Here is a pic of the potentiometer....
    This is worth trying before doing anything else. it worked when my tacho started reading low.
    Here is a link to another thread on this subject:
    https://www.ferrarichat.com/forum/threads/88-mondial-tachometer.262520/
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  10. johnk...

    johnk... F1 World Champ
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    #10 johnk..., Apr 23, 2024
  11. Michael DB

    Michael DB Karting

    Dec 22, 2023
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    #11 Michael DB, Jun 10, 2024
    This is embarrassing.....where are the Digiplexes?!! I can see the coils but.....(308 GTBQV Euro)
    The workshop allegedly fixed the tacho under warranty, but it's no better. I'd like to swap the feed from the sender to the "other" Digiplex as suggested. That should confirm it's the tacho and if so I can get it rebuilt
     
  12. Rifledriver

    Rifledriver Three Time F1 World Champ

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    #12 Rifledriver, Jun 10, 2024
    In the trunk on the left side.
     
  13. Rifledriver

    Rifledriver Three Time F1 World Champ

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    #13 Rifledriver, Jun 10, 2024
    Mondial manual is the only comprehensive V8 injected manual they ever produced
     
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  14. Michael DB

    Michael DB Karting

    Dec 22, 2023
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    #14 Michael DB, Jun 10, 2024
    Thanks!
     
  15. Michael DB

    Michael DB Karting

    Dec 22, 2023
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    Michael D Beswick
    #15 Michael DB, Jun 11, 2024
    Weather precludes work....
    Meantime; I understand the output to the tach is pulsed 12vDC but can be measured using AC meter range (probably comparatively rather than actually). If the output is the same from each pin 10 of the Digiplex then that should confirm it is the tach, I think?
    I also have a period Bosch (Fluke) Automotive Oscilloscope but the manual doesn't mention testing this. Though I suppose it's just measuring voltage on a screen!

    The tach can be rebuilt but it will take 8-10 weeks so I'm inclined to wait till the end of the "season". As the workshop won't want the car sitting for that time I will need to remove the binnacle to send the tach off and keep the car at home-the workshop is 2 hours away.
     

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