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Headlights causing false Tachometer indication

Discussion in 'Mondial' started by Louis Brown, Oct 17, 2020 at 11:46 PM.

  1. Louis Brown

    Louis Brown Karting
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    Dec 24, 2019
    61
    Richardson Texas
    Full Name:
    Louis Brown
    Anyone ever have their tachometer jump up to a higher constant (erroneous) rpm reading at idle once headlights are turned on?!!!
    My dash instrumentation lighting is very dim as well with no known rheostat to adjust to brighter. Sounds like I have a poor ground somewhere. Any known history with any of this? Thanks
     
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  3. Incaico

    Incaico Karting

    Jun 13, 2020
    50
    SoCal
    Full Name:
    Juan Rafael Sandoval
    happens in my ‘89 as well. It affect oil pressure gauge also

    Have not found an answer. Maybe someone will let us know?..


    Sent from my iPhone using FerrariChat
     
  4. donkost

    donkost Karting
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    Dec 1, 2016
    156
    Blandon, PA
    Consider it part of the Italian charm? Sorry, not too helpful, but my '85 QV Cab does the headlight thing as well. I really only ever drive it during the day so it isn't an issue for me.
     
  5. Steve Magnusson

    Steve Magnusson F1 World Champ
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    Jan 11, 2001
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    Steve Magnusson
    +1 -- but what is the magnitude? Your use of the word "jump" implies it's quite large. I'd say it's "normal" for the gauges to move up maybe 1/2~1 needle width higher when the parking/headlight are turned on (but I haven't noted any tach effect). Do your gauges also move up just slightly or a lot when you turn on the lights?
     
  6. JLF

    JLF Formula 3
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    Sep 8, 2009
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    Dallas
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    JERRY
    You don’t have the little pole sticking out of the tach to adjust brightness?
     
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  8. Incaico

    Incaico Karting

    Jun 13, 2020
    50
    SoCal
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    Juan Rafael Sandoval
    Dinner adjustment is there. Just makes gauges dimmer but did not affect the jumping.


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  9. Louis Brown

    Louis Brown Karting
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    Dec 24, 2019
    61
    Richardson Texas
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    Louis Brown
    With headlights to on, my Tach jumps +1000rpm and stays there +/- 1-2mm movement. The car does not change idle to 2000rpm so its clearly some sort of short. I haven’t noticed any gauge movement but will check when able.
     
  10. Louis Brown

    Louis Brown Karting
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    Dec 24, 2019
    61
    Richardson Texas
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    Louis Brown
    I do have the pole but never messed with it, assuming it was the odometer trip reset like in other cars! Thanks
     
  11. Incaico

    Incaico Karting

    Jun 13, 2020
    50
    SoCal
    Full Name:
    Juan Rafael Sandoval
    Same on mine.
     
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  13. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,342
    Texas
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    Steve Magnusson
    Just checked mine again today, and get a small movement of the gauges (maybe 1/2~1 needle width) but no change in the tach at idle at all. Can you be more specific about what model, year, and version Mondial that you have? (I'll have a peek at the corresponding wiring diagram.)
     
  14. Louis Brown

    Louis Brown Karting
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    Dec 24, 2019
    61
    Richardson Texas
    Full Name:
    Louis Brown
    Sure Thanks

    1989 T Cabriolet US version
    Last 6 digits vin: 0082519
     

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