Headlights causing false Tachometer indication | FerrariChat

Headlights causing false Tachometer indication

Discussion in 'Mondial' started by Louis Brown, Oct 17, 2020.

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  1. Louis Brown

    Louis Brown Karting

    Dec 24, 2019
    83
    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
     
  2. Incaico

    Incaico Karting

    Jun 13, 2020
    69
    SoCal
    Full Name:
    JR 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
     
  3. donkost

    donkost Formula Junior
    Silver Subscribed

    Dec 1, 2016
    912
    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.
     
  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,040
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    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?
     
  5. JLF

    JLF Formula 3
    Silver Subscribed

    Sep 8, 2009
    1,635
    You don’t have the little pole sticking out of the tach to adjust brightness?
     
  6. Incaico

    Incaico Karting

    Jun 13, 2020
    69
    SoCal
    Full Name:
    JR Sandoval
    Dinner adjustment is there. Just makes gauges dimmer but did not affect the jumping.


    Sent from my iPhone using FerrariChat
     
  7. Louis Brown

    Louis Brown Karting

    Dec 24, 2019
    83
    Richardson Texas
    Full Name:
    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.
     
  8. Louis Brown

    Louis Brown Karting

    Dec 24, 2019
    83
    Richardson Texas
    Full Name:
    Louis Brown
    I do have the pole but never messed with it, assuming it was the odometer trip reset like in other cars! Thanks
     
  9. Incaico

    Incaico Karting

    Jun 13, 2020
    69
    SoCal
    Full Name:
    JR Sandoval
    Same on mine.
     
  10. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,040
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    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.)
     
  11. Louis Brown

    Louis Brown Karting

    Dec 24, 2019
    83
    Richardson Texas
    Full Name:
    Louis Brown
    Sure Thanks

    1989 T Cabriolet US version
    Last 6 digits vin: 0082519
     
  12. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,040
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    I can tell you what to check from looking at the schematic as a likely cause for this tach problem, but, unfortunately, I can't tell you where it is physically located (most of the useful spatial information that used to exist on the schematics up to the mid-80s has been lost on the later schematics). The electrical architecture of the tach and its illuminating bulbs strongly suggests that it is a ground problem so first things I would check would be:

    1. The connection of the N 0.8 wire at the "B" connector of the 70092 common ground block, and

    2. The ground connection between the 70092 common ground block and the chassis.


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    Based on the location of 70092 on the schematic, I would guess that it might be located on the floor of the passenger footwell, but could be in the forward floor section of the central tunnel or even on the Driver's side or anywhere ;). Good Hunting!
     
  13. Louis Brown

    Louis Brown Karting

    Dec 24, 2019
    83
    Richardson Texas
    Full Name:
    Louis Brown
    Alright then! Thank you I’ll begin the hunt this evening and report any positive findings.
     
  14. Cafe racer

    Cafe racer Rookie

    Mar 22, 2020
    24
    London
    Full Name:
    Nigel Waterfall
    My 90 T does a similar thing when the lights are on, adjusting the dash brightness will change some of the readings - it's on my list of things to do, but my guess is that its a grounding issue in the dash itself.
    The 12v from the lights is going through the instruments to ground which gives the false readings.
    I had to add a ground to my oil temperature sensor to get that to work as there wasn’t one fitted, this may be a similar thing that it’s not a bad ground, you just need to fit an extra ground wire to the lighting circuit in the dash
     
  15. GordonC

    GordonC F1 Rookie
    Owner Rossa Subscribed

    Aug 28, 2005
    4,119
    Calgary, AB, Canada
    Full Name:
    Gordon
    Not a Mondial, but my 84 308 QV exhibited similar needle jumps when turning on the park or headlights - about 2 needle widths on fuel, coolant temp, and oil pressure. I would gain 1/8 tank, 20*F, and 20 psi! I just did a cleanup project last week to address this, and now have zero movement of the gauge needles when I turn on the lights.

    I removed the instrument panel, and with emery board or sandpaper cleaned the whitish corrosion off of every single male blade connector in the 3 main multi-connectors for the panel harness; in the separate speedo and tach multi-connectors, on each of the wires on the back of each of the gauges; and on each of the light bulb blade connectors. After cleaning, I applied a very light coat of Stabilant-22A electrical contact enhancer (https://www.amazon.com/Standard-Motor-Products-SL5-Stabilant/dp/B000O3N0U0/ref=sr_1_1?dchild=1&keywords=stabilant+22a&qid=1603981146&sr=8-1) to every wire connection point. The little 15ml bottle of 22a costs around $80, but it goes a long way, and it's very effective - I'd dare to suggest it's like a budget version of Dave Helms' gold contact kits, and as such it's a genuine bargain!

    I also bypassed the dimmer (just needed a wire with two male blade connectors to jump between the two female harness connectors that connect on the dimmer), and replaced all the lights with LEDs - that brightened up the panel significantly so it's actually visible and not just like a glow worm.

    Along with cleaning and Stabilant-22a on all the connections, I also added a second ground wire - all the gauge and lighting ground wires are on a single chain through one wire into one of the 3 main panel connectors, so I tapped one of the wires at the opposite end of the chain, and ran a 14 ga wire to an unused bolt under the dashboard.

    End result, the instrument panel is now readable at night, and the gauge needles don't move at all when turning the lights on.
     
    afterburner, ronfrohock and Incaico like this.
  16. Louis Brown

    Louis Brown Karting

    Dec 24, 2019
    83
    Richardson Texas
    Full Name:
    Louis Brown
    Wow. Great explanation! I’ve got the S-22a so going in headfirst.
     
    Incaico likes this.
  17. caconavajas

    caconavajas Rookie

    Nov 18, 2021
    2
    Full Name:
    Antonio navajas gonzalez
    I own a mondial T from 91, I had the exact same problem. The solution add a new ground from tach to chassis. SOLVED,!! Now all readings perfect! Look for bad ground to dash, or in my case better add a new reinforcement ground to tach ground!,
     
    JLF likes this.
  18. theunissenguido

    theunissenguido Formula 3
    Owner Silver Subscribed

    Jan 21, 2004
    2,371
    Argent/Brasil
    Full Name:
    Guido
  19. Cafe racer

    Cafe racer Rookie

    Mar 22, 2020
    24
    London
    Full Name:
    Nigel Waterfall
    My T is currently in for its Major service & the technician suspects a grounding issue with the headlights causing my Tachometer issue as the fog lights come on when the headlights are on (I hadn’t noticed that)

    As i said in an earlier post all my dash instruments would change readings with the lights on, there has been a lot of work done to the electrical system on the car (not good work either)

    I’m fairly sure that it’s a bad ground somewhere & because the headlights seem to be the main problem that’s where I will start
     
  20. afterburner

    afterburner F1 Rookie
    Silver Subscribed

    Jun 20, 2008
    3,197
    Hong Kong
    Since the all the instruments and the light switch are daisy-chain grounded - wouldn't it be logical to isolate the light switch ground from the others and see if that cures the issue?
     
    Cafe racer likes this.

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