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Tachometer reads high at idle then goes down on revving!

Discussion in 'Mondial' started by Capri335, Apr 13, 2019.

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  1. Capri335

    Capri335 Rookie

    Jun 18, 2015
    23
    Perth, Australia
    Full Name:
    John
    Hi Guys
    On Sunday I went racing and had to pull out because I thought that my engine was over-heating. I should have realised that it wasn't at 250F, but anyway I was giving it a bit of a flogging and immediately before that I noticed two stutters in the engine. Anyway, I found that the sender wire for the temp gauge had touched the water pump and gone to the core (which is + and therefore live). I have fixed this, but my tachometer is reading all over the place. On start-up it goes straight to 4-5000 rpm. If I rev the engine the tacho reading falls and then climbs again. The engine is happy and running well. I don't even know whether these two things are related, as I am sure that the tacho failed to read correctly before the temp gauge went up full. I think that the 2 misses were as a result of the live wire touching the engine though. Any suggestions? I feel that it may be a blown resistor. But the falling reading on revving has me miffed!
     
  2. mike32

    mike32 F1 Rookie

    May 13, 2016
    2,788
    Uk
    If i remember correctly the pick up is somewhere on the clutch housing, if some knows better than me. Also the plug on the sensor might need a clean or not fitting correctly. I recall a prob with mine not reading but cannot recall exactly
     
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  3. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,159
    Texas
    Full Name:
    Steve Magnusson
    The wire going to the temp sensor is a ground wire (not a live wire). Having it short to ground just makes the temp gauge go to maximum with no ill effects on anything else.

    If your engine ignition system is still working correctly, and the engine runs well, that basically absolves all of the sensors from fault and most of the Motronic ECU ignition function. The mondial t schematic indicates the tach signal comes from one of the Motronic ECUs so I'd put most suspicion on that Motronic ECU or the tachometer itself. The mondial t SPC indicates that the two Motronic ECUs are the same part so maybe you could try swapping those for a test (although they'd need to relearn their calibrations IIRC to remain swapped). Good Hunting!
     
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  4. Capri335

    Capri335 Rookie

    Jun 18, 2015
    23
    Perth, Australia
    Full Name:
    John
    The wire that dropped down the front and wore part way through is a constant live from the battery (cooling fan feed). We've done checks on the alternator and it isn't that. It is still charging OK (14.5v). So I think that the live wire shorting onto the engine caused the stutter as no voltage was left to run the engine momentarily (split second). The tacho has had a "feed back" through the harness and stuffed it up. The oil pressure gauge also isn't reading as high as it was (70 cold) and I also have the brake warning light on (in the tacho). So it's out with the dash to see what we can find there. I am fairly sure that the problems lie there.
     
  5. paulchua

    paulchua Cat Herder
    Moderator Owner

    Jul 1, 2013
    7,825
    Menlo Park, CA
    Full Name:
    Paul Chua
    Hi John, the issue you described happen to me a few years ago. Although not sure in your case, it turned out to be the tach itself for me. Eventually the tach itself failed. Good news is if it is the tach, there are many and they are not too expensive. I've seen for about $150 USD, which I think was about the same cost as the repair, if not cheaper.

    I would check that too, they are easy to remove.
     
  6. mike32

    mike32 F1 Rookie

    May 13, 2016
    2,788
    Uk
    I saw one being tested on the tv. They connected an ociliscope to the tacho signal cable on the back of the rev counter- this one was picking up pulses rather than voltage, so when the engine was reved up the pulse rate increased. If that does not work then try mearsuring volts
     
  7. Capri335

    Capri335 Rookie

    Jun 18, 2015
    23
    Perth, Australia
    Full Name:
    John
    I'll be removing and taking to a specialist repairer. He told me he can test if for me while I wait, if it needs repair about $150 aussie max. Sounds OK. Does anyone know what the potentiometer does on the side of the Tacho? Is is supposed to be a trim pot for calibration? Listed in the parts wiring diagram as a reostat.
     
  8. Capri335

    Capri335 Rookie

    Jun 18, 2015
    23
    Perth, Australia
    Full Name:
    John
    I've just realised that this must the instrument lamp dimmer. I haven't driven the car at night much and it's still pretty new to me.
     
  9. paulchua

    paulchua Cat Herder
    Moderator Owner

    Jul 1, 2013
    7,825
    Menlo Park, CA
    Full Name:
    Paul Chua
  10. Capri335

    Capri335 Rookie

    Jun 18, 2015
    23
    Perth, Australia
    Full Name:
    John
    SOLVED. It was the alternator. The tacho was good. We did lots of 2nd guessing, swapped the ECU's over and had a really strange reading when testing the tacho signal with Hz on the multimeter, the tacho worked fine. So we measured the Ohms on the mutlimeter with another and put a 100 ohm resistor in line... nothing! The Tacho behaved the same. I even took the Tacho apart and desoldered a suspect resistor, it measured ok and so put it back. We were dumfounded. I said to my mate who was helping me, "I still think that it's the alternator" Right lets pull all the wires off, was the reply, stuff me the tacho read correct straight away. So it seems that this is how it went wrong: the live (positive) touched the engine, and shorted the diodes out on the alternator. This then sent an erroneous voltage to the tacho guage. The fact that I got no ignition light except when the engine was cranking was the big indicator. So now wait until everyone comes back from Easter break, and get the spare parts. Thanks all for your input.
     
  11. Alden

    Alden F1 Rookie
    Owner

    Apr 25, 2010
    2,708
    Central Florida
    Glad you found the problem!

    For future reference, if your tach is reading more or less than true RPM, check post #12+ in this thread:
    https://www.ferrarichat.com/forum/threads/88-mondial-tachometer.262520/
    There is an adjustment potentiometer on the tach itself than can get corroded and cause errors.

    Mine had started reading 1/2 the true RPM's, this fix solved the problem.
    Alden
     
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