Alternator/Tacho Query

Discussion in 'Diesel Engines' started by darrenyorston, Nov 24, 2006.

  1. darrenyorston
    Joined: Sep 2006
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    Location: Sydney, Australia

    darrenyorston Junior Member

    I think either my tacho has given up the ghost or the alternator is not outputting correctly. Is there a way I can test the output from the tacho post on the alternator to see where the problem is coming from. There seems to be no pattern to the gauge. When djsting the throttle or turning on some electrical component the gauge seems to move randomly to a new number.
     
  2. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Dirty ground.

    But remember a tach driven by an alt has almost no repeatable accuracy.
    The belt temp, glazing , alt load and engine room temp all will change the reading.

    FAST FRED
     
  3. darrenyorston
    Joined: Sep 2006
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    Location: Sydney, Australia

    darrenyorston Junior Member

    What other options are there for a reliable tacho?
     
  4. Frosty

    Frosty Previous Member

    Alt tachs are not accurate at all. There is an adjustable screw on the back that allows you to adjust the tach to the nearest settings. Obviously it is alt speed. They are just good enought for a repeater situation where calibration of say 2 engines was more important than actuall RPM.

    You can then make a mental calibration from your known good ones.

    As for your problem,-- if the adjustable screw is not making a good contact this would give the simptoms you describe
     
  5. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    For a gas engine some car makers offere tachs driven from the distributor.

    For a diesel a tach pulse sender can be installed to a cable tach drive.

    Ot the pulse sender can go on an aux power pad.

    FAST FRED
     
  6. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Have a look at tinytach.com. They make an accurate, economical diesel tach that senses the pulsation in a high pressure fuel line. No magnets, no moving parts, simple and effective. You can calibrate your tach with a phototach, but as mentioned above, an alternator tach can be fickle for a number of reasons. Fred's mention of dirty ground is where I'd start.
     
  7. Carioca
    Joined: Aug 2005
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    Location: BRAZIL

    Carioca Junior Member

    kapnD,

    Looked at Tiny Tach, yes. Gret t face value.But...

    What is the accuracy ?

    Attach it over any one of the injection-lines, between injection pump and cylinder nozzle ?

    How do you adjust for differing number of cylinders, 2-stroke/4-stroke etc. ?
     
  8. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    The sensor is attached to any convenient high pressure fuel line between the pump and the injector. Since each cylinder fires one time per revolution of the motor whether 2 or 4 stroke, the sensor reads one fuel pulse per revolution. Much simpler and straightforward than the belt driven alternator sender, or the other options. I installed one in my engine room along with a set of mechanical engine guages and a start/kill swith. The first time I ran it, I compared it to the tach at the controls and found quite a lot of discrepancy in the mid to high rpm range. This worried me, so I borrowed a photo tach to compare to, and found the tiny tach to be spot on, while the instrument in the bridge panel was incorrect. The only drawback I see is that the tiny tach has a non replacable battery that lasts only about 5 yrs.
     
  9. Carioca
    Joined: Aug 2005
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    Carioca Junior Member

    Spot on kapnD !

    At 65 bucks a piece and a battery lifetime of 5 years, the alternator sender option with its attendant drawbacks is definitely worse
     
  10. Capt. Mike
    Joined: Feb 2006
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    Capt. Mike Junior Member

    According to the manufacturer's description the diesel version of Tiny Tach uses either a replaceable 9v battery or the ship's power.
     

  11. Carioca
    Joined: Aug 2005
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    Location: BRAZIL

    Carioca Junior Member

    Capt Mike,
    You are right. A 9 V external battery (or boat battery) is required to supply the power when the diesel engine is turned on.

    An internal - and unreplaceable - lithium battery supplies the power to the instrument´s microprocessor and digital display at all times.

    The former power source would seem to address the needs of the (proprietary) transducer and associated signal processing electronics.

    The internal lithium power source probably has a low-power consumption mode when the diesel engine is turned off, which keeps the digital display operational.
     
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