Tachometer doesn't work

Discussion in 'The Projects Forum' started by Jack_K, Feb 26, 2014.

  1. Jack_K

    Thread Starter Senior Member

    May 13, 2009
    I have a tachometer from a boat with a V8 engine. When power is applied the needle spins and stops at the 6 o'clock position and will never move.
    I'll attach the schematic and explain what I have done so far. I'll also attach the data sheet on the IC.
    1. I measured the resistance of both meter coils at 200 ohms each (to common)
    2. I replaced the LM1819 with no change in operation
    3. I measured all the resistors and diodes
    4. Using a signal generator connected to the 'coil' input I see pulses on pin 9, R8-9, and to the top of the .0068 cap. Nothing on pin 5, 6,or 8.
    5. I replaced the .0068 cap with no change noted
    6. 8.5 volts on pin 11, 13.8 volts on pin 13, and 5.1 volts on pin 1.
    7. I forget which, but one coil of the tach meter has 0.0 volts and the other has 10 volts. Common has 5.1 volts.
    8. The 1N4007 on the 13.8 volt input gets hot, so does the 20 ohm resistor. I'm guessing tach meter is probably bad?
    Last edited: Feb 26, 2014
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    If it is fixing to one position is sounds like the chip has a short in the sine or cosine driver FET, which is running that meter coil at full current.

    That would fix it's needle position and cause the overcurrent situation where the 13.8v input diode and resistor get hot.

    If you measure the voltage across the 20 ohm resistor you can calculate the current the entire device is drawing;
    I = E/R
    I = measured_volts / 20 ohms