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Tachometers

Operation:

Electronic tachometers work by counting pulses generated by the alternator, tach signal generator, ignition system or by a magnetic pick-up sender. By choosing the correct tachometer for your application and setting the ranging switch to the right position, you let the tachometer know how many pulses are sent for each revolution of the engine. From this information, the tachometer will display the correct engine speed. The tachometer is connected to 12 or 24 VDC, Ground and to one of the signal sources previously listed. Reference the installation instructions for your tachometer for the proper connections.

Application:

On 4 cycle gas engines the tachometer sender terminal is connected to the negative terminal on the ignition coil or to a transistorized tachometer driver circuit connected to the ignition system. The tachometer ranging switch should be set to the 4, 6, or 8 cylinder position, depending on the amount of cylinders in the engine.

On 2 cycle gas engines the tachometer sender terminal is connected to the unrectified AC output of the alternator/lighting coil. Sometimes it is connected directly to the stator output wire. The tachometer-ranging switch should be set according to the amount of poles in the engines alternator. The amount of poles in the alternator can be found by checking the Outboard Tachometer Application page.

On diesel engines tachometer signal is generated by one of 3 things: 1) the alternator, 2) the magnetic sensor, which counts gear teeth, or 3) a tachometer signal generator that is spun by the mechanical take-off.

  1. The diesel alternator tachometer is hooked up the AC tap of the alternator. This connection can be marked as: AC, AUX, S, R, TACH, or nothing at all. Once installed, the tachometer can be calibrated to the engine by using a shop tach, or a known “no load” governed speed. The tachometer can be bench-calibrated by using a frequency generator and the following formula:
    Full scale frequency = full scale RPM x ½ poles in the alternator x pulley ratio
    60

    There are 2 versions of the alternator tachometer the 300-900 Hz model and the 700-2100 Hz model. By using the above formula you can choose the correct tachometer for your application.

  2. The magnetic sensor tachometer is hooked up to a hall-effect sensor that counts gear teeth as they pass by the sensor. Both wires from the sensor are routed to the tachometer as a twisted pair. One wire is connected to the signal input and the other to the ground terminal on the back of the tachometer. The ranging switch code for the amount of teeth on the wheel can be found in your tachometer’s installation instructions.

  3. The generator driven tachometer is connected to a pulse generator, which is spun by the engine’s mechanical take-off. One of the wires is connected to chassis ground and the other is routed to the signal input of the tachometer. The switch for this tachometer is marked as .5:1, 1:1, 1.5:1 and 2:1. Set the selector switch to the position that corresponds to the mechanical take-off ratio of your vehicle.

Troubleshooting:

Symptom recognition and visual inspection are the steps in effective troubleshooting. A multimeter is a helpful tool when troubleshooting instrumentation, and will make the task much easier. Tachometers usually exhibit the following symptoms: 1. Dead, 2. Reading high or low, 3. Erratic.

  1. Dead - this is usually caused by no power applied or by no signal applied. First, with the installation instructions, make sure the tachometer is connected properly. If the tachometer is connected properly, check the voltage between the “B+” and “G” terminals on the tachometer. 12-volt systems will have a voltage between 12-16 VDC; 24-volt systems will have a voltage between 22-28 VDC. If these voltages are not present check the ground and ignition connections to the tachometer and the continuity of the wires making these connections. If the voltages are present, check for the presence of a signal. Measure the signal between the “SIG” and “G” terminals on the tachometer. Signal strength should be in excess of 2 VDC.

  2. Reading high or low
  3. - This is usually caused by the ranging switch on the tachometer set in the wrong position or the wrong tachometer is being used for your application. Check to be sure the switch is in the correct position for your engine.

  4. Erratic
  5. - This is usually caused by a loose or intermittent connection on the back of the tachometer. Check to see if the connections are tight and that the terminal hardware is making contact with the wires’ conductor. In the case of the magnetic pick-up tachometer, erratic action can be caused by the wrong air-gap between the sensor and the gear teeth. A .050” air-gap is what Beede Electrical Instruments recommends. To achieve a .050” air-gap, turn the sensor into the Housing until it touches the gear teeth and then back the sensor out ¼ turn.

If after you have performed these troubleshooting steps the tachometer is still not working, please log onto our Online Support Forum for further technical assistance.