Standard Ignition Coil
If you have a standard ignition coil you can use the negative terminal of the coil for the signal source. Use the SIG 2 (In-Dash Tach) or green wire (Pedestal Mount Tach) input (see "Signal Hookup" in the instructions). If you have a high output capacitive energy discharge type coil it is recommended that you install a tachometer filter to prevent damage to the tachometer.
If you have an ignition box (i.e. MSD 6AL) you can use the "Tach Output" from the ignition box. Use the SIG 1 (In-Dash Tach) or purple wire (Pedestal Mount Tach) input (see "Signal Hookup" in the instructions).
12V ECU Tachometer Signal
If you have a modern ignition system and your ECU (computer) has a dedicated 12V tachometer signal output you can use this to drive the tachometer. Use the SIG 1 (In-Dash Tach) or purple wire (Pedestal Mount Tach) input (see "Signal Hookup" in the instructions).
5V ECU Tachometer Signal
Many ECU outputs are low voltage (4-5V) which will not drive the tachometer without modification. If you have a low voltage (4-5V) tachometer signal you can install a 680 ohm "pull up" resister from the 12V gauge power wire (ACC) to the signal wire to increase the signal strength. Connect one end of the resister to the 12V ACC wire and the other end of the resister to the signal input wire. A 680 ohm resister will work for signals in the 4V range. A higher value resister (i.e. 1000 ohm) can be used if your signal has a higher strength (i.e. 5-6V). NOTE: Make sure your tachometer has a common ground with the signal source (ECU).
If you have an HEI distributor with a "Tach Output" you can use the "Tach Output" from the distributor. Due to the high voltage of the signal from the HEI you should use a tachometer filter to prevent damage to the tachometer.
Coil Pack or Individual Coils (Distributorless Ignitions)
If you have a modern vehicle with "coil pack" or "individual ignition coils" distributorless igntion you will need a tach driver like the MSD 8913 to drive the tachometer. Connect the signal from the tach driver to SIG 1 (In-Dash Tach) or purple wire (Pedestal Mount Tach) input.
If your tachometer has no signal or an erratic signal (pointer jumping all over) you first need to verify that you have correct power, ground and signal connections.
- Verify the gauge has 12V power. The gauge should perform a self calibration check when turned on (pointer moves to full scale, then back to zero).
- Verify you have a good common ground. A bad ground connection is often the cause of many gauge issues. Make sure the gauge ground connection is connected directly to a good ground on the vehicle chassis. We recommend using the same location as the battery negative terminal ground.
- Verify you are using a proper signal source. The most common locations for a tach signal are the negative terminal of the ignition coil or tachometer output terminal (HEI, Ignition Control Boxes or ECU).
No Pointer Movement
- Verify 12V ACC, signal and ground connections (see above).
- Verify you have a good signal that is at least 8V peak to peak in amplitude (requires an oscilloscope). Some ECU signals are not strong enough to drive the tachometer.
- Try switching the signal input. Marshall tachometers come with 2 signal input options.
Erratic Pointer Movement
- Verify correct signal and ground connections (see above).
- Verify your signal wire is not routed close to any high voltage sources (i.e. spark plug wires, distributor). The signal wire can pick up electrical interference (signal noise) if routed close to a high voltage source.
- Try switching the signal input. Marshall tachometers come with 2 signal input options, each input utilizes a different signal filter.
- Install a tachometer signal filter.
- Add a 10K Ohm resistor inline with the signal. Adding a resistor will increase the level of signal filtering. Try this on both signal inputs. Use a higher ohm resistor for more filtering, a lower value ohm resistor for less filtering. /ul>
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