If your tachometer doesn't read correctly after installing an MSD, you may need a Tach Adapter. On GM vehicles, first remove the tach filter. There are two MSD Tach Adapters. The chart below displays the most common Tach Adapter options:
On the chart above, the trigger wire on tachometers that are marked None may be connedted to the Tach Output Terminal on the MSD 6 Series Ignition Unit using the supplied Female Faston Receptacle.
Note #1: There are basically two types of tachometers: Current Triggered Tachometers and Voltage Triggered Tachometers. These type of tachometer that is on the vehicle may be recognized by the way it is connected to the ignition coil. The drawings below show two wiring diagrams to help in determining which type of tachometer is on the vehicle.
Note #2: These auto manufacturers use tachometers made by different tachometer manufacturers. Some will work when connected to the MSD 6 Series Ignition Unit Tach Output Terminal. Otherse may require an MSD Tach Adapter. Connect the tachometer trigger wire to the Tach Output Terminal as shown below to determine if the tachometer is operating correctly. If it does not operate correctly, go back to the Chart to determine which MSD Tach Adapter should be used.
Tachometer Trigger Wire - This is the wire that went from the Tachometer to the Negative side of the coil. Remove the end of the wire that is connected to the coil and connect it to the MSD Tach Output Terminal as shown:
Note #3: General Motors Corporation vehicles have an inline filter that should be bypassed when the factory tachometer drops back to zero as the engine RPM is going up. The drawings at the right show what the filter might look like. Located the filter by tracing the wire from the Tach Terminal on Vehicles equipped with an HEI Ignition system. On vehicles equipped without an HEI, trace the wire from the coil negative terminal until the filter is found. Disconnect both wires from the filter and leave disconnected. Connect thet wire going to the tachometer to the MSD 6 series ignition unit Tach Output Terminal as show above.
Note #4: If the Tachometer is connected to the positive side of the ignition coil, do not attempt to connect this tachometer to the tach ouput terminal on the MSD 6 series unit.
See Note #1 for a diagram of a current triggered tach. The tachometer is connected to the positive side of the ignition coil. The ballast resistor or resistance wire can be on either side of the coil.