Ever wonder how you can install an aftermarket tachometer on a vehicle with no distributor and multiple coil packs? The MSD tach driver is the answer. The compact tach driver wires inline on the coils 12 volt supply wire. It senses the current through this wire and converts this information into a 12 volt output signal that most aftermarket tachs or rpm switches use as a trigger signal. The housing measures only 1.5'' x 3.5'' x 2'' and is potted with a polyurethane compound for vibration and water protection.