If the e6k is anything like the 8860 tach output, it applies/switches ground, not B+. You can get it to read properly by wiring it up as the 8860 would be wired up to factory tach.
Wire it up to an output set to "Tach out" on the e6k. After that you have to put ignition switched power to the factory tach wire threw a 2.2kohm resistor.
The power will run threw the resistor into the stock tach, when the ground is applied via EMS, its an easier path then threw the tach circuitry and brings the tach wire to 0 volts.
This makes the circuit essentially switched power as opposed to switched ground and should make the stock tach function correctly.
WARNING: FIGURE OUT HOW THE TACH OUTPUT CIRCUIT WORKS AND VERIFY YOUR WIRING BEFORE TURNING ON POWER. IF YOU WIRE IT WRONG YOU COULD/WILL DAMAGE ELECTRICAL COMPONENTS.
IF you cant hook it up threw the EMS, You might need an MSD tach adaptor.