RCGF 21CC-Twin Operator's Manual page 17

Many of the RCGF 8.4 V Ignition modules have an additional lead to plug into the Digital
Tachometer. If your 6.V old ignition module does not have this additional lead, the Digital
Tachometer can still be used. Simply use the Y-harness (included with the Digital
Tachometer) to connect to the pick-up lead from the engine. Be sure to secure all
connections and to secure the Digital Tachometer or leads on the aircraft used.
RCGF 8.4 V Ignition modules use the Digital Tachometer