Some time I can shake all the wiring harness and connectors and the RPM guage works again

NORMALLY YOUR TACHOMETER'S SIGNAL COMES FROM THE "NEGATIVE SIDE OF THE COIL"

NOT ONLY RECONNECT CONNECTORS, BUT WITH THEM DISCONNECTED, INSURE THE CONTACTS HAVE NOT "BEEN SHOVED REARWARD" IN THEIR CONNECTOR, AND DO NOT CONNECT PROPERLY. DIELECTRIC GREASE NEVER HURTS, ON CONNECTIONS EITHER!

IF YOU STILL CANNOT FIND THE PROBLEM, LET ME KNOW, I'LL TRY TO GET YOU A WIRE DIAGRAM. TO DO SO, I NEED MORE SPECIFIC INFO ON YOUR RIG AND ENGINE SIZE

THE MEDIC