The 2-terminal electromechanical flasher's flash rate is directly dependent on the lamp current, 2-terminal electronic ones have a more constant flash rate. The electronic ones, however, start dark; ie when you switch on the turn signalr there's no signal light for a 1/2 second or so before the flashing starts... this bothers me especially in highway lane changes where I want the signal NOW. Electromechanical ones start ON.
3-terminal flashers with a ground connection have a rate effectively independent of lamp current, usually used on trucks or whatever with a lot of signal lamps. There are 3-terminal flashers that are different, they have 2 outputs for alternating outputs (or something) and do not connect to ground. They are really 2-terminal models with an extra lamp output. Avoid these!
If you want to use LED signals (or just like a constant flashing speed) the electronic grounded 3-terminal flasher is the way to go. LED lamps don't take enough power to activate an electromechanical 2-terminal flasher so they don't flash, just stay on. A 3-terminal grounded electromechanical one wastes power heating the flasher mechanism, the eelctronic ones use less juice..
