It is generated by a magnet and pick-up in the speedo, 240,260s did not have the pick-ups, I had to install my own. It is a 2p pulse, two pulses per rotation. The magnet is located in the speedo case and is what causes your speedo needle to move. Rip out the the pick-up from a 280 and find a spot to put it near the magnet, hook up a meter on continueity and spin the speedo cable. You should get 2 beeps per rotation. Hook 12 volts to one wire and run the other to your sorce,ie timer boost controler.