circuit: adjustable negative or low pulse

Hello, I'm trying to duplicate negative pulses from a motorcycle speedometer's reed switch. This low pulse was roughly 4 X a second & signaled my directional module to cancel directional flash output. My new speedometer has no switch. I would like to trick the module with maybe a directional flash: (high pulse) to start a series of unrelated negative pulses that I would feed in as one lead to the reed switch input. So simple it's complicated! Ideas appreciated.A mind is a terrible thing to waste

Been a while- still looking to do this? Should be easy enough with an IC, resistor and cap. Schmitt 2 input NAND gate connected as an oscillator by feeding the output back to the input, and the other input coming from the directional signal. When the directional signal is high, the gate will output pulses, and when the signal is turned off, the pulses will stop.