A telemetry and control system is disclosed for use in direct current power wiring applications, such as those used in automobiles or other vehicles. The system includes a master transmitter which encodes data as a train of current pulses which modulate the d.c. supply voltage. A slave receiver at another location, connected to the same power source through pre-existing wiring, amplifies the supply voltage modulation, recovers the data waveform, and decodes it, to effect an action or display a measurement or status indication. A second transmitter combined with the slave receiver may transmit data similarly for reception by a second receiver combined with the master transmitter, to enable two-way communication therebetween. Applications of the system include an emergency battery booster, an anti-theft battery disconnection device, a distress alert system, activation and reception of data from outboard devices, and remote measurement of fluid levels, temperatures and pressures.