Speedometer Head: Description and Operation

SPEEDOMETER

The instrument cluster receives vehicle speed information from the Powertrain Control Module (PCM) via the speedometer signal circuit. The PCM pulses this circuit to ground. The number of pulses varies with tire size. The PCM receives vehicle speed information from the vehicle speed sensor located in the transmission assembly. Voltage is produced by the speed sensor and is fed to the PCM, which filters and shapes the signal. The PCM then counts the number of pulses received in a given time to determine the vehicle speed. Once the PCM has calculated the vehicle speed, it then pulses the speedometer signal circuit to ground. This causes the 12 volts at instrument cluster to be pulled down to less than 0.5 volt. The instrument cluster then determines the vehicle speed and the miles/kilometers travelled from the number of pulses it receives.