Speedometer Head: Description and Operation

SYSTEM OPERATION
The speedometer gives an indication of the current vehicle speed. The instrument cluster circuitry controls the gauge pointer position. The instrument cluster circuitry calculates the proper gauge pointer position based upon vehicle speed messages received from the Powertrain Control Module (PCM) on the Chrysler Collision Detection (CCD) data bus.

The PCM uses a speed pulse input received from the Controller Anti-lock Brake (CAB) and internal programming to decide what vehicle speed and distance messages are required. The PCM then sends the proper messages to the instrument cluster circuitry on the CCD data bus. The CAB uses an input from the rear wheel speed sensor and internal programming, which includes a programmable pinion factor to compensate for different axle gear ratios and tire sizes, to determine what speed pulse output is required. The CAB then sends the proper speed pulse to the PCM on a hard wired circuit.

The rear wheel speed sensor is excited by an exciter ring on the differential case within the rear axle housing. Incorrect tire size, incorrect axle ratio, an incorrect pinion factor, a faulty or improperly installed rear wheel speed sensor, a faulty rear axle exciter ring, or a faulty CAB can each result in inaccurate speedometer readings. For diagnosis of the speedometer inputs, a DRB scan tool and the proper Diagnostic Procedures are recommended.