Vehicle Speed Sensor: Description and Operation

Undefined Ratio Circuit:

Purpose:
The PCM/TCM uses speed information from the input/output speed sensors to determine the following:
^ Engine is Running
^ Vehicle Speed
^ Calculate Gear Ratio
^ Calculate TCC Slip
^ Calculate Turbine Speed


Operation:
Both the input and output speed sensors are of the magnetic induction type. Voltage is induced in the output sensor by gear teeth pressed on the outside diameter of the rear carrier assembly. The output sensor may be thought of as replacing the governor. Output speed may differ from speedometer readings because the output speed sensor reading is not corrected for minor variations in axle ratio or tire size.

If the input speed sensor is not operational at start up and reads 0, this will cause the output sensor to also read 0. There is no direct code associated with an input sensor fault, however, an inaccurate input sensor could cause other codes to set especially those associated with ratios or slip.

Location:
Both speed sensors are accessible from the left hand side of the transmission. The input speed sensor is just forward of center and the output speed sensor is near the rear.