K 3500 Truck 4WD V8-393 6.5L DSL Turbo (1993)
Transmission 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 of center.
