Transmission Speed Sensor: Description and Operation

The Automatic Transmission Input Shaft Speed (AT ISS) sensor is a magnetic inductive pickup that relays turbine shaft speed information to the PCM. The PCM uses AT ISS sensor information in order to control line pressure, transmission shift patterns and TCC apply and release. This information is also used in order to calculate the appropriate operating gear ratios and TCC slippage.

The AT ISS sensor mounts in the case cover, next to the automatic transmission input shaft speed sensor reluctor wheel assembly. An air gap of 0.08-2.12 mm (0.0032-0.0834 inch) occurs between the sensor and the teeth on the speed sensor reluctor wheel as the drive sprocket rotates. The speed sensor reluctor wheel is secured to and turns with the drive sprocket by the tangs on the drive sprocket forward thrust washer.

The sensor consists of a permanent magnet surrounded by a coil of wire. As the turbine shaft rotates the speed sensor reluctor wheel and the drive sprocket, an AC signal is produced by the AT ISS sensor. This AC signal consists of a voltage and frequency that changes based on vehicle speed. The PCM uses the frequency portion of this signal to determine input shaft speed. Higher input shaft speeds induce a higher frequency and a higher voltage measurement at the sensor. The voltage portion of the signal is used in diagnostic procedures.

Sensor resistance should measure between 893-1127 ohms at 20°C (68°F). Output voltage will vary with the vehicle speed from a minimum of 0.5 volts AC at 300 RPM to 200 volts at 6000 RPM.