Automatic Transmission Input (Shaft) Speed Sensor

WARNING: This page is about a different car, the 2008 Buick Lucerne. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: View Of Automatic Transmission Input Speed Sensor

Courtesy of GENERAL MOTORS CORP.

The automatic transmission input (shaft) speed sensor is mounted in the case cover facing the drive sprocket. The sensor consists of a permanent magnet surrounded by a coil of wire. The sensor mounts into the case and maintains a slight air gap (0.045-0.109 in.) between itself and the drive sprocket. An AC voltage signal is induced in the input sensor by rotating the drive sprocket, which contains 39 teeth cut in its outside diameter. The voltage output and frequency varies with drive sprocket speed:

Low speed = low Hz and voltage amplitude
High speed = high Hz and voltage amplitude

Inside the transmission control module (TCM), this analog signal changes to a digital signal. The processor then compares this digital signal to a fixed clock signal within the TCM in order to determine the actual turbine speed. The TCM uses input speed for control of line pressure, speed calculation and gear ratio.

Sensor resistance should be 1300-1950 ohms when measured at 20-100°C (68-212°F). Output voltage will vary with speed from a minimum of 0.5 volts AC at 100 RPM, to more than 100 volts AC at 8000 RPM.