BREATHER AS 9C4937 - Caterpillar

Each speed sensor has two connections to the ECM (+ and −). For all of the speed sensors, connector contact "2" is the signal line and connector contact "1" is the return line. The speed sensors send an AC frequency signal (HZ) that matches the number of gear teeth passing the sensor per second to the ECM. The ECM compares the frequency from the speed sensor (HZ) to the number of pulses per revolution in order to determine the speed of the rotating components. The ECM uses the input from the speed sensors in order to regulate transmission shifts.The speed sensors are used to cross check and diagnose each other during normal operation. Crosscheck uses two transmission output speeds and the torque converter output speed to validate and determine which transmission output speed sensor to use for shifting. Crosscheck compares the three speeds. If at least two speeds agree, the transmission shifts as expected. Otherwise shifting is inhibited and the lockup clutch is disengaged.The ECM periodically checks the value from the speed sensor, and if an incorrect, erratic, or absent value is found, the ECM will log a service code (speed sensor diagnostic) that indicates a fault for a speed sensor circuit.Straight Through Transmission
Illustration 1 g02017533
(Left View and Rear View)
(1) Engine output speed sensor
(2) Torque converter output speed sensor
(3) Transmission output speed sensor There is one engine output speed sensor (1), one torque converter output speed sensor (2), and two transmission output speed sensors (3) on a straight through transmission. All speed sensors are used to determine if any individual speed sensor has failed. Dropbox Transmission
There is one engine output speed sensor (1), one torque converter output speed sensor (2), one transmission output speed sensor (3), one work mode output speed sensor (4), one rear wheel drive disconnect speed sensor (5), and one front wheel drive disconnect speed sensor (6) on a dropbox transmission. During work mode the transmission output speed sensor (3) and work mode output speed sensor (4) are active and are used to crosscheck transmission output speed and to diagnose each other. During road mode the transmission output speed sensor (3) and rear wheel drive disconnect speed sensor (5) are active and are used to crosscheck transmission output speed and to diagnose each other. The speed from the rear wheel drive disconnect speed sensor (5) and front wheel drive disconnect speed sensor (6) are adjusted to eliminate the effect of a speed change due to dropbox gearing before comparison to the speed from the transmission output speed sensor (3). The front wheel drive disconnect speed sensor (6) equipped on "4WD" dropbox transmissions is used for information only.
Illustration 2 g02017633
(3) Transmission output speed sensor
(4) Work mode output speed sensor
Illustration 3 g02017813
(5) Rear wheel drive disconnect speed sensor
Illustration 4 g02056479
4WD Dropbox
(6) Front wheel drive disconnect speed sensor