what is a countershaft speed sensor on an auto transmission?

Answer 1

the On-Board Diagnosis II system requires that the transmission be monitored for slippage as a slipping transmission can cause excessive emissions.

honda uses two sensors to do that. The engine rpm is monitored through the signal from the crankshaft position sensor. the engine speed is reduced and torque increased through the gear reduction in the transmission. As each gear has a ratio the on-board computer can calculate the speed of the transmission output shaft based on the engine rpm going to the input shaft. The rpm of the transmission output shaft is monitiored by the transmission speed sensor.

Some manual transmissions also use a transmission input shaft speed sensor as well as the output shaft speed sensor. The input speed sensor would be the countershaft sensor when used.

Both the automatic and manual transmission shaft speeds are monitored in the same way. If the output shaft spins significantly slower than the calculated speed based on engine rpm then the OBD II system determines that there is a loss of power through transmission slippage and turns on the Check Engine Light (Malfunction Indicator Lamp).

There is also a Vehicle Speed Sensor that monitors transmission output shaft speed and that value is used for the speedometer and engine fuel management functions and calculations.

hope that answers your question