Transmission issue

[Sltn]

My car has transmission, when i drive above 120 mph it change down the gear 3 gear and it gave me the code which is 37 gear monitoring and when i drive below that speed it doesn't do remain stable not only tht but it also have reverse issue most of the time it doesn't work

[bmwdirtracer]

Welcome to the forum.

Why would you change down to 3rd gear with a 5 speed automatic 323i, at 120 mph? Are you joking?

The reverse drum on that transmission is a known failure item. You should be looking for a replacement transmission.

[bmwdirtracer]

The data I find for this car show maximum speed in 3rd gear is 92 mph. I can't tell you that's correct, but I can tell you you should not be trying to shift to third gear at 120 mph.

[shogun]

if it automatically switches down to 3rd gear, check the turbine speed sensors (input and output) inside the transmission, explained here https://www.meeknet.co.uk/E38/BMW_Tr...damental_1.pdf
Turbine Speed Sensor
The Turbine Speed Sensor is used to provide input (turbine) shaft speed information to the TCM (EGS). The input shaft speed signal is used in conjunction with the output shaft speed signal to determine gear range and slip time information for processing in the TCM. Not all BMW transmissions use a turbine speed sensor. Some TCM’s use the TD (engine speed) signal to determine input shaft speed.