Hi everyone , I figured Id start a discussion regarding the issue I'm having with a clients bmw. I did do a tech help but wanted to get some more interaction . Im pasting the same post I have over there. Since then I have tested and replaced the turbine/input speed sensor (was reading 100 ohms higher than spec)along with adding new fluid as well. I am still having the same issue and it only happens when in drive and vehicle is stationary. I can watch the turbine speed jump erratically from say 32 rpm all the way up to 3300rpm . These are the readings Im seeing using my autel 708 scanner. THe readings will be normal(0) sometimes but if you let it idle too long in drive you will see the readings start to jump and that's when it will go into fail safe mode and throw the 0720 code. Im still thinking this is an electrical issue because I don't understand how the turbine speed would reach 3000rpm when im idling at 670. Crankshaft speed and turbine speed should be relatively close but only in neutral or when the car is moving. I am somewhat new to diagnosing electromechanical transmission issues so this is somewhat of a learning process for me. Im wondering if Im dealing with an ecu that is creating an erroneous signal.

original post in tech help..........

"Hi everyone. We have a customers bmw in the shop for service and need some help.The transmission is going into fail safe mode. We first checked the fluid level and had to add two quarts.There were no external leaks detected. We checked faults and gear monitoring 4 limp home mode(bmw code 41)and p0720 (bmw code 3b) were present. WE cleared the faults and gave the car back to the customer . A week or two later they returned with car in limp home mode again and this time fault p0720 only. We have checked all electrical plug connections for corrosion or contamination at egs module and at trans plug. We replaced the output shaft speed sensor and serviced the trans, cleared faults. I drove the car for a few days and trans indicator came on and went into fail safe. This happened to me twice and only when idling in drive .I cycled the key and trans light went off and no longer in fail safe mode. I then began to monitor sensor speeds in trans with scanner. I haven't been able to get it to go into fail safe again yet. I notice that the turbine(input shaft) speed , while in drive with my foot on the brake ,will erratically jump from 0-32-0, 0-96-0, 0-1440-0, 0-2200-0 rpms randomly. This is far as I have gone. Im feeling that this is an electrical issue (possibly turbine speed sensor failing or ecu). If the converter was locking up at idle , the engine would try to stall. I don't feel this. If there was transmission input shaft slippage I don't think its speed would be greater than engine speed at idle. Im hoping someone has had experience with this . Maybe I should have replaced the turbine sensor instead of the output speed sensor. The transmission otherwise seems to be in good working order . This is a ZF 5HP transmission."

thanks Justin

Justin Mancino
Technician
Bridgehampton Auto Sport
Bridgehampton, New York, USA