BMW E38 automatic gearbox in limp mode
Why does the E38 automatic gearbox go into limp mode?
In limp mode the gearbox works only in a limited way, for example only in 4th gear. The cause can be the speed sensor, a solenoid (electric valve), the gearbox control unit, the wiring harness or too little oil. A camshaft sensor fault and tyres of different sizes can also trigger it.
Where do you start diagnosing limp mode on an E38?
1. Read the fault codes from both the engine and the gearbox control unit. 2. Disconnect and clean the connectors of the control unit and the engine management, because rust and oil cause intermittent faults. 3. Check the oil level: the gearbox has no dipstick, so the level is checked from below with the engine running. 4. Make sure all tyres are the same size.
Does limp mode go away by itself, and how do you reset the gearbox's learned values?
If the fault has gone away, limp mode usually resets by switching the car off and starting it again. The gearbox's learned values are reset with a fault code reader; just disconnecting the battery terminal is not enough. Relearning requires a longer drive.