In 1996, BMW M introduced a manual transmission that was capable of shifting gears automatically without the use of a conventional clutch pedal or mechanical shifter lever. The system was called the SMG I. Derived from Formula 1 racing technology; SMG stands for Sequential Manual Gearbox.
The U.S. received the first SMG in the 2001 E46 M3. This system was referred to as SMG 2. The 6-speed manual gearbox manufactured by Getrag, operated in automated “A” mode and performed similar to an automatic transmission or in sequential “S” mode, which allows the operator to shift manually through the gears.
In sequential mode, the driver shifts up and down through the gears by simply operating the shifter lever or a pair of paddle shifters mounted on either side of the steering wheel. Individual gear selection is realized by electrical/hydraulic actuation of both, the gearbox selector forks and the (conventional) clutch assembly.
The SMG II system is consists of the following components:
- Basic Gearbox (S6S420G, Getrag type D)
- SMG II Control Unit (Siemens)
- SMG CAN Bus
- DME Control Unit (MSS54)
- Hydraulic unit and solenoids
- Gearbox Actuator
- Clutch Slave Cylinder with PLCD (Permanent Linear Contactless displacement)
- Shift Lever Module with Shift Lock
- Steering Wheel Paddle mounted switches
- DRIVELOGIC Control
- Display in instrument cluster
- DSC control unit
- SAC dry single disc clutch
SMG II Control Unit
The SMG II Control Unit, installed in the E-Box next to the DME, is a single board module with SKE (134 pin) gray colored connectors. The unit contains protection against reverse polarity and overvoltage.
Communication with the DME is via a dedicated CAN bus (SMG II CAN Bus). Based on instructions received from the DME the SMG II control unit manages the clutch solenoid valve, the selector shaft up and down solenoids, and the selection angle solenoid. Safety functions and limp home capabilities are also part of the SMG II programming.
While the SMG II control unit receives many of the inputs and manages the control of the hydraulic system, the DME is responsible for and controls all gearshifts. Sensor inputs received by the SMG II control unit are relayed to the DME for processing and monitoring.
SMG CAN Bus
The SMG CAN Bus is the link for signal exchange between the DME and SMG II Control Unit. It allows the DME to issue command instructions for clutch and gearbox control as well as shift sequence and enables SMG II communication with the vehicle CAN BUS via the DME.
Signals exchanged via SMG CAN Bus:
Engine Speed, Engine Coolant Temp, Engine Oil Temp, Intake Air Temp, PWG, EDR Feedback Pots, Wheel Speed, Transverse Acceleration, Steering Angle, Cruise Status, Parking Brake, Door Contacts, Brake Light Switch, Key Memory
SMG II Diagnosis
The SMG II control unit is fully diagnosable and can be checked using ISTA/D. The first step involves the fault memory being read out. Malfunctions are communicated as part of the SMG II control unit’s self-diagnosis. The second step involves appropriate operator prompting to facilitate diagnosis with simple measuring equipment using test modules.
Repairs can be made once the fault in question has been found.
The diagnosis including self-diagnosis can only be carried out when the ignition (terminal 15) is switched on and there is a supply voltage of at least 10 volts. Erroneous fault entries may be recorded if the supply voltage drops below the threshold of approx. 10 volts.
A test program (service functions) must be carried out upon completion of various types of work on the SMG system – see the following table. The test programs serve to implement test, initialize and adjustment functions.
The values determined are permanently stored in the non-volatile memory only after the test program has been success-fully completed.
Checking & topping up fluid level in hydraulic system (S6S 420G SMG)
- Turn on ignition
- Press brake pedal
- Move selector lever from position “0” to “R” or “0” to “E” until hydraulic pump is activated
- Wait until hydraulic pump shuts down
- Open expansion tank (1)
- Fill expansion tank up to MAX marking (2) on dipstick (1)
NOTE: Check when cold. Check with dipstick on cap; for this, screw in cap.