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RPM Control, Function - GF07.10-P-1008MRA

Engine 274.9 in model 205 (except 205.047/053/054/147/247/253) 

Engine 274.9 in model 253 (except 253.354/954) 

Function requirements for RPM control, general points 

RPM control, general 

RPM control stabilizes the idle speed and limits the motor speed to protect the engine, the drivetrain and the vehicle. The ME-SFI [ME] control unit (N3/10) detects the engine speed via the crankshaft Hall sensor (B70) signal.

For RPM limitation, the ME-SFI [ME] control unit reads in the following signals:

The ME-SFI [ME] control unit limits the engine speed by the following measures:

Function sequence for RPM control 

Regulation of the engine speed is described in the following points:

Torque converter protection function sequence (for automatic transmission) 

To protect the torque converter in selector lever positions "P" or "N" (no power flow) from excessive static internal pressure, the maximum engine speed is limited to approx. 4200 RPM. The ME-SFI [ME] control unit receives information about the current selector lever position from the fully integrated transmission control unit via the drive train CAN.

Function sequence for manual transmission protection (for a manual transmission) 

To prevent damage to the manual transmission, the maximum engine speed is limited to about 4000 RPM when the vehicle is stationary.

Function sequence for RPM limitation 

In order to protect the engine from inadmissible engine speeds during driving mode, the maximum engine speed is limited to about 6000 RPM. Driving operation is detected by the ME-SFI [ME] control unit by means of the "vehicle speed" CAN signal, which is transmitted by the Electronic Stability Program control unit via the chassis FlexRay, powertrain control unit and drive train CAN.

Function sequence for maximum speed limit 

Upon reaching maximum speed, the opening angle of the throttle valve and the injection period of the fuel injectors are reduced by the ME-SFI [ME] control unit. These measures ensure simple maximum speed limit through the reduction of the engine torque.

Function sequence for throttle valve actuator defective 

In the event of a defective throttle valve actuator, the maximum engine speed is limited in idle to approx. 1400 RPM and in driving mode to approx. 1800 RPM.

Function sequence for missing variant coding 

If no variant coding exists, maximum engine speed is limited to 2400 RPM.

Function sequence for idle speed control 

The idle speed control regulates and stabilizes the idle speed depending on operating conditions and engine loads when the accelerator pedal is not actuated.

For idle speed control the ME-SFI [ME] control unit also reads in the following signals:

The ME-SFI [ME] control unit controls the idle speed via the throttle valve actuator and adapts the injection period of the fuel injectors in accordance with the modified air mass. To assist with idle speed control, the ME-SFI [ME] control unit can adjust the ignition angle of the cylinder 1 ignition coil to cylinder 4 ignition coil up to 36°CKA (C  rank A  ngle) towards "retarded" or 20° CKA towards "advanced".

IMPORTANT The ignition angle initiates a faster response through the ignition angle setting than the throttle valve adjustment.

The ME-SFI [ME] control unit regulates the idle speed in the following operating conditions and engine load conditions:

Warming-up phase 

In the warm-up phase, the ME-SFI [ME] control unit controls the idle speed according to coolant temperature, in order to bring the catalytic converter more quickly up to operating temperature after engine start.

Additional function requirements in the warm-up phase are:

The ME-SFI [ME] control unit raises the idle speed to about 1100 to 1300 RPM for about 20 s. After this time has elapsed, the idle speed is increased at a coolant temperature of <70°C for about 50 s by the ME-SFI [ME] control unit by about 50 to 100 RPM.

Engagement of the gear range "D" or "R" (for an automatic transmission) 

When gear range "D" or "R" is selected, the idle speed is set to a lower value to reduce the tendency of the vehicle to creep forward. For this purpose, the ME-SFI [ME] control unit evaluates the "selected gear range" CAN signal via the fully integrated transmission control unit.

Actuation of the clutch pedal (for a manual transmission) 

When actuating the clutch pedal the idle speed is stabilized. Clutch pedal actuation is detected by the ME-SFI [ME] control unit over the clutch pedal switch.

Switching in the A/C compressor 

In order to prevent an engine RPM reduction when the A/C compressor is started up in neutral, the Climate Control control unit sends a status signal to the ME-SFI [ME] control unit before starting up. The ME-SFI [ME] control unit adapts the idle speed appropriately.

Undervoltage of the on-board electrical system battery 

If the ME-SFI [ME] control unit detects an undervoltage through "circuit 30", the idle speed is raised by about 100 RPM when gear range "D" or "R" is engaged, until the ignition is switched off.

Purge control 

The idle speed control prevents any change in the engine speed due to purging.

IMPORTANT Diagnosis

The basic setting of the idle speed can be read out and influenced using the Xentry Diagnostics.

  Electrical function schematic for RPM control   PE07.10-P-2708-97FBA
  Overview of system components for gasoline injection and ignition system with direct injection   GF07.70-P-9998MRA 
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