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

ENGINE 157.9, 278.9 in MODEL 166 

as of model year 2016 

ENGINE 157.9, 278.9 in MODEL 292 

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:

Function sequence for torque converter protection (for engine 278) or start-up clutch (for engine 157) 

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. 5000 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 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. The ME-SFI [ME] control unit detects the driving mode on the "wheel speed" CAN signal, which is sent from the Electronic Stability Program control unit via the chassis CAN 1.

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 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 ignition coils as far as 36° CKA (C  rank A  ngle) toward "retarded" or 20° CKA toward "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 dependent on the coolant temperature, in order to bring the catalytic converters 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 gear range "D" or "R"

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. To do this, the ME-MFI [ME] control unit evaluates the "gear range engaged" CAN signal from the fully integrated transmission control unit.

Switch-in of the A/C

When the engine is at idle, so that engine speed will not fall when the refrigerant compressor is engaged, the automatic air conditioning control and operating unit sends a status signal to the ME-SFI [ME] control unit before the AAC is engaged.

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.

Purging

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-2708-97NBB
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