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Rpm control, function - GF07.10-P-1008OGA

ENGINE 642.8 in MODEL 207 as of 6/1/13, 212, 218 as of model year 2014 

Function requirements for RPM control, general points 

Rpm control, general 

Rpm control stabilizes the idle speed and limits the engine speed to protect the engine and the drivetrain. The CDI control unit (N3/9) detects the engine speed over the crankshaft Hall sensor signal (B70). For RPM limitation the CDI control unit reads in the following signals:

Function sequence for RPM control 

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

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 CDI control unit receives information about the current selector lever position from the fully integrated transmission control controller unit via the drive train CAN.

Function sequence for RPM limitation 

During travel, the maximum engine speed is limited to 4500 RPM for a speed >0 in order to protect the engine from the effects of inadmissibly high engine speeds.

The CDI control unit detects the driving mode on the "vehicle speed" CAN signal, which is sent by the ESP® control unit via the chassis CAN 1.

Function sequence for maximum speed limit 

Upon reaching 250 km/h the injection period of the fuel injectors are reduced by the CDI control unit and the fuel pressure in the rails is reduced via the quantity control valve and the pressure regulating valve. These measures ensure simple maximum speed limit through the reduction of the engine torque.

IMPORTANT The fuel pressure in the rails and the injection period of the fuel injectors determine the injection quantity.

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 CDI control unit also reads in the following sensors and signals:

The idle speed can increased to about 1600 RPM for a limited period depending on the atmospheric pressure and ambient temperature.

The CDI control unit regulates the idle speed in the following operating conditions and engine load conditions:

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. To do this, the CDI control unit evaluates the "gear range engaged" CAN signal from the fully integrated transmission control controller unit.

IMPORTANT For vehicles with an automatic transmission the idle speed is 750 RPM and for vehicles with a manual transmission the idle speed is 810 RPM.

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 CDI control unit through the clutch pedal switch.

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 CDI control unit before the AAC is engaged. The CDI control unit reacts by adapting the idle speed appropriately.

IMPORTANT If the following conditions exist, the CDI control unit interrupts switching on of the A/C compressor:

Undervoltage of the on-board electrical system battery 

The CDI control unit recognizes an undervoltage of the on-board electrical system battery via the "circuit 30" and increases the idle speed in combination with the automatic transmission for an engaged gear range "D" or "R" up to switch off of the engine at around 100 RPM.

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 ENGINE 642.8 in MODEL 207 as of 6/1/13 as of model year 2014 PE07.10-P-2708-97EAJ
ENGINE 642.8 in MODEL 212 as of model year 2014 PE07.10-P-2708-97DAM
ENGINE 642.8 in MODEL 218 as of model year 2014 PE07.10-P-2708-97XAE
  Overview of system components for common rail diesel injection (CDI)   GF07.16-P-9997OG
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