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

ENGINE 651.9 in MODEL 207, 212 (except 212.098/298) as of model year 2014 

ENGINE 651.9 in MODEL 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:

The engine speed is regulated by the CDI control unit by the following measures:

The natural vibration of the engine at idle is regulated in the following manner by the CDI control unit:

Function sequence for RPM control 

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

Function sequence for torque converter protection 

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 via the drive train CAN from the fully integrated transmission control unit (with code 427 (7-speed A/T)) or (with code 421 (9-speed automatic transmission)) as of 01.09.14.

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 idle speed control 

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

For idle speed control the CDI control unit also reads in the following signals:

The CDI control unit regulates the fuel pressure in the rail over the pressure regulating valve and the pressure regulator valve and the injection period via actuations of the fuel injectors.

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

IMPORTANT The switchable engine mounts prevent transfer of low frequency natural vibration from the engine when idling. When driving they act as hydraulic engine mounts. They are actuated by the CDI control unit. The CDI control unit regulates the idle speed in the following operating and engine load conditions:

Engagement of the gear range "D" or "R" (with 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 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 (with a manual transmission) 

The CDI control unit recognizes actuation of the clutch pedal or clutch pedal switch and regulates the engine RPM to obtain a stable idle speed.

Switching in the A/C compressor 

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 adapts 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 MODEL 207 PE07.10-P-2708-97EAN
MODEL 212 PE07.10-P-2708-97DAR
MODEL 218 PE07.10-P-2708-97XAH
  Overview of system components for common rail diesel injection (CDI)   GF07.16-P-9997OM
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