Rpm Control, Function - GF07.10-D-1008TSC

Engine 642.8 in model 907 

Block diagram 

Function requirements for RPM control, general points 

• Circuit 87 (Engine management ON)
• Engine running.

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 (N3/28) detects the engine speed via the signal from the crankshaft position sensor (B70/1).

For RPM limitation the CDI control unit reads in the following signals:

• Rpm sensor (B6/24), engine speed
• Fuel pressure sensor (B4/17)
• Fully integrated transmission control (VGS) electric controller unit (N1511) with code G42 (7G-TRONIC PLUS)), drive range
• ESP control unit® (N30/4), vehicle speed

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

• Actuation of the pressure regulating valve (Y74/6) (fuel pressure control)
• No actuation of the fuel injectors (Y76) (for RPM limitation)
• Adaptation of the injection duration of the fuel injectors (Y76) (with idle speed control)
• Actuation of the quantity control valve (Y94/4) (fuel pressure control)

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

• Boost pressure sensor (B5/8), engine load
• Coolant temperature sensor (B11/19)
• Charge air temperature sensor (B17/15)
• SAM control unit (N10), outside temperature
• Clutch travel sensor (B109) (for manual transmission with code MJ8 (ECO start/stop function))
• Clutch pedal switch (for manual transmission without code MJ8 (ECO start/stop function))
• Clutch pedal switch, upper (for manual transmission without code MJ8 (ECO start/stop function))
• Automatic air conditioning (N22/2) control and operating unit (with code HH4 (THERMOTRONIC automatic climate control) or with code HH9 (TEMPMATIC semi-automatically controlled air conditioning)), air conditioning system status

Function sequence for RPM control 

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

• Function sequence for torque converter protection
• Function sequence for RPM limitation
• Function sequence for idle speed control

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. 3000 RPM. The CDI control unit receives information about the current selector lever position via the drive CAN (CAN C) from the fully integrated transmission control (VGS) electric controller unit (with code G42 (7G-TRONIC PLUS)).

Function sequence for RPM limitation 

In order to protect the engine from the consequences of an impermissibly high engine speed, at a speed of > 0 km/h the maximum engine speed is limited to approx. 4500 RPM.

The CDI control unit detects the operating status on the "vehicle speed" CAN signal, which is sent by the ESP® control unit via the suspension FlexRay (Flex E).

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.

The CDI control unit regulates the fuel pressure in the rails via the pressure regulating valve (Y74/6) and controls the injection duration via actuations of the fuel injectors (Y76).

The fuel pressure in the rails and the injection duration of the fuel injectors (Y76) determine the injection quantity. The idle speed can be raised for a limited time to approx. 1,600 RPM 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)
• Actuation of the clutch pedal (for a manual transmission)
• Switching in the refrigerant compressor (A9) (with A/C)
• On-board electrical system battery undervoltage (G1)

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

Engagement of drive 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 "drive range engaged" CAN signal from the fully integrated transmission control (VGS) electric controller unit (with code G42 (7G-TRONIC PLUS)).

For vehicles with an automatic transmission, the idle speed is approximately 750 RPM.

Actuation of the clutch pedal (with a manual transmission) 

When actuating the clutch pedal the idle speed is stabilized. At the same time, the signals of the two clutch pedal switches are detected by the powertrain control unit and transmitted to the CDI control unit via the CAN connection.

Switching in the refrigerant compressor (with A/C) 

When the engine is at idle, so that engine speed will not fall when the refrigerant compressor (A9) switches in, the automatic air conditioning control and operating unit (N22/2) sends a status signal to the CDI control unit before switching-in. The CDI control unit adapts the idle speed appropriately.

Undervoltage of the on-board electrical system battery 

The CDI control unit recognizes undervoltage of the on-board electrical system battery via "circuit 30" and increases the idle speed by about 100 RPM until the engine switches off.

Electrical function schematic for RPM control	PE07.10-D-2708-97TSC
Overview of system components for common rail diesel injection (CDI)	GF07.16-D-9997TSC