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 (N3/9) detects the engine speed over the crankshaft Hall sensor signal (B70).
For rpm limitation the CDI control unit reads in the following signals:
Crankshaft Hall sensor, engine speed
High pressure fuel pressure sensor (B4/6)
Fully integrated transmission control controller unit (Y3/8), gear range over drive train CAN (CAN C)
Electronic Stability Program control unit (N304), vehicle speed via chassis CAN 1 (CAN E1)
The engine speed is regulated by the CDI control unit by the following measures:
Actuation of the pressure regulating valve (Y74) (fuel pressure control)
No actuation of the fuel injectors (Y76) (vehicles with rpm limitation)
Adaptation of the injection period of the fuel injectors (with idle speed control)
Actuation of the quantity control valve (Y94) (fuel pressure control)
The natural vibration of the engine at idle is regulated in the following manner by the CDI control unit:
Actuation of the switchable engine mounts on the left and right (Y123, Y1231)
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
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. 3000 rpm. The CDI control unit receives information about the current selector lever position from the fully integrated transmission control unit (Y38n4/) via the drive train CAN.
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 driving mode based on the "vehicle speed" CAN signal, which is transmitted from the Electronic Stability Program control unit via 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 sensors and signals:
Boost pressure sensor (B5/1), engine load
Coolant temperature sensor (B11/4)
Charge air temperature sensor (B17/8)
SAM control unit (N10), outside temperature via the interior CAN (CAN B), electronic ignition lock control unit (N73) and chassis CAN 1
Automatic air conditioning control and operating unit (N22/7), status of the A/C via the interior CAN, electronic ignition lock control unit and chassis CAN 1
The CDI control unit regulates the fuel pressure in the rails via the pressure regulating valve and regulates the injection period by actuating the fuel injectors.
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 fuel pressure in the rails and the injection period of the fuel injectors determine the injection quantity. 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 gear range "D" or "R"
Switching in the A/C compressor (A9)
On-board electrical system battery undervoltage (G1)
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 CDI control unit evaluates the "gear range engaged" CAN signal from the fully integrated transmission control unit.
For vehicles with an A/T the idle speed is about 750 rpm.
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.
Diagnosis
The basic setting of the idle speed can be read out and influenced using the Xentry Diagnostics.
Undervoltage of the on-board electrical system battery
The CDI control unit recognizes an undervoltage of the on-board electrical system battery through "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 to approx. 100 rpm.
Referenced documents
- gf0716p9997ogs Overview of system components for common rail diesel injection (CDI)