RPM Control, Function - GF07.10-P-1008MNC
Engine 276.8 in model 172.4
Function requirements for RPM control, general points
- Circuit 87M (Engine management ON)
- Engine runs
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 (B70) detects the engine speed from the signal from the crankshaft Hall sensor signal (N3/10).
For RPM limitation, the ME-SFI [ME] control unit reads in the following signals:
- Crankshaft Hall sensor, engine speed
- Actual value potentiometer 1 and 2 (M16/6r1, M16/6r2), throttle valve position
- Electronic Stability Program control unit (N30/4) wheel speed via the chassis CAN 1 (CAN E1)
- Fully integrated transmission control unit (Y3/8n4), gear range via drive train CAN (CAN C)
The ME-SFI [ME] control unit limits the engine speed by the following measures:
- Adjustment of throttle valve opening angle by actuation of actuator motor (M16/6m1) for throttle valve
- Adjust the ignition angle of the ignition coils for cylinders 1 to 6 (T1/1 to T1/6)
- Cutout of fuel injectors for cylinder 1 to 6 (Y76/1 to Y76/6) (for speed control)
- Adaptation of injection period for the cylinder 1 to 6 fuel injectors (for idle speed control)
Function sequence for RPM control
The RPM control is described at the following points:
- Function sequence for torque converter protection
- Function sequence for throttle valve actuator defective
- Function sequence for RPM limitation
- Function sequence for maximum speed limit
- Function sequence for missing variant coding
- 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. 4200 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 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 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 dependent on the model, the opening angle of the throttle valve and the injection period of the cylinder 1 to 6 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 missing variant coding
If no variant coding exists, maximum engine speed is limited to 2400 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:
- Coolant temperature sensor (B11/4), coolant temperature
- Pressure sensor downstream of throttle valve (B28/7), engine load
- On-board electrical system battery (G1), on-board electrical system voltage (undervoltage)
- AAC control and operating unit (N22/7), status of A/C via the interior CAN (CAN B), front SAM control unit with fuse and relay module (N10/1) and chassis CAN 1
The ME-SFI [ME] control unit controls the idle speed via the throttle valve actuator and adapts the injection period of the fuel injectors for cylinders 1 to 6 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 for cylinder 1 to 6 by up to 20°CKA (C rank A ngle) towards "retarded" or 20°CKA towards "advanced".
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
- Engagement of gear range "D" or "R"
- Switch-in of the A/C
- Purging
Warming-up phase
In the warm-up phase, the ME-SFI [ME] control unit controls the idle speed according to coolant temperature, in order to bring the catalytic converter more quickly up to operating temperature after engine start.
Additional function requirements in the warm-up phase are:
- Coolant temperature T < 40°C
- Gear range "N" or "P" engaged
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 approx. 50 seconds by approx. 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-SFI [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.
Purging
The idle speed control prevents any change in the engine speed due to purging.
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-2708-97TAH | ||
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