Electrical Accelerator Control Unit

Electronic Accelerator Control Unit:

The electronic accelerator control unit (N4/1) contains two printed circuits with electronic components. On models 124 and 129 it is waterproof. The housing consists of an aluminum alloy with a 35-pole plug connector. The connection with the wiring harness is through a 35-pole connection attached with a spring bracket (arrow).

The electronic accelerator control unit supplies voltage to the potentiometers at the accelerator actuator and accelerator pedal position sensor. These components return signals that are then processed. The electronic control unit is functionally divided into the following component groups:

^ System control with input amplifier (signal conditioning part),
^ Output drivers,
^ Monitoring equipment (safety circuit),
^ Cruise control circuit,
^ ASR signal processing.

The electronic accelerator control unit, with the exception of the cruise control circuitry, is designed as an analog processor.

SYSTEM CONTROL
The input signals to the system control are transformed, filtered and amplified in their respective signal conditioning components. Subsequently, these signals are processed in the system control, which generates a specific signal for the output driver.

OUTPUT DRIVER
The output signal of the system control is sent to the output driver and there amplified. In addition, this signal determines the adjustment direction (open or closed throttle) for the actuator DC motor.

If the DC motor is locked, a constant DC current is briefly applied. If it remains locked, then the control unit turns off the output driver. The output driver is protected from short circuits to ground.

CRUISE CONTROL SYSTEM
The cruise control function is integrated into the electronic accelerator control unit. It is turned on or turned off by the cruise control switch. If the cruise control switch is held on below approx. 40 km/h (25 mph) and the vehicle is then accelerated above that speed, the cruise control still does not turn on. The cruise control also does not activate when the switch is turned on during braking. During SET cruise control driving, the system control unit receives a corresponding input signal. This determines the setting of the throttle valve. If the driver exceeds the set speed using the accelerator pedal, the cruise control system is idle. If the ASR system identifies drive wheel spin during cruise control driving, then reduction of engine power (and reduced drive wheel speed) takes precedence over the cruise control operation. Once the drive spin disappears, then the cruise control adjusts vehicle speed to the previously stored vehicle speed as long as the driver does not employ another function.

MONITORING ELEMENTS
The monitoring elements performs safety operations. They test the following functions:

^ The working ranges of the accelerator pedal position sensor and accelerator actuator potentiometers,
^ The wires to the accelerator pedal position sensor and accelerator actuator potentiometers for short or open circuits,
^ Operation of the safety switches in the accelerator pedal position sensor and in the accelerator actuator,
^ The adjustment accuracy of the actuator, relative to the nominal position that either the accelerator pedal (accelerator pedal position sensor), cruise control or the ABS/ASR control unit has determined,
^ Short circuit at the actuator DC motor output driver,
^ Battery voltage at the electronic accelerator control unit. Below 6 V the control unit turns off. It turns on again when the voltage returns to normal.

SAFETY SWITCH (Fuel Pump Control)
There are two safety switches, one in the accelerator pedal position sensor and one in the actuator. These two switches, in combination with a switching transistor using circuit 15 to control power to the fuel pumps via the fuel pump relay or Engine System Control Unit (MAS). The purpose of these switches is to shut off power to the fuel pumps (thus stopping the engine) if the throttle setting is higher than idle while the accelerator setting is idle. Thus, if a malfunction occurs where the driver has his foot off the accelerator pedal (thus requesting the engine to idle) but the actuator output shaft is not at idle, the engine shuts off.

The operation of the safety switch is independent of the electronic control function of the drive torque control circuit. The control unit routes current through the safety switches of the accelerator pedal position sensor and the accelerator actuator. When driving and/or at idle, the fuel pump relay or the Engine Systems Control Unit (MAS) is supplied with voltage. The safety switch in the accelerator pedal position sensor closes when the accelerator pedal is depressed, before the safety switch in the accelerator actuator opens. When the accelerator pedal is released, the sequence is reversed. Whenever the accelerator pedal is moved, the contact in either the accelerator pedal position sensor or in the accelerator actuator is closed. To prevent an open circuit of the voltage supply during a fast accelerator pedal movement toward idle, there is in addition a switch transistor connected on the control unit output circuit. When the safety switch in the accelerator pedal position sensor opens, this transistor controls the voltage supply to the fuel pump relay or the engine systems control unit for a maximum of 150 ms. During cruise control operation, the safety switch in the accelerator pedal position sensor is open, and the same is true (most of the time) for the one in the accelerator actuator. So that the fuel pump relay (or engine systems control unit) can continue the voltage supply to the fuel pump (circuits 30 and 87, bridged by the relay contact), the switch transistor takes over the function of the safety switch from the accelerator pedal position sensor. The monitoring elements control the safety switches as well as the switch transistor.