Note: this info was collected from various websites over a period of years. I am not liable for any errors contained herein, nor is the PNG.
Bosch Motronic 4.1: GM (Early 20XE pre 1990)
Motronic 4.1 operation
The electronic system used to control the GM 8 valve engines (1987 to 1990) is labelled Motronic 4.1 and is a fully integrated EMS that controls primary ignition, fuelling and idle control from within the same ECU. In addition the ignition point and injection duration are jointly processed by the ECU so that the best moment for ignition and fuelling are determined for every operating
condition. The injection function of the Motronic system is based on the well tried 'L' jetronic system, although a number of refinements have improved operation. A 35 pin connector and multi-plug connects the ECU to the battery, sensors and actuators.
Basic ECU operation
A permanent voltage supply is made from the vehicle battery to pin 18 of the ECU. This allows the self-diagnostic function to retain data of an intermittent nature. Once the ignition is switched on, a voltage supply to the ignition coil and to ECU pin 35 is made from terminal 87 of the main fuel injection relay.
The majority of sensors (other than those that generate a voltage such the CAS and OS), are now provided with a 5.0 volt reference supply from a relevant pin on the ECU. When the engine is cranked or run, a speed signal from the CAS causes the ECU to earth pin 20 so that the fuel pump will run. Ignition and injection functions are also activated. All actuators (Injectors,
ISCV, FTVV etc), are supplied with nbv from the main relay and the ECU completes the circuit by pulsing the relevant actuator wire to earth.
Signal processing
Basic ignition timing is stored in a two dimensional map and the engine load and speed signals determines the ignition timing.
The main engine load sensor is the AFS and engine speed is determined from the CAS signal.
Correction factors are then applied for starting, idle, deceleration and part and full-load operation. The main correction factor is engine temperature (CTS). Minor correction to timing and AFR are made with reference to the ATS and TS signals.
The basic AFR is also stored in a two dimensional map and the engine load and speed signals determines the basic injection pulse value. Motronic calculates the AFR from the AFS signal and the speed of the engine (CAS).
The AFR and the pulse duration are then corrected on reference to ATS, CTS, battery voltage and position of the TS. Other controlling factors are determined by operating conditions such as cold start and warm-up, idle condition, acceleration and deceleration.
Motronic accesses a different map for idle running conditions and this map is implemented whenever the engine speed is at idle.
Idle speed during all warm-up and normal hot running conditions are maintained by the ISCV. However, Motronic makes small adjustments to the idle speed by advancing or retarding the timing, and this results in an ignition timing that is forever changing during engine idle.
Self Diagnostic function
The Motronic 4.1 system has a self-test capability that regularly examines the signals from engine sensors and internally logs a code in the event of a fault being present. This code can be extracted from the Motronic Serial Port by a suitable Fault Code Reader. When the ECU detects that a fault is present, it earths pin 17 and the warning lamp on the dash will light. The lamp will
stay lit until the fault is no longer present. If the fault clears, the code will remain logged until wiped clean with a suitable FCR, or until the engine has been started for more than 20 times when the fault code is self initialising. An ECU that retains codes for faults of an intermittent nature is a valuable aid to fault diagnosis.
The codes emitted by the Motronic 4.1 ECU fitted to GM vehicles emit codes of the 'slow code' variety. This means that the codes can be extracted by connecting two pins in the SD (ALDL) plug together.
In addition to the self-test capability, Motronic 4.1 has full limp home facilities. In the event of a serious fault in one or more of the sensors, the EMS will substitute a fixed default value in place of the defective sensor.
This means that the engine may actually run quite well with failure of one or more minor sensors. Since the substituted values are those of a hot engine, cold starting and running during the warm-up period may be less than satisfactory. Also, failure of a major sensor, i.e. the AFS, will tend to make driving conditions less easy.
Reference voltage
Voltage supply from the ECU to many of the engine sensors is at a 5.0 volt reference level. This ensures a stable working voltage unaffected by variations in system voltage.
The earth return connection for some engine sensors is made through an ECU pin that is not directly connected to earth. The ECU internally connects that pin to earth via one of the ECU pins that are directly connected to earth.
Signal shielding
To reduce RFI, the CAS uses a shielded cable. The shielded cable is connected to the main ECU earth wire at terminal 27 to reduce interference to a minimum.
CAS
The primary signal to initiate both ignition and fuelling emanates from a CAS mounted in proximity to the crankshaft at the front of the cylinder block. The CAS consists of an inductive magnet that radiates a magnetic field and a toothed disk. The disk is attached to the crankshaft or pull. The disk is attached to the crankshaft and theoretically comprises 60 teeth set at 3° intervals around its circumference; each tooth being 3° wide. At a position some distance BTDC, two teeth are omitted as a reference to TDC and so a total of 58 teeth remain on the disk. As the crankshaft spins, and the teeth are rotated in the magnetic field, an AC voltage signal is generated and delivered to the ECU to indicate speed of crankshaft rotation. In addition, as the engine spins,
the missing teeth generate a variation of the signal that serves as a reference to TDC to indicate crankshaft position.
The peak to peak voltage of the speed signal (when viewed upon an oscilloscope) can vary from 5 volts at idle to over 100 volts at 6000 rpm. Because computers prefer their data as on/ off signals, the ECU utilises an analogue to digital converter (ADC) to transform the AC pulse into a digital signal.
Ignition
Data on load (AFS), engine speed (CAS), engine temperature (CTS) and throttle position (TS) are collected by the ECU, which then refers to a three dimensional digital map stored within its microprocessor. This map contains an advance angle for each operating condition, and thus the best ignition advance angle for a particular operating condition can be determined.
Amplifier
The Motronic amplifier contains the circuitry for switching the coil negative terminal at the correct moment to instigate ignition.
The amplifier circuitry is contained within the ECU itself and the microprocessor contains a map containing the correct ignition dwell period for each condition of engine speed and battery voltage. One disadvantage of an internal amplifier, is that if the amplifier fails, the whole ECU must be renewed.
Dwell
Dwell operation in Motronic is based upon the principle of the 'constant energy current limiting' system. This means that the dwell period remains constant at around 4.0 to 5.0 ms, at virtually all engine running speeds. However, the dwell duty cycle, when measured in percent or degrees, will vary as the engine speed varies. A current limiting hump is not visible when viewing an oscilloscope waveform.
Ignition coil
The ignition coil utilises low primary resistance in order to increase primary current and primary energy. The amplifier limits the primary current to around 8 amps and this permits a reserve of energy to maintain the required spark burn time (duration).
Distributor
In the Motronic system, the distributor only contains secondary HT components (distributor cap, rotor and HT leads) and serves to distribute the HT current from the coil secondary terminal to each spark plug in firing order.