Electronic control unit MM 8P (see illustration 12.1) includes three main areas of control: the ignition system, the multiport fuel injection system and the idle speed. The correct ignition timing and ignition timing for all engine operating conditions is calculated from data received from the crank angle sensor (position and speed of rotation of the crankshaft) and intake manifold absolute pressure sensor (engine load). Using a 35-pin connector and multi-pin connectors, the electronic control unit is connected to the battery, sensors and actuators.
12.1 Electronic control unit Magneti Marelli 8P
Main functions of the electronic control unit (ECM)
DC voltage is supplied from the battery to pin #4 of the ECU. The connection is made via a 15-pin contact relay. With the ignition on, power is supplied to the main relay, and the relay, in turn, supplies switching voltage to pin #35 of the ECU. Power is now supplied from pin #35 to the electronic control unit.
Most sensors, for example: intake air temperature sensor (ATS), coolant temperature sensor (CTS), throttle position sensor (TPS), intake manifold absolute pressure sensor (IDA) now receive a voltage of 5.0 Volts from the corresponding contact pin of the electronic control unit. When the crankshaft is cranked or the engine is running, the signal from the crankshaft angle sensor (CAS) causes the ECM to ground pin #23 so that the fuel pump starts working. The ignition and fuel injection functions are also activated. All drives (ignition coil, injector, carbon filter solenoid valve, etc.) provided with nominal battery voltage through the fuel pump relay and the ECM closes the circuit.
Reference voltage
The initial voltage level supplied from the electronic control unit to many sensors is 5.0 volts.
Power supplies for the electronic control unit (ECM) and grounding checks
Rated battery voltage (nbv)
| State | Volts |
| Ignition on | 11.5-13.5 |
| When turning the crankshaft | 8.0 + |
| On a running engine | 13.0-15.9 |
Voltage values of the electronic control unit (ECM)
Terminal numbers
| ECM | Element | State | Volts |
| 4 | Battery: t30 (via fuel injection relay) | Ignition off | nbv |
| 35 | Relay output terminal | Ignition on | nbv |
| 4 | Relay control circuit | Ignition off | nbv |
| 4 | Relay control circuit | Ignition on | 1.25 (max) |
| 23 | Relay control circuit | Ignition on | nbv |
| 23 | Relay control circuit | When cranking/engine running | 1.25 (max) |
| 14, 34 | ECM ground | Ignition on | 0.25 (max) |
Note. nbv - rated battery voltage (see illustrations 12.2)
12.2 Wiring diagram ECM/ISSM/CFSV/OS/FP/IMH/relay
Checking the electronic control unit ECM (general check)
1. Inspect the multi-pin connector of the electronic control unit for signs of corrosion and damage.
2. Make sure that the connector terminal pins are properly installed and make good contact with the multi-pin connector of the electronic control unit.
Voltage values for the connections of the electronic control unit (ECM)
Note. Contact pins No. 17 and No. 34 are the grounding of the electronic control unit for the MM 8P engine control system. For this reason, if possible, pins Ns 17 or No. 34 should be used to connect a voltmeter or oscilloscope.
Pin No. 4 ECM
Note 1. Pin No. 4 is connected to the terminal (+) battery through the fuel injection relay, so voltage is available when the ignition is off.
Note 2. When measuring voltage at a multi-pin connector, remember that the terminals are reversed. The multi-pin connector remains connected to the component, and the insulation of the multi-pin connector of the electronic control unit should be bent. The positive probe of the voltmeter is attached to the corresponding contact pin.
Electronic control unit connected
1. Connect the positive lead of a voltmeter to pin #4. The voltmeter should indicate the nominal battery voltage (see note 2).
Electronic control unit disconnected
2. Attach the negative lead of a voltmeter to the ECU ground pin.
3. Attach the positive lead of the voltmeter to pin #4. The voltmeter should indicate the nominal voltage of the battery (nbv).Contact pin No. 35 ECM
Note. The Ns 35 pin of the ECM is connected to the main relay output terminal and voltage must be available when the ignition is on.
Electronic control unit connected
1. Ignition on.
2. Connect the positive lead of the voltmeter to pin #35. The voltmeter should indicate the nominal battery voltage (see note 2).
Electronic control unit disconnected
3. Ignition on.
4. Attach the negative voltmeter lead to ground on the electronic control box.
5. Connect the positive voltmeter lead to pin #35. The voltmeter should indicate the nominal battery voltage.
Pin No. 4 ECM (relay main control circuit)
Relay and ECU connected
1. Ignition off.
2. Connect the positive probe of the voltmeter to pin #4, the meter should show the nominal voltage of the battery (see note 2).
3. If there is no voltage, check the relay and relay wiring.
4. Ignition on. The voltage should drop to almost zero.
5. If not, disconnect the ECU multi-pin connector and connect a temporary jumper wire from pin #4 to ground.
6. Relay working: Check the voltage supply to the electronic control unit and its grounding. If the wiring is good, then the electronic control unit may be faulty (ECM).
7 Relay not working: check relay and relay wiring.
Pin No. 23 ECM (fuel pump relay control circuit)
Relay and ECU connected
1. Before starting this test, check the operation of pin #23.
2. Ignition on.
3. Connect the positive probe of the voltmeter to pin #23, the meter should show the nominal voltage of the battery (see note 2).
4. If there is no voltage, check the relay and relay wiring.
5. Turn the crankshaft or start the engine, the voltage should drop to almost zero.
6. If not, disconnect the ECU multi-pin connector and connect a temporary jumper wire from pin #23 to ground.
7. Relay working: Check the voltage supply to the electronic control unit and its grounding. If the wiring is good, then the electronic control unit may be faulty (ECM).
8. Relay not working: Check relay and relay wiring.
ECM ground pins #17 and #34
Multi-pin connector of the electronic control unit is connected
1. Ignition on.
2. Attach the negative voltmeter lead to ground on the motor.
3. Attach the positive probe of the voltmeter to the ground terminal, the voltmeter should indicate 0.25 volts (maximum).
Multi-pin connector of the electronic control unit is disconnected
1. Ignition on or off.
2. Attach the negative lead of the voltmeter to the ground terminal.
3. Attach a positive test lead to pin #4 of the ECU, the voltmeter should indicate the nominal battery voltage if grounding is within specifications.
Shielded wire signal
To reduce radio frequency interference (RFI) on some sensors (those. CAS, KS and OS) shielded wire is installed. The shielded wire for the crank angle sensor is connected to terminal No. 17 of the ECM main ground wire to keep noise to a minimum. The other shielded wires are connected to ground pin No. 34 of the ECM.
Self-diagnosis function
The MM 8P system has a serial port for diagnostics and system configuration. Thanks to the self-diagnosis function, some parameters can be changed (those. CO value on models not equipped with a catalytic converter, or ignition timing).
In addition, the system has the ability to self-test, which consists in the continuous evaluation of the signals received from the sensors, and in the event of a malfunction, it enters the appropriate code into the memory specially allocated for this. Appropriate DTC Reader (FCR) can extract this code from MM 8P serial port (see illustration 12.3). If a malfunction is detected, the code will remain registered until it is erased from memory by the appropriate DTC reader.
12.3 Electrical connector for self-diagnosis (SD)
If the electronic control unit (ECM) detects a malfunction, it grounds the No. 6 pin, and the malfunction indicator lamp on the instrument panel lights up (MIL). The lamp remains on until the problem is corrected. However, if the fault is minor, the warning lamp will not come on, although the electronic control unit will register a fault code. A DTC will be logged under these circumstances, and the presence of a DTC indicates that a fault has been detected in the appropriate section of the circuit.
In addition to the self-test function, the MM 8P has a special device (LOS - a mode that allows you to get to the nearest workshop). If one or more sensors fail, the ECM will set the fault instead of the faulty sensor. In LOS mode, the engine will continue to run, albeit inefficiently. However, a faulty crank angle sensor will, of course, shut down the engine altogether.
This means that the engine can run quite well if the fault occurs in one or more of the auxiliary sensors. If the main sensor is damaged, such as the intake manifold absolute pressure sensor (IDA), the driving efficiency will be reduced.
A fault detected by the self-diagnosis function remains in the computer's memory until the fault code is cleared from memory by the corresponding device or when the battery is disconnected.
Adaptive memory
Over a period of time, the electronic control unit «will remember» optimal idle speed for a specific engine - regardless of age, engine condition and engine load, so that the correct idle speed is always maintained. Adaptive idle settings are stored in memory.
Adaptive dimensions and fault codes stored in memory will be lost when the battery is removed from the vehicle. This means that idle quality will deteriorate until the ECM is re-adjusted «will remember» its optimal setting.