Motor Control Unit

How Does a Motor Control Unit Work?


    – Characteristics of the motor control unit

    – Role of the motor control unit

    – Maintenance of the motor control unit

The motor control unit is the central element of the motor management system; it is the brain that processes the various information it receives to control the electrical operation of the motor.

Characteristics of the motor control unit

The engine control unit (ECU) has several names depending on the manufacturer: engine control unit (ECU), engine control unit (ECU), etc. It is a metal box containing electronic components (processors, EEPROM, semiconductors, transistors, diode resistors, etc.).

It communicates with external elements via electrical links and has one or more connectors with an average of 110 to 120 channels.

Role of the motor control unit

The motor control unit can be divided into three main stages: reception of input signals, data processing and output.

Input signal reception stage

There are two types of input signals, analog and digital:

    – Analog input signals are generated by inductive type sensors (engine speed sensor, old generation wheel sensor, etc.), pressure sensors, electrochemical sensors (oxygen sensor) or temperature sensors. They are also voltage information (positive supply, ground…). They require an analog/digital converter at the input of the box to be used.

    – The digital input signals are square and have a binary coding (position 0 or 1), allowing the motor box to use them directly. These are hall effect sensors (camshaft sensor, speed sensor) or piezoelectric sensors (knocking sensor, new generation wheel sensors, etc.).

Also included in this category is information from ON / OFF switches (accelerator pedal position sensor, brake pedal, etc.).

Data processing stage

Motor Control Unit

A set of microprocessors allows the processing of information received with the following functions:

    – The computing power that allows the exploitation of data for the elaboration of output signals.

    – Memory storage (EEPROM and flash EEPROM) integrates the engine mapping and its possible reprogramming.

    – The communication of data internally and externally, with the coding and decoding necessary for the use of multiplexed lines (bus of multiple data transmitted in series on the same wire support). It also allows the exchange of data with, mainly, the interior service box (ISB), the central brain of the vehicle.

Output stage

The engine control unit sends control signals to various actuators, including :

    – the injectors for air/fuel metering (injection timing) ;

    – the turbo pressure control, for air intake regulation;

    – anti-pollution functions (EGR valve, particulate filter regeneration).

Note: it also includes onboard diagnostics, software standardized for all manufacturers, memorizing malfunctions imputing engine pollution management.

Maintenance of the engine control unit


Causes of failure

If the engine control unit’s peripheral elements (sensors, actuators, electrical harnesses…) are potential sources of failure, the unit itself is exceptionally reliable.

Replacements are rare and often linked to inappropriate interventions (overvoltage, short circuit, etc.).

The diagnostic methodology involves 1. checking all its peripheral elements, 2. external power supplies, and finally multiplexed links (only if those two diagnostic checks do not reveal any anomalies).

Intervention with the diagnostic tool

Troubleshooting requires the use of a diagnostic tool to communicate with the engine control unit for reading of the memorized defects, deleting faults (after they have been rectified), analysis of parameters and components (temperature, speed, pressures, status, etc.), activation of actuators to check their operation (solenoid valves, relays, injectors, etc.), and resets and updates.

Replacing of the motor control unit

In the event of a failure of the engine control unit, various operations are proposed, such as the replacement of the servo motor by a new one. This operation will then require programming, mainly carried out by the manufacturer’s network (cost of the replacement: approximately from $800 to $1 500). 

A standard exchange usually allows for substantial savings (30 to 50% of the new one). Some companies offer an intervention on the dismantled ECU; an estimate is given to you before the repair, the cost being proportional to the seriousness of the breakdown (generally from $100 to $200).

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