Engine Control Module: Description

The electronic control module (ECM) is mounted to the left lower dash panel and consists of an electronic printed circuit board which is designed as a milliliter board assembly fitted on both sides. The routing of the wiring harness connector at the ECM connector are split into interfering cables and sensitive cables in order to achieve improved electromagnetic compatibility. The smaller wiring harness connector is used for the vehicle wiring harness and the larger harness is used for the engine wiring harness. The ECM stores engine specific data, monitors the connected sensor and analyzes their measurement. See Fig 1.

Its task consists in controlling the following systems in line with the analysis of the input signals:

1. Fuel Supply System
2. Injected Quantity Control
3. Emission Control System
4. Charge Pressure Control
5. Cruise Control
6. A/C Compressor Shut-Off
7. Pre-Heating Output Relay for the Glow Plugs
8. Vehicle Theft
9. Air Bag
10. Monitors inputs/outputs, checks plausibility and stores faults
11. Share information with other control modules
12. Diagnosis

If a sensor should fail, provided the fault is not serious, the ECM will continue to operate the engine in Limp-Home Mode (emergency mode) using a default value for the missing signal. The ECM ensures that, continuing to operate the engine will not cause damage or effect safety, otherwise a Engine shut-off process will be carried out. See Fig 2.

Fig 1: ECM

Courtesy of DAIMLERCHRYSLER CORP.

ECM Control Strategy 

The engine control module is involved with a variety of functions such as:

• Individual injector activation
• Engine idle speed control to ensure smooth engine idling independent of engine load
• Ride comfort function such as anti jerk control: The CDI control module detects irregularities in engine speed (resulting, for example, from load changes or gear shift) from the signal supplied by the crankshaft position sensor and reduces them by adjusting the quantity injected into each of the cylinders
• Constant RPM (high idle feature) for ambulance vehicle bodies equipped with electrical appliances
• Starter control, immobilizer, cruise control, kick down, air conditioner
• Maintenance computer ASSYST (optional)
• Glow plug for pre-heating, post heating and intermittent heating
• Error code memory/diagnostics, communication interface for diagnosis and handling the fault codes
• The maximum vehicle speed is programmable from 19-82 m.p.h. The standard is 82 m.p.h.

See Fig 2

Fig 2: ECM Control

Courtesy of DAIMLERCHRYSLER CORP.

New software has been loaded to the ECM for EGR control. This is due to the wider operating range and larger volume of recirculated gas. There is a considerable number of new, adapted, and optimized functions, particularly with regard to injection, EGR, boost control, sensing of the input parameters and the signaling of the actuators. See Fig 2.

• The rail pressure control achieved by signaling the quantity control valve in the high pressure pump and the pressure regulator results in reduced power consumption of high pressure pump and in lowering fuel pressures
• Individual cylinder torque control for smooth engine running: using the crankshaft position sensor signals, the ECM detect non-uniform engine running results from uneven torque contributions of the individual cylinders and adjust the injection quantities of the individual injectors so that all cylinders make the same torque contribution
• A relay is used for activating the electric in-tank fuel pump
• Heated crankcase ventilation to ensure pressure compensation even at low temperatures
• Improved boost pressure control using an electric variable nozzle turbine actuator with position feed back
• Controlled fuel heating using the high pressure pump closed-loop control
• Translation of the drive input received from the accelerator pedal module which is equipped with dual hall sensors
• Measurement of the intake air mass using new mass air flow (MAF) sensor with increase precision and extended measuring range
• O2 sensor for measuring the amount of oxygen in the exhaust in order to calculate the air to fuel ratio. With the intake air mass being known, the injected fuel quantity can be calculated from the air to fuel ratio
• Activation of the O2 sensor heater to burn off deposits
• Full load EGR with a more precise, model based EGR closed-loop control. The ECM calculates the EGR rate from the various sensor signals. Using the calculate EGR rate in percent instead of the fresh air mass flow as a control parameter enables a more precise control of the EGR rate as well as better correction of the target value.

The oxygen sensor signal can be used in combination with the mass air flow signal, the injection quantity signal and pressure and temperature signals to perform the following functions for optimized closed loop control and monitoring of emissions related components:

• Injection valve quantity drift compensation in partial load range: the oxygen content in the exhaust is calculated from the air mass and from injection quantity signal and is compared to the air-fuel ratio as measured by the sensor. If the calculated air-fuel ratio differs from the measured air-fuel ratio, the is no correction of the injection quantity but the EGR rate and boost pressure are adjusted to the actual injection quantity.
• Injection valve quantity drift compensation in full load range: this function is to limit the maximum injection quantity for engine protection. The injection quantity signal is compared to the injection quantity calculated from the oxygen sensor signal and MAF signal. If the comparison shows that the actual injection quantity is too high, it is limited to the maximum permissible injection quantity
• Air-fuel ratio controlled smoke limiter (full load): the smoke limiter limits the injection quantity on the basis of the air-fuel ratio permissible at the smoke limit depending on the measure mass air flow and the calculated EGR rate. As a consequence, the generation of smoke due to an excess injection quantity is avoided under all operating conditions. At the same time, the oxygen sensor signal is used to ensure that the air-fuel ratio is adjusted accordingly

A function referred to as air flow sensor drift compensation detects and corrects the possible drifting of the MAF sensor by comparing the air mass measured by the MAF with the projected air mass as it is calculated by the ECM in consideration of various influencing conditions. It is the air flow drift compensation that gives the MAF air mass measurement the precision needed to use it for the function mentioned above. The high precision of the MAF measurement enables the calculation of the actual injection quantity from the measured air mass and from the oxygen sensor signal in order to correct injection quantity. The MAF signal can also be used as a input parameter for the smoke limiter.