VEHICLE DIAGNOSTICS : Engine Management

Modern vehicles have complex engine management systems designed to increase engine efficiency and reduce harmful exhaust gas emissions. The engine control unit compares values from input sensors to calculate;

  • Driver demand from
  • Throttle position sensor, brake pedal switch, clutch switch,etc

  • Position and rotational speed of mechanical engine components from
  • Crankshaft position sensor , Cam position sensor

  • Engine Temperature :
  • from intake air temperature sensor,coolant temperaure sensor, cylinder head temperature sensor

  • Engine speed and load
  • from Mass air flow sensor (MAF), manifold absolute pressure sensor (MAP or vacuum) , turbocharger boost pressure sensor,

  • Exhaust emissions
  • detected from (Lambda or Oxygen sensor) measuring Oxygen content of exhaust gas before catalytic converter.

  • Exhaust Gas Recirculation (EGR)
  • Monitoring of Exhaust gas recirculation flow , position monitoring of flap or valve.

    Engine Fault Codes

    The engine control unit of a modern vehicle is capable of condition monitoring of system components. Faults may be logged by the engine management control unit and stored in memory , which can later be read using diagnostic code reading tool. Faults may be indicated to the vehicle driver by MIL engine management light illumination on instrument panel.

    Test and Repair

    A scan for fault codes is only the beginning of a repair to identify what the engine management computer detects as a fault by comparison to programmed parameters. Further testing may involve wiring checks with multimeter , signal testing with oscilloscope, pressure testing with guages.

    Following testing, a faulty wiring circuit may be repaired or component cleaned or replaced.

    Diagnostic Equipment Functions

    We have vehicle diagnostic equipment capable of testing Car, Van, Commercial HGV vehicle, Construction plant and Tractor Engines

    Engine fault diagnostic testing equipment is capable of

  • Fault code reading
  • Actuator testing
  • Actuator testing is used to verify wiring circuits and component operation by operating suspect component. The actuator test function can allow some components to be thoroughly cleaned which may save replacement cost.

    Programming and Coding

    Components such as electronic fuel injectors , throttle bodies etc may need coding adaptation to the vehicle ecu .

    Advanced system testing

    Advanced system testing can be carried out using oscilloscope signal test methods,/p>

    Reset Procedures

    Following repair work the fault code memory of the vehicle computer will need to be reset to clear stored fault codes.

    Common Faults : Sensors and Actuators

    Mechanical Position and Speed sensors :

    Camshaft position sensor :

    Function : Identify cylinder phasing. A faulty cam position sensor may prevent engine starting

    Crankshaft (CKP)position sensor

    Function : Identification of engine crankshaft position and to allow ecu calculation of engine speed and combustion forces. often AC signal with Frequency and amplitude signal proportional to engine rotational speed.

    Typical failure : Signal errors can occur due to failed sensor , phonic wheel damage, or wiring connection faults . cranksensor.jpg

    Vehicle speed sensor or VSS

    Function: Input to control unit network to sense vehicle speed which may be taken from gearbox sender, axle speed sender,or antilock braking wheel speed sensor. Vehicle speed signal may be used at several modules eg engine control unit, speedometer, tachograph, also for vehicle tracking

    Driver Input Sensors

    Throttle position sensor (TPS)

    Accelerator Position sensor (APP) function: Driver demand signal. TPS failure may occur due to wear of potentiometer track causing driverability issues such as flat spots and hesitation when accelerating.

    Brake Pedal Position & Clutch Pedal Position sensors

    Function : Stabilise idle speed and fuel delivery quantity when engine is off load . Faults may occur due to worn out switches resulting in false (implausible) signals.

    Temperature Sensors

    >Intake Air Temperature Sensor

    IAT may be incorporated with air mass meter , with MAP sensor (T-MAP) or seperate.

    Coolant Temperature Sensor

    Function: Engine temperature measurement by engine coolant temperature. May be used to provide signal to control unit to activate cooling fan or adjust fuelling from cold start to warm up. Temperature signal is processed to compensate engine smooth running by control of fuel delivery  egr position, & diesel pre heat function.

    Flow Sensors

    Mass Air Flow (MAF) sensor

    Function: Measures volume of air entering engine air intake and combustion chambers.

    Air Mass Meter

    Function: Measures weight of air entering intake tract. Engine load sensor which can effect fuel metering (petrol engine) or EGR position (diesel engines) . Typical fault code P0100

    Exhaust gas recirculation ( EGR )

    EGR system functions to reduce combustion temperature and oxides of Nitrogen ( Note : not Nitrous oxide !) EGR and Idle speed control ( ISC ) devices are particularly prone to failure due to build up of carbon and soot deposits which can restrict the air inlet system of the engine. EGR systems may cause the MIL warning lamp to illuminate intermittently and flag a P0400 EOBD fault code due to "critical system malfunction" as described on certain diagnostic equipment !

    Pressure and Vacuum Sensors

    Manifold Absolute Pressure

    Function: MAP sensor measures engine load by vacuum (petrol engines) or boost pressure (turbocharged engines) at intake manifold. Split or blocked pipes can effect operation. MAP signal can influence fuel control, EGR, and turbo boost.

    Fuel Delivery System

    Low and high pressure fuel injection pumps, pressure regulator, fuel injectors. Prone to faults caused by fuel contamination , and general wear.

    Boost pressure control :

    Turbocharger waste gate position and hence boost pressure control is often actuated by electrical solenoids controlling valves in vacuum circuits.

    Boost pressure control Common faults:

  • Loss of vacuum due to vacuum pump wear, split or blocked vacuum pipes
  • Electrical circuit faults with control,solenoid or wiring.
  • Exhaust Emissions Gas Sensor

    Lambda Sensor (fitted to Petrol & LPG fuel systems)

    Voltage signal to ecu allowing measurement of oxygen content of exhaust gas. Used as a feedback signal to control fuel quantity delivery.

    Lambda Sensor Faults: Can deteriorate due to age and contamination causing slow switching or total failure. Lambda sensor fault codes may relate to problems other than the sensor itself.

    Lambda sensor operation can be affected by sensor temperature

    Lambda sensor feedback signal is only used when engine operating temperature is sufficient to enable closed loop control status.

    A second lambda sensor may be fitted in the exhaust system after the catalytic converter to monitor condition of the exhaust catalyst.

    Actuators

    Fuel injectors

    EGR valve

    Throttle Motor

    Solenoids

    Turbocharger wastegate controller