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Electronic and Computer Systems Service. Chapter 46. Objectives. Diagnose related engine and electrical problems prior to computer repair Describe the theory and operation of on-board diagnostics Read trouble codes Use a scan tool Confirm closed loop Test sensors and actuators

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Electronic and Computer Systems Service

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  • Diagnose related engine and electrical problems prior to computer repair
  • Describe the theory and operation of on-board diagnostics
  • Read trouble codes
  • Use a scan tool
  • Confirm closed loop
  • Test sensors and actuators
  • Diagnose computer wiring problems
  • Diagnose and replace a computer
  • Computer systems have become sophisticated
    • Necessary to consult model-specific service information
      • Step-by-step procedures for troubleshooting
    • Today's technicians must be able to read a wiring schematic
      • In order to diagnose and repair modern computerized vehicles
inspection sequence
Inspection Sequence
  • Computers have self-diagnostic ability
    • Logical diagnosis sequence must be followed before checking computer
  • Digital multimeter
    • Used to measure electricity in electronic circuits
  • Visual inspection
    • Can often determine problem cause
perform diagnostic tests
Perform Diagnostic Tests
  • Analyze the cause of the problem rather than just fixing the problem’s result
    • Listen during cranking for an even rhythm and then for a smooth idle
    • Check base timing setting on engines with a distributor ignition
    • Do a charging system test before beginning a diagnostic procedure
on board diagnostics
On-Board Diagnostics
  • Computers detect incorrect electrical conditions
    • Save trouble codes to memory
      • Key is turned on: computer does a self-check of its circuits
  • Diagnostic tree
    • Provides a step-by-step diagnostic procedure
  • Sensors
    • Cause electronic control problems more often than actuators
reading trouble codes
Reading Trouble Codes
  • Different ways to read trouble codes
    • OBD II systems have standardized connectors and procedures
    • Most systems have a diagnostic link connector (DLC)
      • Scan tool can be connected to it to read codes
      • Procedure for retrieving fault codes varies
scan tools
Scan Tools
  • Portable computer
    • Reads data from the on-board computer
    • Have specific software cartridges
    • Handheld and can be taken on a road test
    • Limited to diagnosing computer problems
  • Communication between scan tool and computer
    • Unidirectional or bidirectional
  • Parameter identification data
    • Included in on-board diagnostics
breakout box
Breakout Box
  • Diagnoses problems
    • Probes inserted into pin holes access sensors and actuators
    • Reads raw system values
retrieving trouble codes
Retrieving Trouble Codes
  • OBD I
    • Scan tools were not so widely owned by automotive technicians
  • OBD II scan tools
    • Powered through the DLC
    • Do not disconnect or connect while the key is on
    • Remove before removing connections to electronic components
working with codes
Working with Codes
  • More than one code is given
    • Fix the lower number code first
    • Fix the problem first and then start again
    • Be sure to check power and grounds
    • Erase codes and test drive the car to reset codes
  • OBD II cars
    • Scan tool must be used to read codes
erasing trouble codes
Erasing Trouble Codes
  • Code can remain in memory even though a problem has been corrected
    • Clear codes after the repair has been made
      • Older vehicles: procedure that shuts off power to computer can be used to erase codes
      • OBD II vehicles: scan tool command required
      • Manufacturer's methods vary
  • Scan tool
    • Erases codes without disconnecting anything
    • Test drive car to see if any codes return
scan tool snapshot
Scan Tool Snapshot
  • Scan tool has a feature like an airplane flight recorder
    • Helps catch glitches and intermittent problems
  • Settings
    • Snapshot: series of pictures representing the conditions present when the DTC was set
    • Automatically record when any fault code occurs during the test drive
    • Freeze frame
closed loop
Closed Loop
  • Computers require correct inputs from sensors and correct actions from actuators
    • Several things must occur for a computer system to go into closed loop
  • Methods for confirming closed loop vary
    • Examples: DMM, scan tool, and a lab scope
    • Do not use ohmmeter to test an oxygen sensor
fuel trim diagnosis
Fuel Trim Diagnosis
  • Diagnosing fuel trim with scan tool
    • Drive vehicle under same conditions where problem occurred
    • Restricted fuel filter or low fuel pump output increase fuel trim under load
    • Leaks that allow air into intake system result in higher fuel trim values
    • Plugged or sticking fuel injector affects fuel trim cells equally as rpm increases
digital waveforms
Digital Waveforms
  • Waveform
    • Better diagnostic tool than digital volt-ohmmeter
  • Tools capable of displaying voltage or frequency in waveform
    • Digital storage oscilloscopes
    • Graphing multimeters
    • Four channel scopes
      • Can display four waveforms
logic probe power probe
Logic Probe/Power Probe
  • Three colored LEDs instead of one bulb
    • Touching probe to ground lights green LED
    • Red LED illuminates when touched to a power source
    • Yellow light comes on when a pulsed voltage is sensed
sensor and actuator testing
Sensor and Actuator Testing
  • Sensor testing strategy
    • No-start condition occurs if a distributor reference or crankshaft sensor signal is lost
    • Defective or misadjusted TPS can send an excessive voltage to computer as WOT
    • With the key on, do not disconnect any electrical components unless instruction says to do this
diagnosing sensor problems
Diagnosing Sensor Problems
  • When using a scan tool to diagnose sensors and actuators, use the following sequence:
    • Check input sensors
    • Perform a quick check of input switches
    • Check outputs
  • Sensor types
    • High authority sensors
    • Passive sensors
    • Active sensors
sensor tests
Sensor Tests
  • Vehicle speed sensor
    • Supplies input for electronic speedometers and cruise control systems
      • Also controls torque converter clutch
  • Types of speed sensors
    • Photoelectric and magnetic AC generator
  • Failed sensor can cause:
    • Premature or no converter clutch lockup
    • Lack of change in steering assist
    • Inoperative cruise control and speedometer
oxygen sensor
Oxygen Sensor
  • Enriches mixture so reduction catalyst can work
    • Must provide a lean enough mixture for hydrocarbons and carbon monoxide to oxidize
  • Characteristics
    • Start-up varies
    • Lazy sensor produces voltage slowly and does not change back and forth
    • Range is tested by creating full rich and full lean conditions
    • Wide range oxygen sensors can accurately detect air-fuel ratios over a wide range
load sensors
Load Sensors
  • Include MAP, vacuum, and MAF
    • Tell computer how much air is entering engine
    • Affect ignition timing and air-fuel ratios
map sensor and baro sensors
MAP Sensor and BARO Sensors
  • MAP sensor
    • Basic fuel delivery to the engine is determined by MAP sensor, CKP, and ECT
    • Engine load is high: fuel injectors are on longer
    • Vacuum higher: MAP sensor voltage drops
  • BARO sensors
    • Monitor changes in weather or altitude
    • Several different types
    • Defective sensor causes poor high-altitude performance or spark knock
vacuum sensors
Vacuum Sensors
  • Measure difference between atmospheric pressure and intake manifold pressure
    • Systems that use vacuum sensors must also use BARO sensors
    • BMAP is a combination barometric and MAP sensor
throttle position sensor
Throttle Position Sensor
  • Potentiometer mounted on throttle shaft
    • Defective or misadjusted TPS causes hesitation when accelerating
      • Check with voltmeter or ohmmeter
coolant temperature sensor and air temperature sensors
Coolant Temperature Sensor and Air Temperature Sensors
  • Coolant temperature sensor
    • Affects how the engine operates in all conditions
    • Common problem: computer system will not go into closed loop when engine is warm
    • Use ohmmeter or voltmeter to test
  • Air temperature sensors
    • IAT sensor works like a coolant temperature sensor
      • Fine tunes air-fuel mixture
      • Compensates for air density
airflow sensor service
Airflow Sensor Service
  • Fuel systems controlled by an airflow sensor
    • React poorly to vacuum leaks
    • Dirt causes problems in vane airflow sensor
    • Intake manifold popback causes the door to bend or break
  • MAF sensors have no moving parts
    • Hot film MAF sensors produce a variable frequency instead of voltage
knock sensor service
Knock Sensor Service
  • Help prevent engine knock
    • Loose bracket or other vibration causes retarded timing
    • Computer senses an inoperative knock sensor: P0324 through P0334 code will set
    • Broken or damaged knock sensor wiring is often the cause of a knock sensor code
    • Test by rapping on the engine near sensor with a metal tool
    • Many engines use a newer style of knock sensor, called a resonance knock sensor
actuator service
Actuator Service
  • Actuators include:
    • Solenoids
    • Fuel injectors
    • Stepper motors
    • Motors for electronic suspension hydraulic controls
  • Test an actuator
    • Done by checking for voltage at actuator control terminal
    • Test according to individual service instructions
repair the problem
Repair the Problem
  • After repairing problem
    • Road test vehicle again
      • Test drive allows a late-model computer to relearn its best adjustments
    • Use scan tool to erase codes
  • After replacement of the computer or when a battery has been disconnected
    • Poor drivability and performance can result until computer relearns best drivability settings
computer wiring service
Computer Wiring Service
  • Common cause of problems in computer systems
    • Poor electrical connections
      • Include loose or corroded connections and grounded wires
  • Always use a wiring diagram when working on computer systems
    • Computer must have good power and ground connections
    • Twisted pair wiring carries very small amounts of current
computer wiring service cont d
Computer Wiring Service (cont’d.)
  • Electronic updates
    • Regular occurrence among manufacturers
  • Battery voltage must be stable during reprogramming
    • Can take an hour or more to complete
  • Computer location
    • Usually mounted in driver’s compartment
static electricity
Static Electricity
  • Static electricity from the front seat is a concern
    • People who work around sensitive components sometimes wear a ground strap
    • Touch ground before touching computer
    • Do not take the computer out of container until you are already in the front seat
electrical damage to a circuit
Electrical Damage to a Circuit
  • Too much electrical current
    • Causes heat that damages an electrical circuit
    • Damaged connections are usually the reason for failure
    • Semiconductors are designed for only a limited amount of current
    • Bus diagnosis is similar to other electrical system diagnoses