Electronic and Computer Systems Service

1. Electronic and Computer Systems Service Chapter 46

2. 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 • Diagnose computer wiring problems • Diagnose and replace a computer

3. Introduction • 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. Breakout Box • Diagnoses problems • Probes inserted into pin holes access sensors and actuators • Reads raw system values

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. Load Sensors • Include MAP, vacuum, and MAF • Tell computer how much air is entering engine • Affect ignition timing and air-fuel ratios

23. 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

24. 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

25. Throttle Position Sensor • Potentiometer mounted on throttle shaft • Defective or misadjusted TPS causes hesitation when accelerating • Check with voltmeter or ohmmeter

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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