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 OBD II cars
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 Types of speed sensors
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
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
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: Test an actuator 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