Electronic and Computer Systems Service

© 2012 Delmar, Cengage Learning

Electronic and Computer Systems Service

Chapter 46

© 2012 Delmar, Cengage Learning

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

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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

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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

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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

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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

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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

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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

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Breakout Box• Diagnoses

problems– Probes inserted

into pin holes access sensors and actuators

– Reads raw system values

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Load Sensors• Include MAP, vacuum, and MAF

– Tell computer how much air is entering engine– Affect ignition timing and air-fuel ratios

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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

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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

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Throttle Position Sensor• Potentiometer mounted on throttle shaft

– Defective or misadjusted TPS causes hesitation when accelerating

• Check with voltmeter or ohmmeter

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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

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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

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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

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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

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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

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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

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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

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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

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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

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