Lexus RX300 (MCU15). Service manual - part 78

Important Points in the Customer Problem Analysis

D What ––––– Vehicle model, system name
D When ––––– Date, time, occurrence frequency
D Where ––––– Road conditions
D Under what conditions? ––––– Running conditions, driving conditions, weather conditions

D How did it happen? ––––– Problem symptoms

(Sample) Engine control system check sheet.

ENGINE CONTROL SYSTEM Check Sheet

Customer’s Name

Driver’s Name

Data Vehicle

Brought in

License No.

Model and Model
Year

Frame No.

Engine Model

Odometer Reading

km
miles

Problem Symptoms

Engine does
not Start

Difficult to
Start

Poor Idling

Poor
Drive ability

Engine Stall

Others

Engine does not crank

No initial combustion

No complete combustion

Engine cranks slowly
Other

Incorrect first idle

Idling rpm is abnormal

High (                    rpm)

Low (                    rpm)

Rough idling

Other

Hesitation

Back fire

Muffler explosion (after–fire)

Surging

Knocking

Other

Soon after starting

After accelerator pedal depressed

After accelerator pedal released

During A/C operation

Shifting from N to D

Other

Data Problem

Constant

Sometimes (          times per          day/month)

Inspector’s
Name

CUSTOMER PROBLEM ANALYSIS CHECK

–

INTRODUCTION

HOW TO TROUBLESHOOT ECU CONTROLLED 
SYSTEMS

IN–23

LEXUS RX300   (RM785E)

1.

CUSTOMER PROBLEM ANALYSIS

In troubleshooting, the problem symptoms must be confirmed accurately and all preconceptions must be
cleared away in order to give an accurate judgment. To ascertain just what the problem symptoms are, it is
extremely important to ask the customer about the problem and the conditions at the time it occurred.
Important Point in the Problem Analysis:
The following 5 items are important points in the problem analysis. Past problems which are thought to be
unrelated and the repair history, etc. may also help in some cases, so as much information as possible should
be gathered and its relationship with the problem symptoms should be correctly ascertained for reference
in troubleshooting. A customer problem analysis table is provided in Diagnostics section for each system
for your use.

DIAGNOSTIC TROUBLE CODE CHECK PROCEDURE

Diagnostic Trouble 
Code Check (Make a
note of and then clear)

Confirmation
of Symptoms

Diagnostic Trouble
Code Check

Problem Condition

Diagnostic Trouble 
Code Display

Problem symptoms
exist

Same diagnostic
trouble code is
displayed

Problem is still occurring in the diagnostic
circuit

Normal code is
displayed

The problem is still occurring in a place
other than in the diagnostic circuit
(The diagnostic trouble code displayed
first is either for a past problem or it is a
secondary problem)

No problem 
symptoms exist

The problem occurred in the diagnostic
circuit in the past

Normal Code Display

Problem symptoms
exist

Normal code is 
displayed

The problem is still occurring in a place
other than in the diagnostic circuit

No problem 
symptoms exist

Normal code is 
displayed

The problem occurred in a place other
than in the diagnostic circuit in the past

IN–24

–

INTRODUCTION

HOW TO TROUBLESHOOT ECU CONTROLLED 
SYSTEMS

LEXUS RX300   (RM785E)

2.

SYMPTOM CONFIRMATION AND DIAGNOSTIC TROUBLE CODE CHECK

The diagnostic system in the LEXUS RX300 fulfills various functions. The first function is the Diagnostic
Trouble Code Check in which a malfunction in the signal circuits to the ECU is stored in code in the ECU
memory at the time of occurrence, to be output by the technician during troubleshooting. Another function
is the Input Signal Check which checks if the signals from various switches are sent to the ECU correctly.
By using these check functions, the problem areas can be narrowed down quickly and troubleshooting can
be performed effectively. Diagnostic functions are incorporated in the following systems in the LEXUS
RX300.

System

Diagnostic Trouble

Code Check

Input Signal Check

(Sensor Check)

Diagnostic Test Mode

(Active Test)

Engine

Automatic Transaxle

ABS & BA & TRC & VSC System

Supplemental Restraint System

Theft Deterrent System

Cruise Control System

Engine Immobiliser System

Multiplex Communication System

LEXUS Navigation System

Air Conditioning System

f

(with Check

Mode)

f

(with Check

Mode)

f

f

f

f

f

f

f

f

f

f

f

f

f

f

In diagnostic trouble code check, it is very important to determine whether the problem indicated by the diag-
nostic trouble code is still occurring or occurred in the past but returned to normal at present. In addition,
it must be checked in the problem symptom check whether the malfunction indicated by the diagnostic
trouble code is directly related to the problem symptom or not. For this reason, the diagnostic trouble codes
should be checked before and after the symptom confirmation to determine the current conditions, as shown
in the table below. If this is not done, it may, depending on the case, result in unnecessary troubleshooting
for normally operating systems, thus making it more difficult to locate the problem, or in repairs not pertinent
to the problem. Therefore, always follow the procedure in correct order and perform the diagnostic trouble
code check.

Diagnostic trouble code check

Making a note of and clearing of the diagnostic trouble codes displayed

Symptom confirmation

No problem symptoms
exist

Problem symptoms 
exist

Simulation test using the symptom 
simulation methods

D Normal code displayed
D Problem symptoms exist

D Normal code displayed
D No problem symptoms exist

Diagnostic trouble code check

Troubleshooting of problem indicated
by diagnostic trouble code

D Diagnostic trouble code displayed
D Problem symptoms exist

System Normal

Troubleshooting of each 
problem symptom

If a diagnostic trouble code was 
displayed in the initial diagnostic
trouble code check, it indicates
that the trouble may have occurred
in a wire harness or connector in
that circuit in the past. Therefore,
check the wire harness and con-
nectors (See page IN–32).

–

INTRODUCTION

HOW TO TROUBLESHOOT ECU CONTROLLED 
SYSTEMS

IN–25

LEXUS RX300   (RM785E)

Taking into account the points on the previous page, a flow chart showing how to proceed with troubleshoot-
ing using the diagnostic trouble code check is shown below. This flow chart shows how to utilize the diagnos-
tic trouble code check effectively, then by carefully checking the results, indicates how to proceed either to
diagnostic trouble code troubleshooting or to troubleshooting of problem symptoms table.

V07268

VIBRATION METHOD: When vibration seems to be the major cause.

CONNECTORS

WIRE HARNESS

PARTS AND SENSOR

1

Slightly shake the connector vertically and horizontally.

Slightly shake the wire harness vertically and horizontally.
The connector joint, fulcrum of the vibration, and body
through portion are the major areas to be checked thorough-
ly.

Apply slight vibration with a finger to the part of the sensor
considered to be the problem cause and check that the mal-
function occurs.

Shake Slightly

Swing Slightly

Vibrate Slightly

HINT:
Applying strong vibration to relays may result in open relays.

IN–26

–

INTRODUCTION

HOW TO TROUBLESHOOT ECU CONTROLLED 
SYSTEMS

LEXUS RX300   (RM785E)

3.

SYMPTOM SIMULATION

The most difficult case in troubleshooting is when there are no problem symptoms occurring. In such cases,
a thorough customer problem analysis must be carried out, then simulate the same or similar conditions and
environment in which the problem occurred in the customer’s vehicle. No matter how much experience a
technician has, or how skilled he may be, if he proceeds to troubleshoot without confirming the problem
symptoms he will tend to overlook something important in the repair operation and make a wrong guess
somewhere, which will only lead to a standstill. For example, for a problem which only occurs when the en-
gine is cold, or for a problem which occurs due to vibration caused by the road during driving, etc., the prob-
lem can never be determined so long as the symptoms are confirmed with the engine hot condition or the
vehicle at a standstill. Since vibration, heat or water penetration (moisture) is likely cause for problem which
is difficult to reproduce, the symptom simulation tests introduced here are effective measures in that the ex-
ternal causes are applied to the vehicle in a stopped condition.
Important Points in the Symptom Simulation Test:
In the symptom simulation test, the problem symptoms should of course be confirmed, but the problem area
or parts must also be found out. To do this, narrow down the possible problem circuits according to the symp-
toms before starting this test and connect a tester beforehand. After that, carry out the symptom simulation
test, judging whether the circuit being tested is defective or normal and also confirming the problem symp-
toms at the same time. Refer to the problem symptoms table for each system to narrow down the possible
causes of the symptom.