Lexus SC300 / Lexus SC400. Manual - part 6

[2] SYMPTOM CONFIRMATION AND DIAGNOSTIC CODE CHECK
The diagnostic system in the LEXUS SC400 fulfills various functions. The first function is the Diagnostic 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.
The air conditioner system has an Actuator Check function whereby the ECU automatically operates the actua-
tors of the damper and blowermotor, etc. to check the operation. The cruise control system has a Cancel Signal
Check function which memorizes and displays what sort of signal it was that last cancelled the cruise control.
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 SC400.

System

Diagnostic Code

Check

Input Signal Check

(Sensor Check)

Other Diagnosis

Function

Engine

 (with Test Mode)

Automatic Transmission

 (with Test Mode)

Anti–Lock Brake

Traction Control (Option)

SRS Airbag

Power Seat (only vehicles with memory function)

Wireless Door Lock Control

Cruise Control

Cancel Signal

Check

Air Conditioner

Actuator Check

In diagnostic code check, it is very important to determine whether the problem indicated by the diagnostic 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 code is directly related to the prob-
lem symptom or not. For this reason, the diagnostic 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, depend-
ing on the case, result in unnecessary troubleshooting for normally operating systems, thus making it more diffi-
cult to locate the problem, or in repairs not pertinent to the problem. Therefore, always follow the procedure in
correct order and perform the diagnostic code check.

DIAGNOSTIC CODE CHECK PROCEDURE

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INTRODUCTION

How to Troubleshoot ECU Controlled Systems

Taking into account the above points, a flow chart showing how to proceed with troubleshooting using the diag-
nostic code check is shown below. This flow chart shows how to utilize the diagnostic code check effectively,
then by carefully checking the results, indicates how to proceed either to diagnostic code troubleshooting or to
troubleshooting  of problem symptoms.

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INTRODUCTION

How to Troubleshoot ECU Controlled Systems

IN–19

[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 envi-
ronment in which the problem occurred in the customer’s vehicle. No matter now 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 engine is cold, or for a problem
which occurs due to vibration caused by the road during driving, etc., the problem 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) are likely causes for problems which are difficult to reproduce, the symptom
simulation tests introduced here are effective measures in that the external 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 symptoms
before starting this test and connect a tester beforehand. After that, carry out the symptom simulation test, judg-
ing whether the circuit being tested is defective or normal and also confirming the problem symptoms at the
same time. Refer to the matrix chart of problem symptoms for each system to narrow down the possible causes
of the symptom.

[1

] VIBRATION METHOD: 

When vibration seems to be the major cause.

CONNECTORS

Slightly shake the connector vertically and horizon-
tally.

WIRE HARNESS

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 thor-
oughly.

PARTS AND SENSORS

Apply slight vibration with a finger to the part of the
sensor considered to be the problem cause and
check if the malfunction occurs.
HINT: Applying strong vibration to relays may result
in open relays.

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INTRODUCTION

How to Troubleshoot ECU Controlled Systems

[2

] HEAT METHOD: 

When the problem seems to occur when the suspect area is heated.

Heat the component that is the likely cause of the malfunc-
tion with a hair dryer or similar object. Check to see if the
malfunction occurs.
NOTICE:
(1)

Do not heat to more than 60

°

C (140

°

F).

(Temperature limit that no damage is done to the
component).

(2)

Do not apply heat directly to parts in the ECU.

[3

] WATER SPRINKLING METHOD: 

When the malfunction seems to occur on a

                                                                   rainy day or in a high–humidity condition.

Sprinkle water onto the vehicle and check to see if the mal-
function occurs.
NOTICE:
(1)

Never sprinkle water directly into the engine
compartment, but indirectly change the tempera-
ture and humidity by applying water spray onto
the radiator front surface.

(2)

Never apply water directly onto the electronic
components.

(Service hint)
If a vehicle is subject to water leakage, the leaked water
may contaminate the ECU. When testing a vehicle with a
water leakage problem, special caution must be used.

[4

] OTHER: 

When the malfunction seems to occur when electrical load is excessive.

Turn on all electrical loads including the heater blower,
head lights, rear window defogger, etc. and check to see
if the malfunction occurs.

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INTRODUCTION

TITLE–NAME

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