Lexus SC300 / Lexus SC400. Service manual - part 8

IN–26

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INTRODUCTION

How to Troubleshoot ECU Controlled Systems

HOW TO USE THE DIAGNOSTIC CHART AND INSPECTION
PROCEDURE

1. 

For troubleshooting, diagnostic code charts or problem symptom charts are provided for each circuit with
detailed inspection procedures on the following pages.

2. 

When all the component parts, wire harnesses and connectors of each circuit except the ECU are found
to be normal in troubleshooting, then it is determined that the problem is in the ECU.
Accordingly, if diagnosis is performed without the problem symptoms occurring, the instruction will be to
check and replace the ECU, even if the problem is not in the ECU. So, always confirm that the problem
symptoms are occurring, or proceed with inspection while using the symptom simulation method.

3. 

The instructions ”Check wire harness and connector” and ”Check and replace ECU” which appear in the
inspection procedure, are common and applicable to all diagnostic codes. Follow the procedure outlined
below whenever these instructions appear.

Check Wire Harness and Connector

The problem in the wire harness or connector is an open circuit or a short circuit.
OPEN CIRCUIT:
This could be due to a disconnected wire harness, faulty contact in the connector, a connector terminal pulled
out, etc.

HINT:
1. 

It is rarely the case that a wire is broken in the middle of it. Most cases occur at the connector. In
particular, carefully check the connectors of sensors and actuators.

2. 

Faulty contact could be due to rusting of the connector terminals, to foreign materials entering
terminals or a drop in the contact pressure between the male and female terminals of the connector.
Simply disconnecting and reconnecting the connectors once changes the condition of the
connection and may result in a return to normal operation.

Therefore, in troubleshooting, if no abnormality is found in the wire harness and connector check, but the prob-
lem disappears after the check, then the cause is considered to be in the wire harness or connectors.
SHORT CIRCUIT:
This could be due to a short circuit between the wire harness and the body ground or to a short inside the switch,
etc.
HINT:

•

When there is a short between the wire harness and body ground, check thoroughly whether the wire
harness is caught in the body or is clamped properly.

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INTRODUCTION

How to Troubleshoot ECU Controlled Systems

IN–27

1. 

CONTINUITY CHECK (OPEN CIRCUIT CHECK)
(1) Disconnect the connectors at both ECU and sensor sides.
(2) Measure the resistance between the applicable terminals

of the connectors.

Resistance: 1 

 or less

HINT:

•

Measure the resistance while lightly shaking the wire
harness vertically and horizontally.

•

When tester probes are inserted into a connector, insert
the probes from the back. For waterproof connectors in
which the probes cannot be inserted from the back, be
careful not to bend the terminals when inserting the
tester probes.

2. 

RESISTANCE CHECK (SHORT CIRCUIT CHECK)
(1) Disconnect the connectors at both ends.
(2) Measure the resistance between the applicable terminals

of the connectors and body ground. Be sure to carry out
this check on the connectors on both ends.

Resistance: 1 M

 or higher

HINT: Measure the resistance while lightly shaking the wire
harness vertically and horizontally.

3. 

VISUAL CHECK AND CONTACT PRESSURE CHECK
(a)  Disconnect the connectors at both ends.
(b)  Check for rust or foreign material, etc. on the terminals

of the connectors.

(c)  Check crimped portions for looseness or damage and

check if the terminals are secured in the lock position.

HINT: The terminals should not come out when pulled lightly.

(d)  Prepare a test male terminal and insert it in the female

terminal, then pull it out.

HINT: When the test terminal is pulled out more easily than
others, there may be poor contact in that section.

IN–28

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INTRODUCTION

How to Troubleshoot ECU Controlled Systems

Actual examples of the inspection method for open circuit and short circuit are explained below.
1. 

OPEN CIRCUIT CHECK
For the open circuit in the wire harness in Fig. 1, perform ”(a) Continuity Check” or ”(b) Voltage Check” to
locate the section.

(a) Continuity 

Check

(1)

Disconnect connectors (A) and (C) and measure the resistance between them.
In the case of Fig. 2,
Between terminal 1 of connector (A) and terminal 1 of connector (C) –> No continuity (open)
Between terminal 2 of connector (A) and terminal 2 of connector (C) –> Continuity
Therefore, it is found out that there is an open circuit between terminal 1 of connector (A) and terminal 1
of connector (C).

(2)

Disconnect connector (B) and measure the resistance between connectors (A) and (B), (B) and (C).
In the case of Fig. 3,
Between terminal 1 of connector (A) and terminal 1 of connector (B) –> Continuity
Between terminal 1 of connector (B) and terminal 1 of connector (C) –> No Continuity (open)
Therefore, it is found out that there is an open circuit between terminal 1 of connector (B) and terminal 1
of connector (C).

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INTRODUCTION

How to Troubleshoot ECU Controlled Systems

IN–29