Lexus SC300 / Lexus SC400. Service manual — part 9

(b) Voltage Check

In a circuit in which voltage is applied (to the ECU connector terminal), an open circuit can be checked for
by conducting a voltage check.
(1) As shown in Fig. 4, with each connector still connected, measure the voltage between body ground and

terminal 1 of connector (A) at the ECU 5V output terminal, terminal 1 of connector (B), and terminal
1 of connector (C), in that order.

If the results are:
5 V: Between Terminal 1 of connector (A) and Body Ground
5 V: Between Terminal 1 of connector (B) and Body Ground
0 V: Between Terminal 1 of connector (C) and Body Ground
then it is found out that there is an open circuit in the wire harness between terminal 1 of (B) and termi-
nal 1 of (C).

2.

SHORT CIRCUIT CHECK
If the wire harness is ground shorted as in Fig. 5, locate the section by conducting a ”continuity check with
ground”.

IN–30

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INTRODUCTION

How to Troubleshoot ECU Controlled Systems

(a) Continuity Check with Ground
(1)

Disconnect connectors (A) and (C) and measure the resistance between terminals 1 and 2 of connector
(A) and body ground.
In the case of Fig. 6,
Between terminal 1 of connector (A) and body ground –> Continuity
Between terminal 2 of connector (A) and body ground –> No continuity (open)
Therefore, it is found out that there is a short circuit between terminal 1 of connector (A) and terminal 1 of
connector (C).

(2)

Disconnect connector (B) and measure the resistance between terminal 1 of connector (A) and body
ground, and terminal 1 of connector (B) and body ground.
Between terminal 1 of connector (A) and body ground –> No continuity (open)
Between terminal 1 of connector (B) and body ground –> Continuity
Therefore, it is found out that there is a short 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–31

Check and Replace ECU

First check the ECU ground circuit. If it is faulty, repair it. If it is nor-
mal, the ECU could be faulty, so replace the ECU with a known
good one and check if the symptoms appear.

(1)

Measure the resistance between the ECU ground terminal
and the body ground.

Resistance: 1

or less

(2)

Disconnect the ECU connector, check the ground terminals
on the ECU side and the wire harness side for bend and
check the contact pressure.

IN–32

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INTRODUCTION

How to Troubleshoot ECU Controlled Systems

VEHICLE LIFT AND SUPPORT
LOCATIONS

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INTRODUCTION

Vehicle Lift and Support Locations

IN–33