Lexus ES300 (1997 year). Manual - part 491

IN0379

Sensor Side

ECU Side

IN0378

Sensor Side

ECU Side

IN0380

Sensor Side

ECU Side

IN0381

Pull Lightly

Looseness of Crimping

IN–28

–

INTRODUCTION

HOW TO TROUBLESHOOT ECU CONTROLLED 
SYSTEMS

28

1997 LEXUS ES300   (RM511U)

2.

CONTINUITY CHECK (OPEN CIRCUIT CHECK)

(a)

Disconnect the connectors at both ECU and sensor
sides.

(b)

Measure the resistance between the applicable terminals
of the connectors.
Resistance: 1

Ω

 or less

HINT:

Measure the resistance while lightly shaking the wire har-
ness 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.

3.

RESISTANCE CHECK (SHORT CIRCUIT CHECK)

(a)

Disconnect the connectors on both ends.

(b)

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

Ω

 or higher

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

4.

VISUAL CHECK AND CONTACT PRESSURE CHECK

(a)

Disconnect the connectors at both ends.

(b)

Check for rust or foreign material, etc. in the terminals of
the connectors.

(c)

Check crimped portions for looseness or damage and
check that the terminals are secured in lock portion.

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

Prepare a test male terminal and insert it in the female ter-
minal, then pull it out.

NOTICE:
When testing a gold–plated female terminal, always use a
gold–plated male terminal.
HINT:
When the test terminal is pulled out more easily than others,
there may be poor contact in that section.

FI7187

Z17004

C

B

A

OPEN

Sensor

Fig. 1

ECU

1

2

1

2

1

2

2

1

Z17005

Fig. 2

Sensor

ECU

C

B

A

1

2

1

2

1

2

B04722

Fig. 3

ECU

Sensor

2

1

A

B1

C

1

1

2

2

1

2

B2

–

INTRODUCTION

HOW TO TROUBLESHOOT ECU CONTROLLED 
SYSTEMS

IN–29

29

1997 LEXUS ES300   (RM511U)

5.

CONNECTOR HANDLING

When inserting tester probes into a connector, insert them from
the rear of the connector. When necessary, use mini test leads.
For water resistant connectors which cannot be accessed from
behind, take good care not to deform the connector terminals.

6.

CHECK OPEN CIRCUIT

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

(a)

Check the continuity.
(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 resis-
tance between the connectors.
In the case of Fig. 3,
Between terminal 1 of connector ”A” and terminal 1
of connector ”B1” 

→

 Continuity

Between terminal 1 of connector ”B2” 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 ”B2” and terminal
1 of connector ”C”.

Z17007

Fig. 4

Sensor C

A

1

2

1

2

1

2

ECU

B

5 V

5 V

0 V

5 V

Z17008

C

B

A

SHORT

Fig. 5

1

2

1

2

2

1

Z17009

Fig. 6

Sensor

ECU

C

B

A

1

2

1

2

1

2

IN–30

–

INTRODUCTION

HOW TO TROUBLESHOOT ECU CONTROLLED 
SYSTEMS

30

1997 LEXUS ES300   (RM511U)

(b)

Check the voltage.
In a circuit in which voltage is applied (to the ECU connec-
tor terminal), an open circuit can be checked for by con-
ducting a voltage check.
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:
5V: Between Terminal 1 of connector ”A” and Body Ground
5V: Between Terminal 1 of connector ”B” and Body Ground
0V: Between Terminal 1 of connector ”C” and Body Ground
Then it is found out that there is an open circuit in the wire har-
ness between terminal 1 of ”B” and terminal 1 of ”C”.

7.

CHECK SHORT CIRCUIT

If the wire harness is ground shorted as in Fig. 5, locate the sec-
tion by conducting a ”continuity check with ground”.

Check the continuity with ground.

(1)

Disconnect connectors ”A” and ”C” and measure
the resistance between terminal 1 and 2 of connec-
tor ”A” and body ground.
In the case of Fig. 6
Between terminal 1 of connector ”A” and body
ground 

→

 Continuity (short)

Between terminal 2 of connector ”A” and body
ground 

→

 No continuity

Therefore, it is found out that there is a short circuit
between terminal 1 of connector ”A” and terminal 1
of connector ”C”.

Z17808

Fig. 7

Sensor

ECU

C

A

1

2

1

2

1

2

2

1

B2

B1

IN0383

Example

Ground

IN0384

ECU Side

Ground

W/H Side

Ground

–

INTRODUCTION

HOW TO TROUBLESHOOT ECU CONTROLLED 
SYSTEMS

IN–31

31

1997 LEXUS ES300   (RM511U)

(2)

Disconnect connector ”B” and measure the resis-
tance between terminal 1 of connector ”A” and body
ground, and terminal 1 of connector ”B2” and body
ground.
Between terminal 1 of connector ”A” and body
ground 

→

 No continuity

Between terminal 1 of connector ”B2” and body
ground 

→

 Continuity (short)

Therefore, it is found out that there is a short circuit
between terminal 1 of connector ”B2” and terminal
1 of connector ”C”.

8.

CHECK AND REPLACE ECU

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

(1)

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

Resistance: 1 

Ω

 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.