Lexus RX300 (MCU15). Service manual - part 223

A14522

Switch

Instrument Panel J/B

B

W–R

B–O

W–L(*1)

IGN

1

6

3

12

1C

1C

7

6

1G

2C

2B

2B

2B

2I 1

6

5

4

5

1

2

3

5

EFI

EFI
Relay

W

1

3

F11
FL Block

W

Battery

EC

Engine ECU

J/C

A

A

J4

E9

17

16

E7

A

J10

E1

Engine Room
J/B

B–W

2

8

IGSW

MREL

B+

1G

J2
J/C

ED

E9

E9

I15 Ignition

BR

A

J/C

A

BR

W–R(*2)

(*1): LHD
(*2): RHD

W–B

B–R

B–R

AM2

DI–124

–

DIAGNOSTICS

ENGINE

LEXUS RX300   (RM785E)

ECU Power Source Circuit

CIRCUIT DESCRIPTION

When the ignition switch is turned ON, battery positive voltage is applied to terminal IGSW of the engine ECU
and the EFI main relay (Marking: EFI) control circuit in the engine ECU sends a signal to terminal MREL of
the engine ECU switching on the EFI main relay.
This signal causes current to flow to the coil, closing the contacts of the EFI main relay and supplying power
to terminal +B of the engine ECU.
If the ignition switch is turned off, the engine ECU continues to switch on the EFI main relay for a maximum
of 2 seconds for the initial setting of the IAC valve.

WIRING DIAGRAM

DI6EA–02

A02040

ON

E1

+B

(–)

(+)

A02344

ON

IGSW

(+)

(–)

–

DIAGNOSTICS

ENGINE

DI–125

LEXUS RX300   (RM785E)

INSPECTION PROCEDURE

1

Check voltage between terminals +B and E1 of engine ECU connectors.

PREPARATION:
(a)

Remove the engine ECU with connector still connected
(See page FI–70).

(b)

Turn the ignition switch ON.

CHECK:
Measure the voltage between terminals +B and E1 of the en-
gine ECU connectors.
OK:

Voltage: 9 – 14 V

OK

Proceed to next circuit inspection shown on
problem symptoms table (See page DI–21).

NG

2

Check for open in harness and connector between terminal E1 of engine ECU
and body ground (See page IN–32).

NG

Repair or replace harness or connector.

OK

3

Check voltage between terminal IGSW of engine ECU connector and body
ground.

PREPARATION:
(a)

Remove the engine ECU with connector still connected
(See page FI–70).

(b)

Turn the ignition switch ON.

CHECK:
Measure the voltage between terminal IGSW of the engine
ECU connector and body ground.
OK:

Voltage: 9 – 14 V

OK

Go to step 6.

NG

F05273

A10577

Instrument Panel J/B

IGN Fuse

A02345

ON

MREL

(+)

(–)

DI–126

–

DIAGNOSTICS

ENGINE

LEXUS RX300   (RM785E)

4

Check IGN fuse.

PREPARATION:
Remove the IGN fuse from the instrument panel J/B.
CHECK:
Check the continuity of the IGN fuse.
OK:

Continuity

NG

Check for short in all harness and components
connected to IGN fuse.

OK

5

Check ignition switch (See page BE–26).

NG

Replace ignition switch.

OK

Check and repair harness and connector be-
tween battery and ignition switch, and igni-
tion switch and engine ECU.

6

Check voltage between terminal MREL of engine ECU and body ground.

PREPARATION:
(a)

Remove the engine ECU with connector still connected
(See page FI–70).

(b)

Turn the ignition switch ON.

CHECK:
Measure the voltage between terminal MREL of engine ECU
and body ground.
OK:

Voltage: 9 – 14 V

–

DIAGNOSTICS

ENGINE

DI–127

LEXUS RX300   (RM785E)

NG

Check and replace engine ECU 
(See page IN–32).

OK

7

Check EFI fuse (See page DI–113, step 2).

NG

Check for short in all harness and components
connected to EFI fuse.

OK

8

Check EFI main relay (Marking: EFI) (See page FI–55).

NG

Replace EFI main relay.

OK

9

Check for open and short in harness and connector between terminal MREL of
engine ECU and body ground (See page IN–32).

NG

Repair or replace harness or connector.

OK

Check and repair harness or connector be-
tween EFI fuse and battery.