Lexus SC300 / Lexus SC400. Manual - part 837

OK

NG

OK

NG

4

Check IGN Fuse.

C

OK

P

Remove IGN fuse from J/B No.1.

Check continuity of IGN fuse.

Continuity.

Check for short in all the harness and components con-
nected to IGN fuse (See attached wiring diagram).

5

Check ignition switch.

C

OK

P

Remove under cover and finish panel.

Check continuity between terminals.

continuity

Replace ignition switch.

Check and repair harness and connector between
battery and ignition switch, ignition switch and en-
gine & ECT ECU.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–127

OK

NG

OK

NG

6

Check voltage between terminal M–REL of engine & ECT ECU connector and body ground.

C

OK

P

Turn ignition switch on.

Measure voltage between terminal M–REL of engine &
ECT ECU connection and body ground.

Voltage:

10 – 14 V

Check and replace engine & ECT ECU.

7

Check EFI

C

OK

P

Remove EFI fuse R/B No.2.

Check continuity of EFI fuse.

Continuity

Rep

TR–128

–

ENGINE TROUBLESHOOTING

Circuit Inspection

OK

NG

OK

NG

8

Check EFI main relay.

C

OK

P

C

OK

Remove EFI main relay.

Check continuity between terminals of EFI main relay
shown below.

(2) Apply battery voltage between terminals 1 and 3.

(2) Check continuity between terminals 2 and 4.

Replace EFI main relay.

9

Check for open and short in harness and connector between terminal M–REL of engine & ECT
ECU and body ground (See page 

IN–27

).

Repair or replace harness or connector.

Check and repair harness or connector between
EFI fuse and battery.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–129

Check EFI fuse.

Check voltage of terminal BATT.

Check operation for the back–up.

Proceed to next circuit inspection
shown on matrix chart (See page 

TR–39

).

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

Check and repair harness or connector
between battery, EFI fuse and ECU.

Check and replace fuse.

WIRING DIAGRAM

 Back Up Power Source Circuit

CIRCUIT DESCRIPTION

Battery voltage is supplied to terminal BATT of the ECU even when the ignition switch is off for use
by the diagnostic code memory and air–fuel ratio adaptive control value memory, etc.

DIAGNOSTIC CHART

TR–126

–

ENGINE TROUBLESHOOTING

Circuit Inspection