Lexus SC300 / Lexus SC400. Manual - part 684

ECU Power Source Circuit

CIRCUIT DESCRIPTION

When the ignition switch is turned on, battery voltage
is applied to the terminal IG SW of the ECU, and the
main relay control circuit in the ECU sends a signal to
the terminal M–REL of the ECU, switching on the
main relay. This signal causes current to flow to the
coil, closing the contacts of the main relay and supply-
ing power to the terminals +B and +B1 of the ECU.
If the ignition switch is turned off, the ECU continues
to switch on the main relay for a maximum of 2 se-
conds for the initial setting of the ISC valve.

DIAGNOSTIC CHART

DIAGNOSTIC CHART

See next page for the DIAGNOSTIC CHART.

WIRING DIAGRAM

TR–124

–

ENGINE TROUBLESHOOTING

Circuit Inspection

DIAGNOSTIC CHART

DIAGNOSTIC CHART

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

Check voltage of terminal M–REL.

Check for open in harness and connector
between battery and ECU.

Check and replace ECU.

Replace ignition switch.

Repair or replace harness or
connector.

Repair or replace harness or
connector.

Check for open and short in harness and
connector between terminal M REL and body
ground.

Replace EFT main relay.

Check voltage of terminal IGSW.

Check IGN fuse.

Check continuity between terminals E1 and
body ground.

Check voltage of ECU power source.

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

TR–35

).

Check ignition switch.

Check EFT fuse.

Check EFT main relay.

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

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

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–125

NG

OK

OK

NG

NG

OK

INSPECTION PROCEDURE

1

Check voltage between terminals + B, + B1 and E1 of engine & ECT ECU connector.

C

OK

P

(2) Connect the Check Harness A.

(See page 

TR–30

)

(2) Turn ignition switch on.

Measure voltage between terminals + B, + B1 and E1
of engine & ECT ECU connector.

Voltage:

10 – 14 V

Proceed to next circuit inspection shown on matrix chart

2

Check for open in harness and connector between terminals E1 of engine & ECT ECU and body
ground (See page 

IN–27

).

Repair or replace harness or connector.

3

Check voltage between terminal IG SW of engine & ECT ECU connector and body ground.

C

OK

P

Turn ignition switch on.

Measure voltage between terminal IG SW of engine &
ECT ECU connector and body ground.

Voltage:

10 – 14 V

Go to step 6.

TR–126

–

ENGINE TROUBLESHOOTING

Circuit Inspection

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