Lexus SC300 / Lexus SC400. Manual - part 940

NG

OK

OK

NG

3

Disconnect A/C control assembly connector, check voltage between terminal MGC of A/C con-
trol assembly connector and body ground.

C

OK

P

(2) Remove air conditioner control assembly.

(2) Disconnect air conditioner control assembly con-

nector.

(2) Turn ignition switch on.

Measure voltage between terminal MGC of air condi-
tioner control assembly connector and body ground.

Voltage:

10 – 14 V

Check and replace A/C control assembly.

4

Check for open and short in harness and connector between engine & ECT ECU and A/C control
assembly (See page 

IN–27

)

Repair or replace harness or connector.

Check and replace engine & ECT ECU.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–121

ECU Power Source Circuit

CIRCUIT DESCRIPTION

When the ignition switch is turned on, battery voltage
is applied to the terminal IGSW 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

See next page for the DIAGNOSTIC CHART.

WIRING DIAGRAM

TR–120

–

ENGINE TROUBLESHOOTING

Circuit Inspection

Check voltage of ECU power source.

Check continuity between terminals E1 and
body ground.

Check voltage of terminal IGSW.

Check IGN fuse.

Check ignition switch.

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

Check voltage of terminal M–REL.

Check EFI fuse.

Check EFI main relay.

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

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

Repair or replace harness or
connector.

Replace EFI main relay.

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

Check and replace ECU.

Replace ignition switch.

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

Repair or replace harness or
connector.

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

TR–39

).

DIAGNOSTIC CHART

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–121

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

1.

Connect the Check Harness A. (See page 

TR–34

)

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
(See page 

TR–39

).

2

Check for open in harness and connector between terminal E1 of engine (& ECT) ECU and

body ground. (See page 

IN–27

).

Repair or replace harness or connector.

3

Check voltage between terminal IGSW of engine (& ECT) ECU connector and body ground.

C

OK

P

Turn ignition switch on.

Measure voltage between terminal IGSW of engine
(& ECT) ECU connector and body ground.

Voltage:  10 – 14 V

Go to step 

.

TR–122

–

ENGINE TROUBLESHOOTING

Circuit Inspection