Lexus SC300 / Lexus SC400. Service manual - part 941

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.

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–123

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 connector and body ground.

Voltage:  10 – 14 V

Check and replace engine (& ECT) ECU.

7

Check EFI fuse.

C

OK

P

Remove EFI fuse from R/B No. 2.

Check continuity of EFI fuse.

Continuity

Check for short in all the harness and components
connected to EFI fuse (See attached wiring diagram).

TR–124

–

ENGINE TROUBLESHOOTING

Circuit Inspection

OK

NG

OK

NG

8

Check EFI main relay.

C

OK

P

OK

C

Remove EFI main relay.

Check continuity between terminals of EFI main relay
shown below.

1.

Apply battery positive 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–125

Neutral Start Switch Circuit

CIRCUIT DESCRIPTION

The neutral start switch goes on when the shift lever is in the N or P shift position. When it goes on the terminal
NSW of the ECU is grounded to body ground via the starter relay and theft deterrent ECU, thus the terminal
NSW voltage becomes 0V. When the shift lever is in the D, 2, L or R position, the neutral start switch goes off,
so the voltage of ECU terminal NSW becomes battery voltage, the voltage of the ECU internal power source.
If the shift lever is moved from the N range to the D range, this signal is used for air–fuel ratio correction and
for idle speed control (estimated control), etc.
When the neutral start switch is off, code ”51” is output in the test mode diagnosis. (This is not abnormal.)

DIAGNOSTIC CHART

DIAGNOSTIC CHART

HINT: 

This diagnosis chart is based on premise that the engine is cranked normally. If the engine is not
cranked, proceed to the matrix chart of problem symptoms on page 

TR–35

.

Check output condition of diag. code 51.

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

TR–35

).

Check for open in harness and connector
between neutral start switch and ECU.

Repair or replace harness or connector.

Check and replace ECU.

Check and replace ECU.

Replace neutral start switch.

Check neutral start switch (See page

AT–100

).

TR–122

–

ENGINE TROUBLESHOOTING

Circuit Inspection