Lexus SC300 / Lexus SC400. Manual - part 683

OK

NG

OK

NG

INSPECTION PROCEDURE

1

Check output condition of diag. code 51.

C

OK

Hint

P

Setting the test mode.

(2) Turn ignition switch OFF.

(2) Connect terminals TE2 and E1 and TDCL.

(2) Turn ignition switch ON.

(For checking terminal A/C, start the engine.)

(2) Connect terminals TE1 and E1 of TDCL.

Check if code “51” is output by the “CHECK” engine
warning light.

*: Before the STA signal is input (ST is not ON), 

diagnostic code 43 is also output.

Diag. code 42 is output with vehicle speed 5 km/h (3
mph) or below

IDL1 .... Go to step 2    NSW ... Go to page 

TR–22

.

A/C  ..... Go to step 3

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

TR–35

).

2

Check throttle position sensor.

C

OK

P

Disconnect throttle position sensor connector.

Measure resistance between terminals IDL1 and E2 of
throttle position sensor connector.

Adjust or replace throttle position sensor
See page 

FI–73

).

Check and repair harness or connector between
engine & ECT ECU and throttle position sensor.

TR–120

–

ENGINE TROUBLESHOOTING

Circuit Inspection

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

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

OK

NG

Type I

NG

Type II

NG

OK

OK

NG

INSPECTION PROCEDURE

1

Check output condition of diag. code 51.

C

P

(2) Connect terminals TE2 and E1 of TDCL.

(2) Turn ignition switch on.

(2) Crank the engine.

(2) Connect terminal TE1 and E1 of TDCL.

Check if diagnostic code “51” is output when then shift
level is in the P and D shift positions.

Result:

Go to step 2.

Go to step 3.

Proceed to next circuit inspection shown on ma-

trix chart (See page 

TR–35

).

2

Check for open in harness and connector between engine & ECT ECU and neutral start switch

(See page 

IN–27

).

Check and replace engine & ECT ECU.

Repair or replace harness or connector.

3

Check neutral start switch (See page 

AT–100

).

Replace neutral start switch.

Check and replace engine & ECT ECU.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–123