Lexus SC300 / Lexus SC400. Manual - part 170

WIRING DIAGRAM

Position sensor check.

OK

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

BE–276

).

NG

Check position sensor.

NG

OK

NG

OK

Replace position sensor.

Repair or replace harness or
connector.

Check harness and connectors between
ECU and position sensors.  (See page

IN–27

).

Check and replace ECU.

Position Sensor Circuit

CIRCUIT DESCRIPTION

The position sensor senses movement of the seat and send
pulse signals to the ECU. The position sensor sends pulses
to the ECU in proportion to the amount of seat movement, as
shown in the diagram on the left.
The ECU uses the number of pulses to constantly calculate
the position relative to the memory position and returns the
seat to the memorized position.
If a malfunction occurs in a position sensor and a sensor sig-
nal is not input to the ECU even when the motor operates, the
ECU prohibits return operation.

DIAGNOSTIC CHART

DIAGNOSTIC CHART

BE–294

–

BODY ELECTRICAL SYSTEM

Power Seat Control System (Driver’s Seat)

INSPECTION PROCEDURE

1

Position sensor check

C

P

(1) Remove the driver’s seat.

(2) Remove the bolts for the front vertical adjuster and

lift the seat cushion up a little.

Notice:   If the seat cushion is lifted to high, the har-
ness will be pulled and the clamp may come loose.

(3) Remove the ECU with the connector attached from

where it is installed to the underside of the seat
cushion.

(1) Connect terminal CHK of the ECU to body ground

to put the ECU into check mode.

(2) Measure the voltage between terminal SO of ECU

and body ground.

HINT:  Use an analog type meter.

(3) Check that the “ready” code is output as shown in

the illustration.

–

BODY ELECTRICAL SYSTEM

Power Seat Control System (Driver’s Seat)

BE–295

1

Position sensor check (Cont’d)

OK Code

NG Code

OK

Hint

Hint

(4) Operate the power seat switch and check the

change in the voltage when the seat is moving.

The lumbar support does not have a position sensor, so
the sensor check cannot be done.

(5) The changes in the output voltage when the input

signal is normal and when it is abnormal are shown
in the illustrations.  Compare the results with those
from (4) and diagnose the condition of each circuit.

The OK and NG codes continue to be output while
the manual switch is ON.

When the seat reaches the limit of movement, e.g.
when the slide position reaches the most forward
or back position, the voltage changes from the OK
code to the NG code.

After operating any of the system functions with no
problem and confirming the OK code and NG code
by comparing the amount of wavering of the 
voltmeter needle, inspect the function with a 
problem.

The amount of wavering of the voltmeter needle
depends on the meter.

Output

OK Code

NG Code

OK

NG

BE–296

–

BODY ELECTRICAL SYSTEM

Power Seat Control System (Driver’s Seat)

OK

NG

OK

NG

2

Check position sensor.

C

OK

Hint

P

Disconnect the connector of the sensor and the connec-
tor of the motor leading to the sensor.

(1) Connect positive 

 lead to terminal 3 of sensor and

negative 



 lead to terminal 2.

(2) Measure voltage between terminal 1 of sensor and

body ground when battery voltage is applied be-
tween terminals 1 and 2 of motor connector.

When the battery voltage is applied to the motor con-
nector terminals, 

 and 



 are interchangeable.

Condition

Voltage

Motor stopped

0 V or battery
voltage ac-
cording to stop
position

Motor Turning

(

Check several times with

the motor in a different
position each time

.

)

half of battery
voltage

Replace position sensor.

3

Check for open and short in harness and connectors between ECU and position sensors (See

page 

IN–27

).

Repair or replace harness or connectors.

Check and replace ECU.

–

BODY ELECTRICAL SYSTEM

Power Seat Control System (Driver’s Seat)

BE–297