Lexus SC300 / Lexus SC400. Service manual - part 25

Diag. Code 31, 41

Air Mix Damper Position Sensor Circuit

CIRCUIT DESCRIPTION

This sensor detects the position of the air mix
damper and sends the appropriate signals to the
air conditioner control assembly.
The position sensor is built into the air mix servo-
motor assembly.

Code No.

Diag. Code Detecting Condition

Trouble area

31

Short to ground or power source circuit in air
mix damper position sensor circuit.

Air mix damper position sensor

Harness or connector between air mix
servomotor assembly and A/C control

41

Air mix damper position sensor value does
not change even if A/C control assembly
operates air mix servomotor.

servomotor assembly and A/C control
assembly.

A/C control assembly

DIAGNOSTIC CHART

Check voltage between terminals TP and

SG of  air conditioner control assembly 

connector.

Check air mix damper position sensor.

Check harness and connector between air

conditioner control assembly and air mix
servomotor assembly (See page 

IN-27

).

Check and replace air conditioner control 

assembly.

Proceed to next circuit inspection

shown on matrix chart (See page

AC-36

). However, when Diag.

code31 or 41 is displayed, check

and replace air conditioner control 

assembly.

Replace air mix servomotor 
assembly.

Repair or replace harness or

connector.

AC–58

–

AIR CONDITIONING SYSTEM

Troubleshooting

Check  voltage between terminals TP and SG of air conditioner control

assembly connector.

(1)

Remove console upper panel.
(See page 

BO–111

).

(2)

Remove A/C control assembly with connec–
tors still connected.

(3)

Turn ignition switch ON.

Change the set temperature to activate the air mix
damper, and measure the voltage between termi-
nals TP and SG of air conditioner control assem-
bly connector each time when the set temperature
is changed.

Set Temperature

Voltage

Max. cool

Max. hot

3.70– 4.27 V

0.88 – 1.16 V

In addition, as the set temperature increases,
the voltage decreases gradually without inter-
ruption.

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

AC-36

).However,

when Diag. code 31 or 41 is displayed, check and

replace air conditioner control assembly.

Check air mix damper position sensor.

(1)

Remove heater unit. (See page 

AC-125

)

(2)

Disconnect air mix servomotor assembly 
connector.

Measure resistance between terminals S5 and SG of
air mix servomotor assembly connector.

Resistance:

4.7~7.2  k

While operating air mix servomotor as in the
procedure on page 

AC-61

, measure  resistance

between terminals TP and SG of air mix servo-
motor assembly connector.

Position

Resistance

Max. cool

Max. warm

3.76 ~ 5.76 k

0.94 ~ 1.44 k

As the air mix servomotor moves from cool side
to warm side, the resistance decreases gradually
without interruption.

Replace  air mix servomotor assembly.

Check for open and short in harness and connector between air conditioner
control assembly and air mix servomotor assembly (See page 

IN-27

).

Repair or replace harness or connector.

Check and replace air conditioner control
assembly.

INSPECTION PROCEDURE

–

AIR CONDITIONING SYSTEM

Troubleshooting

AC–59

Diag. Code 41

Air Mix Servomotor Circuit

CIRCUIT DESCRIPTION

The air mix servomotor is controlled by the A/C control assembly and moves the air mix damper to the
desired position.

Code No.

Diag. Code Detecting Condition

Trouble area

41

Air mix damper position sensor value does not
change even if A/C control assembly operates
air mix servomotor.

Air mix servomotor.

Air mix damper position sensor.

Harness or connector between A/C controlas-
sembly and A/M servomotor, A/M damperposi-
tion sensor.

A/C control assembly.

DIAGNOSTIC CHART

Actuator check.

Check air mix servomotor.

Check harness and connector between air

conditioner control assembly and air mix
servomotor assembly (See page 

IN-27

).

Check and replace air conditioner control 

assembly.

Proceed to next circuit inspection

show on matrix chart (See page

AC-36

).

Replace air mix servomotor 
assembly.

Repair or replace harness or

connector.

AC–60

–

AIR CONDITIONING SYSTEM

Troubleshooting

Actuator check.

(1)

Warm up the engine.

(2)

Set to the actuator check mode (See page

AC-30

).

(3)

Press  the  FRS            switch and change it to
step operation.

Press the FRS              switch and check the
operation of the air mix damper and the condition
of the blower.

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

AC-36

).

Check air mix servomotor.

Remove heater unit.

Connect positive 

 lead to terminal 4 and negative



 lead to terminal 5.

The lever turns smoothly to cool side.

Connect negative 



 lead to terminal 4 and positive 

lead to terminal 5.

The lever turns smoothly to hot side.

Replace  air mix servomotor assembly.

Check for open and short in harness and connector between air conditioner
control assembly and air mix servomotor assembly (See page 

IN-27

).

Repair or replace harness or connector.

Check and replace air conditioner control
 assembly.

INSPECTION PROCEDURE

–

AIR CONDITIONING SYSTEM

Troubleshooting

AC–61