1991 LEXUS SC400 ignition

Check voltage between terminals TR and SG of air conditioner control
assembly connector.
(1) Remove console upper panel. (See page BO-111 )
(2) Remove A/C Control Assembly with connectors
still connected.
(3) Turn ignition switch ON.
Check voltage between terminals TAM and SG of air
conditioner control assembly connector at each tem-
perature.
Voltage
at 25°C (77°F) : 1.8~2.2 V
at 40°C (104°F) : 1.2~1.6 V
In addition, as the temperature increases, the
voltage decreases gradually.
Proceed to next circuit inspection shown on
matrix chart (See page AC-36).However,
when Diag. code 11 is displayed, check and
replace air conditioner control assembly.
Check room temperature sensor.
(1) Remove instrument finish lower No. 1 panel.
(See page Be-103).
Resistance
at 25°C (77°F) : 1.6~1.8k

at 50°C (122°F) : 0.5~0.7k

In addition, as the temperature increases, the
voltage decreases gradually.
Replace room temperature sensor.
Check for open and short in harness and connector between air conditioner
control assembly and room temperature sensor (See page IN-27).
Repair or replace harness or connector.
Check and repair air conditioner control
assembly.
(2) Disconnect room0 temperature sensor
connector.
Check voltage between terminals 1 and 2 of
room temperature sensor connector at each tem±
perature.
INSPECTION PROCEDURE
±
AIR CONDITIONING SYSTEM TroubleshootingAC±45
WhereEverybodyKnowsYourName

Check voltage between terminals TAM and SG of air conditioner
control assembly connector.
(1) Remove console upper panel. (See page BO-111
(2) Remove A/C control assembly with
connectors still connected.
(3) Turn ignition switch ON.
Check voltage between terminals TAM and SG of air
conditioner control assembly connector at each tem-
perature.
at 25°C (77°F) : 1.35~1.75 V
at 40°C (104°F) : 0.85~1.25 V
In addition, as the temperature increases, the
voltage decreases gradually.
Proceed to next circuit inspection shown on
matrix chart (See page AC-36).However,
when Diag. code 11 is displayed, check and
replace air conditioner control assembly.
Check ambient temperature sensor.
(1) Remove under cover
(2) disconnect ambient temperature sensor connector.
Check resistance between terminals 1 and 2 of
ambient temperature sensor connector at each
temperature.
Resistance
at 25°C (77°F) : 1.6~1.8k

at 50°C (122°F) : 0.5~0.7k

In addition, as the temperature increases, the
voltage decreases gradually.
Replace ambient temperature sensor.
Check for open and short in harness and connector between air conditioner
control assembly and ambient temperature sensor (See page IN-27).
Repair or replace harness or connector.
Check and repair air conditioner control
assembly.
Voltage
INSPECTION PROCEDURE
±
AIR CONDITIONING SYSTEM TroubleshootingAC±47
WhereEverybodyKnowsYourName

(1) Remove console upper panel. (See page BO±111 )
(2) Remove A/C Control Assembly with connectors still connected.
(3) Turn ignition switch ON.
Measure voltage between terminals TE and SG of air
conditioner control assembly connector at each tem-
perature.
Voltage
at 0°C (32°F) : 2.0~2.4 V
at 15°C (59°F) : 1.4~1.8 V
In addition, as the temperature increases, the
voltage decreases gradually.
Proceed to next circuit inspection shown on
matrix chart (See page AC-36).However,
when Diag. code 11 is displayed, check and
replace air conditioner control assembly.
Check voltage between terminals TE and SG of air conditioner control
assembly connector.
Check evaporator temperature sensor.
Remove evaporator temperature sensor(See page
AC-124 ).
Check resistance between terminals 1 and 2 of evap-
orator temperature sensor connector at each temper-
ature.
In addition, as the temperature increases, the
voltage decreases gradually.
Replace evaporator temperature sensor.
Check for open and short in harness and connector between air conditioner
control assembly and evaporator temperature sensor (See page IN-27).
Repair or replace harness or connector.
Check and replace air conditioner control
assembly.
Resistance
at 15°C (59°F) : 2.1~2.6k

at 0°C (32°F) : 4.6~5.1k

INSPECTION PROCEDURE
±
AIR CONDITIONING SYSTEM TroubleshootingAC±49
WhereEverybodyKnowsYourName

Check voltage between terminals TW and SG of air conditioner control
assembly connector.
(1) Remove console upper panel. (See page BO±111 )
(2) Remove A/C Control Assembly with connectors
still connected.
(3) Turn ignition switch ON.
Measure voltage between terminals TW and SG of air
conditioner control assembly connector at each tem-
perature.
Voltage
at 0°C (32°F) : 2.8~3.2 V
at 40°C (104°F) : 1.8~2.2 V
at 70°C (158°F) : 0.9~1.3 V
In addition, as the temperature increases, the
voltage decreases gradually.
Proceed to next circuit inspection shown on
matrix chart (See page AC-36).However,
when Diag. code 14 is displayed, check and
repair air conditioner control assembly.
Check water temperature sensor.
(1) Remove heater unit.
(2) Remove water temperature sensor.
Measure resistance between terminals 1 and 3 of
water temperature sensor connector at each temper-
ature.
Resistance
at 0°C (32°F) : 16.5~17.5 k

at 40°C (104°F) : 2.4~2.8 k

at 70°C (158°F) : 0.7~1.0 k

In addition, as the temperature increases, the
voltage decreases gradually.
Replace water temperature sensor.
Check for open and short in harness and connector between air conditioner
control assembly and water temperature sensor (See page IN-27).
Repair or replace harness or connector.
Check and replace air conditioner control
assembly.
INSPECTION PROCEDURE
±
AIR CONDITIONING SYSTEM TroubleshootingAC±51
WhereEverybodyKnowsYourName

99
Check voltage between terminals S5 and TS 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.Measure voltage between terminals S5 and TS
of air conditioner control assembly connector
when the solar sensor is subjected to an electric
light, the sensor is covered by a cloth.
ConditionVoltage
Sensor subjected to electric light
Sensor covered by a cloth
Below 4 V
4 ±4.5 V
In addition, when the inspection light is gradu-
ally moved away from the sensor, the voltage
increases.
Proceed to next circuit inspection shown on
matrix chart (See page AC-36).However,
when Diag. code 21 is displayed, check and
replace air conditioner control assembly.
Check solar sensor.
(1) Remove solar sensor from dashboard.
(2) disconnect solar sensor connector.
Connect positive (+) lead of ohmmeter to terminal 2
and negative (±) lead to terminal 1 of the solar sensor.
(1) Cover the sensor by a cloth.
(2) Measure resistance between terminals 1 and 2 of solar sensor connector.
(1) Remove the cloth from the solar sensor andsubject the sensor to electric light.
(2) Measure resistance.
Resistance: Approx, 4 k
(continuity)
In addition, as the electric light is moved gradu-
ally away from the sensor the resistance in-
creases.
Replace solar sensor.
Check for open and short in harness and connector between air conditioner
control assembly and solar sensor (See page IN-27).
Repair or replace harness or connector.
Check and replace air conditioner control
assembly.
INSPECTION PROCEDURE
±
AIR CONDITIONING SYSTEM TroubleshootingAC±53
WhereEverybodyKnowsYourName

9
Check voltage between terminals LP of air conditioner control
assembly and body ground.
(1) Install the manifold gauge set.(See page AC-15).
(2) Turn ignition switch ON.
Check voltage between terminals LP of air conditioner
control assembly connector and body ground when the
air conditioner gas pressure is
changed.
The voltage changes with gas pressure, as shown
in the diagram below.
Low Pressure Cut Side
High pressure Cut Side
Proceed to next circuit inspection shown on
matrix chart (See page AC-36).
Check pressure switch.
Disconnect pressure switch connector.
Check continuity between terminals 1 and 4 of pres-
sure switch when air conditioner gas pressure is
changed.
The continuity changes with gas pressure as
shown below.
Low Pressure Cut Side
High pressure Cut Side
Replace pressure switch
Check for open and short in harness and connector between air conditioner control
assembly and pressure switch, pressure switch and body ground (See page IN-27).
Repair or replace harness or connector.
Check and replace air conditioner control
assembly.
INSPECTION PROCEDURE
±
AIR CONDITIONING SYSTEM TroubleshootingAC±57
WhereEverybodyKnowsYourName

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 TemperatureVoltage
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 TroubleshootingAC±59
WhereEverybodyKnowsYourName

Check voltage between terminals TP1 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.
Press REC/FRS switch to change the air inlet
between fresh and recirculation air, and measure
voltage between terminals TP1 and SG of air
conditioner control assembly when the air inlet
servomotor operates.
In addition, as the air inlet servomotor is moved
from REC side to FRS side, the voltage de-
creases gradually without interruption.
Proceed to next circuit inspection shown on matrix chart
(See page AC-36).However, when Diag. code 32 or 42 is
displayed, check and replace air conditioner control assem-
bly.
Check air inlet damper position sensor.
(1) Remove heater unit. (See page AC-127)
(2) Disconnect air inlet servomotor assembly
connector.
Measure resistance between terminals S5 and SG of
air inlet servomotor assembly connector.
Resistance: 4.7~7.2 k

While operating air inlet servomotor as in the
procedure on page AC-65, measure resistance
between terminals TP1 and SG of air inlet servo-
motor assembly connector.
Resistance:
As the air inlet servomotor moves from REC side
to FRS side, the resistance decreases gradually
without interruption.
Replace air inlet servomotor assembly.
Check for open and short in harness and connector between air conditioner
control assembly and air inlet servomotor assembly (See page IN-27).
Repair or replace harness or connector.
Check and replace air conditioner control assembly.
INSPECTION PROCEDURE
±
AIR CONDITIONING SYSTEM REPAIR INSTRUCTIONSAC±63
WhereEverybodyKnowsYourName