LEXUS SC400 1991 Service Repair Manual

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Igniter Circuit
CIRCUIT DESCRIPTION
The air conditioner control assembly monitors the engine speed through sign\
als sent from the igniter.
The air conditioner control assembly uses these signals and compressor speed signals to detect the
compressor lock condition.
DIAGNOSTIC CHART
WIRING DIAGRAM
Check operation of tachometer.
Check harness and connector between air
conditioner control assembly and igniter
(See page IN-27).
Proceed to combination meter
troubleshooting (See page
BE-125 ).
Proceed to next circuit inspection
shown on matrix chart (See page AC-36).
Repair or replace harness or
connector.
AC±82±
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Check power transistor.
Check that the tachometer operates normally.
Proceed to combination meter trou-
bleshooting (See page BE-125).
Proceed to next circuit inspection shown on matrix
chart (See page AC-36).
Repair or replace harness or connector.
Check for open and short in harness and connector between air
conditioner control assembly and igniter (See page IN-27).
INSPECTION PROCEDURE
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AIR CONDITIONING SYSTEM TroubleshootingAC±83
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Compressor Circuit
CIRCUIT DESCRIPTION
The air conditioner control assembly outputs the magnetic clutch ON signal \
from terminal MGC to the
Engine & ECT ECU.
When the Engine & ECT ECU receives this signal, it sends a signal from terminal A\
CMG and switches
the air conditioner magnetic clutch relay on, thus turning on the air condi\
tioner compressor magnetic
clutch.
The air conditioner control assembly also monitors at terminal A/C IN wheth\
er or not power is being sup-
plied to the magnetic clutch.
DIAGNOSTIC CHART
See next page for the Diagnostic Chart.
WIRING DIAGRAM
AC±84±
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Check voltage between terminals A/C IN of air
conditioner control assembly connector and
body ground .
Check A/C compressor magnetic clutch.
Check harness and connector between
A/C compressor and compressor relay
(See page IN-27).
Proceed to next circuit inspection
shown on matrix chart (See page AC-36).
Check voltage between terminals MGC of air
conditioner control assembly connector and
body ground .
Check voltage between terminals MGC of air
conditioner control assembly harness side con-
nector and body ground .
Check harness and connector between
air conditioner control assembly and
engine & ECT ECU (See page iN±27).
Check and replace engine & ECT ECU.
Check magnetic clutch relay.
Repair A/C compressor magnetic
clutch.
Repair or replace harness or
connector.
Check and replace air conditioner
control assembly.
Repair or replace harness or
connector.
Replace magnetic clutch relay.
Check voltage between terminals ACMG of
engine & ECT ECU and body ground .
Check harness and connector between
engine & ECT ECU and battery (See page
IN-27).
Check and replace engine & ECT ECU.
Repair or replace harness or
connector.
Check harness and connector between
air conditioner control assembly and
compressor relay, compressor relay and
battery (See page IN±27 ).
Repair or replace harness or
connector.
Check and replace air conditioner control
assembly.
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AIR CONDITIONING SYSTEM TroubleshootingAC±85
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Check voltage between terminals A/C IN of air conditioner control
assembly connector and body ground.
(1) Remove console upper panel.(See pageBO-111 ).
(2) Remove A/C control assembly with
connectors still connected.
(3) Start the engine.
Check voltage between terminals A/C IN of air condi-
tioner control assembly and body ground when mag-
netic clutch is on and off by A/C switch
Go to step (4).
Check air conditioner compressor magnetic clutch.
Disconnect magnetic clutch connector.
Connect positive
lead connected to battery to mag-
netic clutch connector terminal 3.
Magnetic clutch is energized.
Repair air conditioner compr essor magnetic
clutch.
Check for open and short in harness and connecter between air
conditioner compressor and compressor relay (See page IN-27).
Repair or replace harness or connector.
Proceed to next circuit inspection shown on ma-
trix chart (See page AC-36).
INSPECTION PROCEDURE
AC±86±
AIR CONDITIONING SYSTEM Troubleshooting
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Check voltage between terminals MGC of air conditioner control assem-
bly connector and body ground.
(1) Remove console upper panel.(See page BO-111 ).
(2) Remove A/C control assembly with connec±
tors still connected.
(3) Start the engine.
Check voltage between terminals MGC of air condi-
tioner control assembly connector and body ground
when magnetic clutch is on and off by A/C
switch.
Go to step (7).
Check voltage between terminals MGC of air conditioner control assem-
bly harness side connector and body ground.
(1) Disconnect air conditioner control assemblyconnector.
(3) Turn ignition switch ON.
Check voltage between terminals MGC of air condi-
tioner control assembly harness side connector and
body ground.
Voltage: 4 ± 6 V
Check and replace air conditioner control
assembly.
Check for open and short in harness and connector between
air conditioner control assembly and engine & ECT ECU (See page IN-27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
±
AIR CONDITIONING SYSTEM TroubleshootingAC±87
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Check magnetic clutch relay.
Remove magnetic clutch relay from R/B No. 2
Check continuity between each pair of terminals
shown below of magnetic clutch relay.
(1) Apply battery voltage between terminals 2 and 3
(2) Check continuity between terminals 4 and 1
Replace magnetic relay.
(1) Remove engine & ECT ECU with connectors still connected.
(2) Turn ignition switch ON.
(1) Push one of the fan speed control switches
(Lo, Med or Hi).
(2) Measure voltage between terminal ACMG of engine & ECT ECU connector and body
ground.
Go to step (10).
Check voltage between terminal ACMG of engine & ECT ECU and
body ground.
Check for open and short in harness and connector between engine &
ECT ECU and battery (See page IN-27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
Check for open and short in harness and connector between air conditioner control
assembly and compressor relay, compressor relay and battery (See page IN-27).
Repair or replace harness or connector.
Check and replace air conditioner control a ssembly.
AC±88±
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Water Valve VSV Circuit
CIRCUIT DESCRIPTION
If the target air mix damper position is on the hot side beyond a predeter\
mined level, the A/C control
assembly turns ON Tr inside the A/C control assembly.
This turns the water valve VSV ON so that engine coolant flows to the he\
ater core.
If the target air mix damper position is on the cool side beyond a prede\
termined level, the A/C control
assembly OFF Tr inside the A/C control assembly.
This turns OFF the VSV and stops circulation of engine coolant to the he\
ater core, thus increasing the
cooling performance.
DIAGNOSTIC CHART
Check voltage between terminal WV of A/C
control assembly connector and body
ground .
Check water valve VSV.
Check for open and short in harness
and connector between VSV and A/C
control assembly .
Check and replace A/C control assembly.
Replace water valve VSV.
Repair or replace harness or
connector.
Proceed to next circuit inspection
shown on matrix chart
(See page
AC-36).
WIRING DIAGRAM
AC±90±
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Check voltage between terminal WV of A/C control assembly connec-
tor body ground.
(1) Remove console upper panel.(See page
BO-111).
(2) Remove A/C control assembly with connec±
tors still connected.
(3) Start the engine.
Measure voltage between terminals WV of A/C con-
trol assembly connector and body ground
when the set temp. is not set to MAX COLD and
MAX HOT.
Proceed to next circuit inspection shown on
matrix chart. ( See page AC-36).
Check water valve VSV.
Water valve VSV connector.
Measure resistance between terminals of VSV con-
nector.
Resistance: 37 ± 47
at 20°C (68°F)
Check operation of water valve VSV when battery
voltage is applied to terminals of VSV connector or
not.

Battery voltage is applied:
The air from A is flowing out through B.

Battery voltage is not applied:
The air from A is flowing out through air filter.
Replace water valve VSV.
Check for open and short in harness and connector between VSV
and A/C control assembly (See page IN-27).
Repair or replace harness or connector.
Check and replace A/C control assembly.
INSPECTION PROCEDURE
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AIR CONDITIONING SYSTEM TroubleshootingAC±91
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Temperature Control Switch Circuit
CIRCUIT DESCRIPTION
By turning the temp. control switch, the set temperature of the
A/C can be adjusted in steps of 1
°F (USA) or 0.5 °C (Canada).
Turning the switch fully will set the A/C to MAX. COLD or MAX.
HOT.
The ECU calculates the set temperature from the combination
of Hi (5 V) or Low (0 V) terminals SET 1 ~ 5 of the temp. control
SW and displays the set temperature on the LCD display.
If a malfunction occurs in the temp. control SW circuit, malfunc-
tions such as not being able to set the desired temperature oc-
cur.
DIAGNOSTIC CHART
WIRING DIAGRAM
Check for open and short in harness and
connector between A/C control assembly and
temp. control SW.
Check temp. control switch.
Proceed to next circuit inspection
shown on matrix chart (See page
AC-36 ).
Replace temp. control switch.
Repair or replace harness or
connector.
Check and replace A/C control assembly.
Check voltage between terminal SET 1 ~ 5
and SG of A/C control assembly.
AC±92±
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