ECU LEXUS SC400 1991 Service Owner's Guide

A14±1
A14±2
A14±3
A14±4
A14±5
A14±6
A14±7
A14±8
A14±9
A14±10
A14±11
A14±12
A14±13
A14±14
A12±1
A14±15
A12±2
A12±3
A12±4
A12±5
A12±6
A12±7
A12±8
A12±9
A12±10A13±25
A13±24
A13±23
A13±22
A13±21
A13±20
A13±19
A13±18
A13±17
A13±16
A13±15
A13±14
A13±13
A13±12
A13±11
A13±10
A13±9
A13±8
A13±7
A13±6
A13±5
A13±A13±4
A13±3
A13±2
A13±1
TERMINALS OF ECU
±
AIR CONDITIONING SYSTEM TroubleshootingAC±31
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STANDARD VALUE OF ECU TERMINAL
TerminalsSymbolsWiringColorConditionStandardValue
A14±1A14±7B ER W±BIG OFF10 ~ 14 V
A14±2 A14±3MH MCV R±YIG OFF13 ~ 19

A14±4A14±5MFRSMRECG G±RIG OFF13 ~ 19

A14±6A14±7IG+ ER±L W±BIG ON10 ~ 14 V
Body
A14±7 
Ground Body
E 
Ground Body
W±B 
GroundAlwaysBelow 1

A14±8A14±9FACE DEFW Y±LIG OFF13 ~ 19

A14±15A14±7ACC EL±R W±BTurn ignition switch ACC10 ~ 14 V
A12±2 A14±7REOST EW±G W±B
IG ON and turn headlight control SW
to TAIL position.
Turn Rheostat volume from most
right to left position.
Most
: 0 ~ 14 V
Right

Most
: 10 ~ 14 V
Left
A12 3 A14 7WV EBY WB
Start
the
MAX
Temp. Control Switch:
COLDBelow 1 V
A12±3 A14±7WV EB±Y W±Btheen-
gine MAX
Temp. Control Switch:
HOT10 ~ 14 V
A12 4 A14 7MGC ELR WB
Start the
engine andA/C switch ONBelow 1 V
A12±4 A14±7MGCEL±R W±Bengine andpush AUTO
SWA/C switch OFF10 ~ 14 V
A12 5 A14 7TC EL WBIG ONHLC SW
Rheostat Most
:
Volume RightBelow 1 V
A12±5 A14±7TCEL W±BH.L.C. SW
: TAIL positionRheostat Turn
:
Volume Left10 ~ 14 V
A12±6 A14±7TAIL EG W±BTurn the light control switch to TAILposition10 ~ 14 V
A12±7A14±7LV EYW±BIG ONPush blower SW to change
the blower speed from 1 to
10.Speed 1: 10 ~ 14 V

Speed 10: Below 2 V
A12±8 A14±7BLR EB±R W±BIG ONTurn the blower motorBelow 1.5 V
A12±9 A14±7HR EL±W W±B
IG ON
Turn the blower motorBelow 1 V
IG ONDon't turn the blower motor10 ~ 14 V
A12 10 A14 7RDFGR ERY WBIG ON
Rear defogger switch ONBelow 1 V
A12±10 A14±7RDFGR ER±Y W±BIG ONRear defogger switch OFF10 ~ 14 V
A13±1 A14±7IGN EBW±BStart the engine10 ~ 14 V
AC±32±
AIR CONDITIONING SYSTEM Troubleshooting
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STANDARD VALUE OF ECU TERMINAL (Cont'd)
AC±34±
AIR CONDITIONING SYSTEM Troubleshooting
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AC±68AC±70AC±72AC±74AC±76AC±78AC±80AC±82AC±84AC±90AC±92AC±96AC±98AC±10
8IN±32AC±100AC±103AC±10
4AC±12
0AC±118AC±122AC±12
5AC±126
HINT:
wIf the instruction ºProceed to next circuit inspection shown on matrix \
chartº is given in the flow chart for each
circuit, proceed to the circuit with the next highest number in the table to con\
tinue the check.
w If the trouble still reappears even though there are no abnormalities in a\
ny of the other circuits, then check
or replace the air conditioner control assembly (contains the ECU).
±
AIR CONDITIONING SYSTEM TroubleshootingAC±37
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Diag. Code 23Compressor Lock Sensor Circuit
CIRCUIT DESCRIPTION
The pressure switch sends the appropriate signals to the air conditioner co\
ntrol assembly when the air
conditioner refrigerant pressure drops too low or rises too high. When the air cond\
itioner control assem-
bly receives these signals, it outputs signals via the Engine & ECT ECU to switch off the compressor
relay and turns the magnet clutch off.
Code No.Diag. Code Detecting ConditionTrouble area
23

Open in pressure sensor circuit.

Abnormal refrigerant pressure.
below 206 kPa (2.1 kgf/cm
2, 30 psi)
over 2,650 kPa (27 kgf/cm2, 385 psi)

Pressure switch.

Harness or connector between pressure switch
and ECU. Refrigerant pipe line.

ECU.
DIAGNOSTIC CHART
Check voltage between terminals LP of air
conditioner control assembly and body
ground.
Check pressure switch.
Check harness and connector between air
conditioner control assembly and pressure
switch and body ground
(See page IN-27).
Check and replace air conditioner control
assembly.
Proceed to next circuit inspection
shown on matrix chart (See page
AC-36 ).
Replace pressure switch
Repair or replace harness or
connector.
AC±56±
AIR CONDITIONING SYSTEM Troubleshooting
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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±
AIR CONDITIONING SYSTEM Troubleshooting
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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.
±
AIR CONDITIONING SYSTEM TroubleshootingAC±85
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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±
AIR CONDITIONING SYSTEM Troubleshooting
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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±
AIR CONDITIONING SYSTEM Troubleshooting
WhereEverybodyKnowsYourName