LEXUS SC400 1991 Service Repair Manual

ISC Valve Circuit
CIRCUIT DESCRIPTION
The ISC valve is provided on the intake air chamber and in-
take air bypassing the throttle valve is directed to the ISC
valve through a hose.
A step motor is built into the ISC valve. It consists of four
coils, the magnetic rotor, valve shaft and valve. When cur-
rent flows to the coils due to signals from the ECU, the rotor
turns and moves the valve shaft forward or backward,
changing the clearance between the valve and the valve
seat.
In this way the intake air volume bypassing the throttle valve
is regulated, controlling the engine speed.
There are 125 possible positions to which the valve can be
opened.
DIAGNOSTIC CHARTDIAGNOSTIC CHART
Check operation for ISC valve.Replace ISC valve.
Repair or replace harness or
connector.
Proceed to next circuit inspection shown on
matrix chart (See page TR±35).
Check harness and connector between EFI
main relay, ISC valve and ECU.
TR±140±
ENGINE TROUBLESHOOTING Circuit Inspection
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OKNG
OKNG
INSPECTION PROCEDURE
1Check ISC value.
C
OK
P
P
OK
C
Disconnect ISC connector.
Measure resistance between terminals shown below.
Remove ISC Valve
(2) Connect the battery positive lead to terminals B1and B2, and the negative lead to terminals
S1±S2±S3±S4 in that order.
(2) Connect the battery positive lead to terminals B1 and B2, and the negative lead to terminals
S4±S3±S2±S1 in that order.
(2) The valve moves in the closing direction.
(2) The value moves in the opening direction.
Replace ISC valve.
2Check for open and short in harness and connector between EFI main relay and \
ISC valve, ISC
valve and engine & ECT ECU (See page IN±27).
Repair or replace harness or connector.
Proceed to next circuit inspection shown on ma-
trix chart (See page TR±35).
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±141
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Fuel Pressure Control VSV Circuit
CIRCUIT DESCRIPTION
The ECU turns on a VSV (Vacuum Switching Valve)
to draw the air into the diaphragm chamber of the
pressure regulator if it detects that the temperature of
the coolant is too high during engine starting.
The air drawn into the chamber increases the fuel
pressure to prevent fuel vapor lock at high engine
temperature in order to help the engine start when it
is warm.
Fuel pressure control ends approx. 100 secs. after
the engine is started.
DIAGNOSTIC CHARTDIAGNOSTIC CHART
Check operation for fuel pressure control
VSV.Replace fuel pressure control VSV.
Repair or replace harness or
connector.
Proceed to next circuit inspection
shown on matrix chart (See page
TR±35).
Check for open and short in harness and
connector between main relay and ECU.
Check voltage of VSV power source.
Check and replace ECU.
WIRING DIAGRAM
TR±142±
ENGINE TROUBLESHOOTING Circuit Inspection
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OKNG
INSPECTION PROCEDURE
1Check fuel pressure control VSV.
C
OK
P
OK
C
(2) Remove fuel pressure control VSV.
(2) Disconnect fuel pressure control VSV connector.
(2) Measure resistance between terminals.
(2) Measure resistance between each terminal and thebody.
1. Resistance: 30 ± 50 at 20C (68F)
2. Resistance: 1M or higher
Check operation of fuel pressure control VSV when bat-
tery voltage is applied to the terminals of fuel pressure
control VSV connector or not.
Battery voltage is applied:
The air from pipe E is flowing out through the air fil-
ter.
Battery voltage is not applied:
The air from pipe E is flowing out through pipe G.
Replace fuel pressure control VSV.
Go to step 2.
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ENGINE TROUBLESHOOTING Circuit InspectionTR±143
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NGOK
OKNG
2Check voltage between terminal FPU of engine & ECT ECU connector and body gro\
und.
C
OK
Hint
P(2) Connect the Check Harness A.(See page TR±30)
(2) Turn ignition switch on.
Measure voltage between terminal FPU of engine &
ECT ECU connector and body ground.
Voltage: 10 ± 14 V
Proceed to next circuit inspection shown on matrix chart
(See page TR±35).
3
Check for open and short in harness and connector between engine & ECT ECU an\
d VSV, VSV
and EFI main relay (See page IN±27).
Repair or replace harness or connector
Check and replace engine & ECT ECU.
TR±144±
ENGINE TROUBLESHOOTING Circuit Inspection
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±MEMO±
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ENGINE TROUBLESHOOTING Circuit InspectionTR±145
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TE1, TE2 Terminal Circuit
CIRCUIT DESCRIPTION
Terminal TE1 is located in the check connector in the engine compartment and terminals T\
E1 and TE2 are lo-
cated in the TDCL in the cabin. When these terminals are connected with \
the E1 terminal, diagnostic codes in
normal mode or test mode can be read from the ºCHECKº engine warning lig\
ht on the combination meter.
DIAGNOSTIC CHARTDIAGNOSTIC CHART
HINT: If terminals TE1 and TE2 are connected with terminal E1, diagnostic code is not output or test mode isnot activated.
Even though terminal TE1 is not connected with terminal E1, the ºCHEC\
Kº engine warning light blinks.
For the above phenomenon, the likely cause is an open or short in the wire \
harness, or malfunction inside
the ECU.
Check continuity between terminal E1 and
body ground.
Repair or replace harness or
connector.Check for open and short in harness and
connector between check connector TDCL
and ECU.
Check voltage of terminals TE1, TE2.
Check and replace ECU.
Repair or replace harness or
connector.
Check and replace ECU.
WIRING DIAGRAM
TR±146±
ENGINE TROUBLESHOOTING
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NGOK
OKNG
OKNG
INSPECTION PROCEDURE
1Check voltage between terminals TE1, TE2 and E1 of check connector, TDCL.
C
OK
PTurn ignition switch on.
Measure voltage between terminals TE1, TE2 and E1
of check connector, TDCL.
Voltage: 10 ± 14 V
Check and replace engine & ECT ECT.
2Check continuity between terminal E1 of check connector, TDCL and body ground.
Repair or replace harness or connector.
3
Check for open and short in harness and connector between engine & ECT ECU and che\
ck con-
nector, TDCL (See page IN±27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
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ENGINE TROUBLESHOOTING Circuit InspectionTR±147
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DEFOGGER SYSTEM
Description
The component parts of this system and their functions are described in the\
following table.
 Parts Name\
 \
Function
 Defogger\
 \
When over current flows in the defogger circuit, the Defogger fuse break\
s the circuit to pro-
tect it against damage.
 


Defogger Relay\
 
\

\

This relay is supplied with current from terminal IG1 of ignition switch\
(Ignition switch
ON) and switches large current from the defogger.
 


A/C Control Panel
Assembly

Defogger Switch\
 
\

\

The defogger switch is built into the A/C Control Panel Assembly. This switch is
supplied with current from the relay and fuse HTR. Grounds current from \
the defogger
relay, turning the defogger relay ON.
 




Defogger

Rear Window
 Outer Mirror\
 
\


\

\

These defoggers generate heat when current is supplied from the defogger\
relay.
Parts Location
±
BODY ELECTRICAL SYSTEM Lighting SystemBE±51
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Wiring and Connector Diagrams
Parts Inspection
(Defogger Switch)
REMOVAL AND INSTALLATION OF AIR
CONDITIONER CONTROL PANEL
ASSEMBLY
(See Instrument Panel on Pub. No. RM260U2, page BO±111)
INSPECTION OF DEFOGGER SWITCH
INSPECT SWITCH OPERATION
(a) Connect the positive (+) lead from the battery to terminal A6and negative (±) lead to terminal A7.
(b) Connect the positive (+) lead from the battery to terminal B10
through a 1.4 W test bulb.
BE±52
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BODY ELECTRICAL SYSTEM Defogger System
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