1991 LEXUS SC400 engine

DIAGNOSTIC CHART
Check fuel pump operation.
Check for open and short in harness and
connector between check connector and
fuel pump ECU.
Repair or replace harness or connector.
Check voltage of terminal +B of check
connector.
Check for open and short in harness and
connector between check connector and
fuel pump.
Repair or replace harness or connector.
Check voltage of terminal FPC.
Check for open and short in harness and
connector between ECU and fuel pump
ECU. ((FPC)
Check for open and short in harness and
connector between ECU and fuel pump
ECU. (DI)
Check and replace ECU.
Check for ECU power source
circuit.
Repair or replace fuel pump.
Replace fuel pump ECU.
Repair or replace harness or
connector.
Repair or replace harness or
connector.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±111
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OKNG
NGOK
OKNG
INSPECTION PROCEDURE
1Check fuel pump operation.
C
OK
P1. Turn ignition switch ON.
2. Using SST, connect terminals +B and FP of checkconnector.
SST 09843±18020
Check that there is pressure in the hose from the fuel
filter.
Fuel pressure can be felt.
Go to step
.
2Check for open and short in harness and connector between terminals +B e+B, FP  FP of
the check connector and fuel pump ECU (See page IN±27).
Go to step .
Repair or replace harness or connector.
3Check voltage of terminal +B of check connector.
C
OK
PTurn ignition switch ON.
Measure voltage between terminal +B of check connec-
tor and body ground.
Voltage: 10 ± 14 V
Check for ECU power source circuit (See page TR±120),
and check for open in harness and connector between
terminal +B of check connector and main relay.
TR±112
±
ENGINE TROUBLESHOOTING Circuit Inspection
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NGOK
NGOK
OKNG
OKNG
4Check for open and short in harness and connector between terminal FP of chec\
k connector,
fuel pump and body ground (See page IN±27).
Repair or replace fuel pump.
Repair or replace harness or connector.
5Check voltage between terminals FPC and E of fuel pump ECU connector.
C
OK
P1. Remove the LH quarter trim panel. (See page FI±81 )
2. Disconnect fuel pump ECU connector.
Measure voltage between terminals FPC and E of fuel
pump ECU connector when ignition switch is turned to
start.
Voltage: 4 ± 6 V
Replace fuel pump ECU.
6Check for open in harness and connector between terminal FPC of engine (& ECT) ECU and ter-
minal FPC of fuel pump ECU, terminal E of fuel pump ECU and body ground (See\
page IN±27).
Repair or replace harness or connector.
7Check for open and short in harness and connector between terminal DI of engin\
e (& ECT) ECU
and terminal DI of fuel pump ECU (See page IN±27).
Repair or replace harness or connector.
Check and replace engine (& ECT) ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±113
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Diag. Code 78Fuel Pump Control Circuit
CIRCUIT DESCRIPTION
The fuel pump speed is controlled at two steps (high speed, low speed) by\
the condition of the engine (starting,
light load, heavy load), when the engine starts (STA ON), the engine & ECT ECU sends a Hi signal to the fuel
pump ECU (FPC terminal).
The fuel pump ECU then outputs Hi voltage (battery voltage) to the fuel p\
ump so that the fuel pump operates
at high speed.
After the engine starts, during idling or light loads, the engine & ECT ECU o\
utputs a High Low signal to the fuel
pump ECU, the fuel pump ECU outputs Lo battery voltage (about 9V) to the f\
uel pump and causes the fuel pump
to operate at low speed.
If the intake air volume increases (high engine load), the engine & ECT \
ECU sends a Hi signal to the fuel pump
ECU and causes the fuel pump to operate at high speed.
Code No.Diagnostic Code Detecting ConditionTrouble Area
(1) Open or short in fuel pump circuit for 1 sec. or more with engine speed 1,000 rpm or less.
(2 trip detection logic)*

Open or short in fuel pump ECU circuit.
78
(2) Open in input circuit of fuel pump ECU(FPC) with
engine speed 1,000 rpm or less.
(2 trip detection logic)*
Fuel pump ECU

Engine & ECT ECU power source circuit.

Fuel pump
(3) Open or short in diagnostic signal line (DI) of fuel
pump ECU with engine speed 1,000 rpm or less.
(2 trip detection logic)*

Fuel um

Engine & ECT ECU
*: See page TR±21.
WIRING DIAGRAM
TR±114±
ENGINE TROUBLESHOOTING Circuit Inspection
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DIAGNOSTIC CHART
DIAGNOSTIC
CHART
Check for open and short in harness and
connector between check connector and fuel
pump.
Check voltage of terminal FPC.Check for open and short in harness and
connector between ECU and fuel pump
ECU. (DI)
Check and replace ECU.
Replace fuel pump ECU.
Repair or replace harness or connector.
Repair or replace harness or
connector.
Check for open and short in harness and
connector between ECU and fuel pump
ECU. (FPC)
Repair or replace harness or
connector.
Repair or replace fuel pump.
Check voltage of terminal +B of check
connector.
Repair or replace harness or connector.
Check for open and short in harness and
connector between check connector and fuel
pump ECU.
Check fuel pump operation.
Check for ECU power source circuit.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±115
WhereEverybodyKnowsYourName

OKNG
NGOK
OKNG
INSPECTION PROCEDURE
1Check fuel pump operation.
C
OK
P(2) Turn ignition switch ON.
(2) Using SST. connect terminals + B and FP of checkconnector.
SST 09843±18020
Check that there is projection of the head of the pulsa-
tion damper screw.
Projection of the head of the pulsation damper
screw.
Go to step 3.
2Check for open and short in harness and connector between terminals + B e + B FP e FP of
the check connector and fuel pump ECU (See page IN±27).
Go to step 5.
Repair or replace harness or connector.
3Check voltage or terminal + B of check connector.
C
OK
PTurn ignition switch ON.
Measure voltage between terminal + B of check connec-
tor and body ground.
Voltage: 10 ± 14 V
Check for ECU power source circuit
(See page TR±124), and check for open in harness and
connector between terminal + B of check connector and
main relay.
TR±116
±
ENGINE TROUBLESHOOTING Circuit Inspection
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NGOK
NGOK
OKNG
OKNG
4Check for open and short in harness and connector between terminal FP of chec\
k connector,
fuel pump and body ground (See page IN±27).
Repair or replace fuel pump.
Repair or replace harness or connector.
5check voltage between terminals FPC and E of fuel pump ECU connector.
C
OK
P(2) Remove the LH quarter trim panel. (See page BO±123)
(2) Disconnect fuel pump ECU connector.
Measure voltage between terminals FPC and E of fuel
pump ECU connector when ignition switch is turned to
start.
Voltage: 4 ± 6 V
Replace fuel pump ECU.
6Check for open in harness and connector between terminal FPC of engine & ECT ECU a\
nd termi-
nal FPC of fuel pump ECU, terminal E of fuel pump ECU and body ground (See\
page IN±27).
Repair or replace harness or connector.
7Check for open in harness and connector between terminal DI of engine & ECT E\
CU and termi-
nal DI of fuel pump ECU (See page IN±27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±117
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Diag. Code 51Switch Condition Signal Circuit
CIRCUIT DESCRIPTION
Neutral Start Switch Signal
The ECU uses the signals from the neutral start switch to determine whether\
the transmission is in park or neu-
tral, or in some other gear.
Air Conditioner Switch Signal
The ECU uses the output from the air conditioner switch to determine whethe\
r or not the air conditioner is operat-
ing so that it can increase the idling speed of the engine if necessary.
Throttle Position Sensor IDL Signal
The IDL contacts are mounted in the throttle position sensor, and detects the idle condition.
Code No.Diagnostic Code Detecting ConditionTrouble Area
51
(1) 3 sec. or more after engine startsidle switch OFF (IDL1).
(2) Neutral start switch OFF (NSW).(Shift position i n ºRº, ºDº, º2ºor º1º ranges).
(3) A/C switch ON.
Throttle position sensor IDL circuit

Accelerator pedal and cable

Neutral start switch circuit

A/C switch circuit

ECU
HINT: In this circuit, diagnosis can only be made in the test mode.
DIAGNOSTIC CHARTDIAGNOSTIC CHART
See next page for DIAGNOSTIC CHART
WIRING DIAGRAM
TR±118±
ENGINE TROUBLESHOOTING Circuit Inspection
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