LEXUS SC400 1991 Service Repair Manual

CIRCUIT DESCRIPTION (Cont'd)
DIAGNOSIS CODE DETECTION DRIVING PATTERN
Purpose of the driving pattern.
(a) To simulate diag. code detecting condition after diag. code is recorded.
(b) To check that the malfunction is corrected when the repair is completed c\
onfirming that diag. code is nolonger detected.

Initiate test mode (See page TR±12).
Start engine and warm up.
After engine is warmed up, let it idle for 3 min.
With the A/C ON and transmission in D range (O/D OFF), drive at 40±\
70 mph for 4 min.
HINT: If a malfunction exists, the ºCHECKº engine warning light will light up at 1±4 min. of driving at 40±70 mph (64±112 km/h).
NOTICE: If the conditions in this test are not strictly followed, detection of the\
malfunction will not be possible.
Malfunction: Open in EGR gas temp. sensor circuit
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±109
WhereEverybodyKnowsYourName

DIAGNOSTIC CHART
WIRING DIAGRAM
Check voltage of terminal EGR 1 ~ 4.
Check voltage of EGR Step motor power
source.
Check resistance of EGR step motor.
Check for open and short in harness and
connector between EGR step motor and
ECU.
Check and replace ECU.
Check operation for EGR step motor.
Check resistance of EGR gas temp. sensor.
Check for open and short in harness and
connector between EGR gas temp. sensor
and ECU.
Check and replace ECU.
Replace EGR gas temp. sensor.
Replace EGR step motor.
Replace EGR step motor.
Repair or replace harness or
connector.
Repair or replace harness or
connector.
Check for ECU power source
circuit.
TR±110±
ENGINE TROUBLESHOOTING
WhereEverybodyKnowsYourName

NGOK
OKNG
INSPECTION PROCEDURE
1Check voltage between terminals EGR 1 ~ 4 of engine & ECT ECU connector and b\
ody ground.
C
OK
P(2) Connect the Check Harness A.(See page TR±30).
(2) Turn ignition switch on.
Measure voltage between terminals EGR 1 ~ 4 of en-
gine & ECT ECU connector and body ground.
Voltage: 10 ± 14 V
Go to step 5.
2Check voltage between terminals B1 and B2 of EGR step motor connector an\
d body ground.
C
OK
P(2) Disconnect EGR step motor connector.
(2) Turn ignition switch on.
Measure voltage between terminals B1 and B2 of EGR
step motor connector and body ground.
Voltage: 10 ± 14 V
Check for ECU power source circuit
(See page TR±124).
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±111
WhereEverybodyKnowsYourName

OKNG
OKNG
OKNG
3Check resistance of EGR step motor.
C
OK
PDisconnect EGR step motor connector.
Measure resistance between terminals shown below.
Replace EGR step motor.
4Check for open and short in harness and connector between EGR step motor and e\
ngine & ECT
ECU (See page IN±27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
5Check operation for EGR step motor.
C
OK
PRemove EGR step motor.
(2) Connect the battery positive lead to terminals B1 and 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 valve moves in the opening direction.
Replace EGR step motor.
TR±112
±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

OKNG
OKNG
6Check resistance of EGR gas temp. sensor.
C
OK
PDisconnect EGR gas temp. sensor connector
(See page FI±103).
Measure resistance between terminals of EGR gas
temp. sensor connector.
Resistance: 69 ± 89 k
at 50 C (122 F)
12 ± 15 k
at 100 C (212 F)
2 ± 4 k
at 150 C (302 F)
Replace EGR gas temp. sensor.
7Check for open and short in harness and connector between EGR gas temp. sensor and\
engine
& ECT ECU (See IN±27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±113
WhereEverybodyKnowsYourName

WIRING DIAGRAM
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 termi-
nal).
The fuel pump ECU then outputs Hi voltage (battery voltage) to the fue\
l pump so that the fuel pump operates at high speed.
After the engine starts, during idling or light loads, the engine (& ECT) EC\
U outputs a High Low signal to the fuel pump ECU,
the fuel pump ECU outputs Lo battery voltage (about 9V) to the fuel pu\
mp and causes the fuel pump to operate at low speed.
If the intake air volume increases (high engine load), the engine (& EC\
T) ECU sends a Hi signal to the fuel pump ECU and
causes the fuel pump to operate at high speed.
DTC No.
DTC Detecting ConditionTrouble Area
(1) Open or short in fuel pump circuit for 1 sec. or more with en- gine 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)*Engine (& ECT) ECU
*: See page TR±25.
TR±110±
ENGINE TROUBLESHOOTING Circuit Inspection
WhereEverybodyKnowsYourName

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
WhereEverybodyKnowsYourName

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
WhereEverybodyKnowsYourName

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
WhereEverybodyKnowsYourName

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
WhereEverybodyKnowsYourName