light LEXUS SC400 1991 Service Repair Manual

INSPECTION PROCEDURE
Check output condition of diag. code 43
Setting the test mode.
6. Turn ignition switch OFF.
7. Connect terminals TE2 and E1 of TDCL.
8. Turn ignition switch ON.(Don't start the engine)
9. Connect terminals TE1 and E1 of TDCL.
Check if code ª43º is output by the ªCHECKº en-
gine warning light.
Code ª43º is output.
Start the engine.
Check if the code ª43º disappear.
Code ª43º is not output.
Proceed to next circuit inspection shown on
matrix chart (See page
TR±39).
Check for open in harness and connector between engine (& ECT) ECU
and starter relay (See page IN±27)
Repair or replace harness or connector.
Check and replace engine (& ECT) ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±97
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NGOK
OKNG
v
INSPECTION PROCEDURE
1Check output condition of diag. code 43.
C
OK
P
OK
C
Setting the test mode.
(2) Turn ignition switch OFF.
(2) Connect terminals TE2 and E1 of TDCL.
(2) Turn ignition switch ON.(Don't start the engine)
(2) Connect terminals TE1 and E1 of TDCL.
Check if code ª43º is output by the ªCHECKº engine
warning light.
Code ª43º is output.
Start the engine.
Check if the code ª43º disapears.
Code ª43º is not output.
Proceed to next circuit inspection shown on maxtric
chart (See page TR±35).
2Check for open in harness and connectors between engine & ECT ECU and st\
arter relay
(See page IN±27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
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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.
Malfunction: Open or Short in Knock Sensor

Start engine and warm up.
After engine is warmed up, let it idle for 3 min.
With the A/C ON, perform quick racing (5,000 rpm) three times.
(Rapidly depress the accelerator pedal and suddenly release it.)
HINT: If a malfunction exists, the ºCHECKº engine warning light will lig\
ht up when sudden racing is performed.
NOTICE: If the conditions in this test are not strictly followed, detection of t\
he malfunction will
not be possible.
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Diag. Code 71
EGR System Malfunction (Only for
USA spec.)
CIRCUIT DESCRIPTION
If even one of the following conditions is ful-
filled, the VSV is turned ON by a signal from
the ECU. This results in atmospheric air act-
ing on the EGR valve, closing the EGR valve
and shutting off the exhaust gas (EGR cut±
OFF).
w
Coolant temp. below 505C (122 5F)
w
During deceleration (throttle valve closed)
w
Light engine load (amount of intake air very
small).
w
Engine speed over 5,200 rpm. The EGR system is designed to recirculate the exhaust gas properly controll\
ed according to the driv-
ing condition back into the intake air±fuel mixture. It helps to slow dow\
n combustion in the cylinder
and thus lower the combustion temperature which, in turn, reduces the amoun\
t of NOx emission. The
amount of EGR is regulated by the EGR vacuum modulator according to the engine\
load.
Code No.Diagnostic Code Detecting ConditionTrouble Area
EGR t i 70
5C (158 5F) b l f 1 4
w
Open in EGR
gas temp. sensor circuit.EGR gas temp. is 70 5C (158 5F) or below for 1 4
min. under conditions (a) and (b).
(2 trip detection logic)*
Oen in EGR gas tem. sensor circuit.
w
Short in VSV circuit for EGR.
71
(2 trip detection logic)*
(a) Coolant temp.: 63 5C (145 5F) or more.
(b) EGR operationpossible (EX ECT in 3rd speed
Short in VSV circuit for EGR.
w
EGR hose disconnected, valve stuck.
(b) EGR operation possible (EX. ECT in 3rd speed
(5th for M/T), A/C ON, 60 mph (96 km/h), Flat
road)
EGR hose disconnected, valve stuck.
w
Clogged EGR gas passage.
road).gg g g
w
ECU
*: See page TR±25.
TR±104
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CIRCUIT DESCRIPTION (Cont'd)
DIAGNOSIS TROUBLE CODE DETECTION DRIVING PATTERN
Purpose of the driving pattern.
(a) To simulate diag. trouble code detecting condition after diag. trouble co\
de is recorded.
(b) To check that the malfunction is corrected when the repair is completed co\
nfirming that diag. trouble codeis no longer detected.
Malfunction: Open in EGR gas temp. sensor circuit
Disconnect the EFI fuse (15 A) for 10 sec. or more, with IG switch OFF\
.
Initiate test mode (Connect terminal TE2 and E1 of check connector or TDCL with\
IG
switch OFF).
Start the engine and warm up.
After the engine is warmed up, let it idle for 3 min.
With the A/C ON and transmission in 5th gear (ECT in 3rd speed) drive at 5\
5 ~ 60 mph
(88 ~ 96 km/h) for 4 min or less.
HINT: If a malfunction exists, the ºCHECKº engine warning light will l\
ight up during step .
NOTICE: If the conditions in this test are not strictly followed, detection of \
the malfunction will not be possible.
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Diag. Code 71
EGR System Malfunction (Only for USA
spec.)
CIRCUIT DESCRIPTION
The EGR system recirculates exhaust gas, which is controlled to the proper quantity to suit the driv\
ing condi-
tions, into the intake air mixture to slow down combustion, reduce the combust\
ion temperature and reduce NOx
emissions.
The lift amount of the EGR valve is controlled by a step motor operated \
by the ECU. The ECU operates the motor
only a preprogrammed amount in response to the engine operating conditions (\
engine rpm, intake air volume
to adjust the EGR volume to the target valve.)
Under the following conditions, EGR is cut to maintain driveability.
wCoolant temp. below 535C (127.45F)
w During deceleration (throttle valve
closed)
w Light engine load (amount of intake air
very small)
w Engine speed over 4,000 rpm
w Engine idling
w Neutral start switch on
Code No.Diagnostic Code Detecting ConditionTrouble Area
(1) Open or short in EGR step motor circuit for 1 sec.
or more.
Open or short in EGR step motor circuit

ECU
71
(2) EGR gas temp. is 65 5C (149 5F) or below for
1 ~ 4 min. under conditions (a) and(b).
(2 trip detection logic)*
(a) Coolant temp.: 65 5C (149 5F) or more.
(b) EGR operation possible (EX. ECT in 3rd speed, A/C ON, 60 mph (96 km/h), Flat road).

Open in EGR gas temp. sensor circuit

ECU
*: See page TR±21.
TR±108
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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
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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.
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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±
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OKNG
OKNG
INSPECTION PROCEDURE
1Check output condition of diag. code 51.
C
OK
Hint
PSetting the test mode.
(2) Turn ignition switch OFF.
(2) Connect terminals TE2 and E1 and TDCL.
(2) Turn ignition switch ON.(For checking terminal A/C, start the engine.)
(2) Connect terminals TE1 and E1 of TDCL.
Check if code ª51º is output by the ªCHECKº engine
warning light.
*: Before the STA signal is input (ST is not ON), diagnostic code 43 is also output.
Diag. code 42 is output with vehicle speed 5 km/h (3
mph) or below
IDL1 .... Go to step 2 NSW ... Go to page TR±22.
A/C ..... Go to step 3
Proceed to next circuit inspection shown on
maxtrix chart (See page TR±35).
2Check throttle position sensor.
C
OK
PDisconnect throttle position sensor connector.
Measure resistance between terminals IDL1 and E2 of
throttle position sensor connector.
Adjust or replace throttle position sensor
See page FI±73).
Check and repair harness or connector between
engine & ECT ECU and throttle position sensor.
TR±120±
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