1991 LEXUS SC400 engine coolant

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±
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Diag. Code 21, 28Main Oxygen Sensor Circuit
CIRCUIT DESCRIPTION
To obtain a high purification rate for the CO, HC and NOx components of th\
e exhaust gas, a three±way catalyst
is used, but for most efficient use of the three±way catalyst, the air±fuel ratio must be \
precisely controlled so
that it is always close to the stoichiometric air±fuel ratio.
The oxygen sensor has the characteristic whereby its output voltage changes\
suddenly in the vicinity of the stoi-
chiometric air±fuel ratio. This characteristic is used to detect the oxygen con\
centration in the exhaust gas and
provide feedback to the computer for control of the air±fuel ratio.
When the air±fuel ratio becomes LEAN, the oxygen concentration in the exh\
aust increases and the oxygen sen-
sor informs the ECU of the LEAN condition (small electromotive force: 0 V)\
.
When the air±fuel ratio is RICHER than the stoichiometric air±fuel rat\
io the oxygen concentration in the exhaust
gas is reduced and the oxygen sensor informs the ECU of the RICH condition \
(large electromotive force: 1V).
The ECU judges by the electromotive force from the oxygen sensor whether th\
e air±fuel ratio is RICH or LEAN
and controls the injection time accordingly. However, if malfunction of the oxygen sensor causes output of ab-
normal electromotive force, the ECU is unable to perform accurate air±fuel \
ratio control.
DTC No.DTC Detecting ConditionTrouble Area
21.
28
(1) Main oxygen sensor signal voltage is reduced to tbetween 0.35 V and 0.70 V for 60 sec. under
conditions (a)
(d). (2 trip detection logic)*
(a) Coolant temp.: Between 805C (176 5F) and
95 5C (203 5F).
(b) Engine speed: 1,500 rpm or more
(c) Load driving (EX. ECT in 4th (5th for M/T) speed, A/C ON, Flat road, 50 mph
(80km/h)).
(d) Main oxygen sensor signal voltage: Alternating above and below 0.45 V.

Main oxygen sensor circuit

Main oxygen sensor
(2) Main oxygen sensor signal voltage exceeds
0.70 V for 3 sec. or more during fuel cut.
*: See page TR±25..
HINT: Diag. trouble code 21 is for the front side main oxygen sensor circuit.\
Diag. trouble code 28 is for the rear side main oxygen sensor circuit.
TR±60
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Diag. Code 21, 28Main Oxygen Sensor Circuit
CIRCUIT DESCRIPTION
To obtain a high purification rate for the CO, HC and NOx components of th\
e exhaust gas, a three±way
catalyst is used, but for most efficient use of the three±way catalyst, the air±fuel ratio must be \
precisely
controlled so that it is always close to the stoichiometric air±fuel \
ratio.
The oxygen sensor has the characteristic whereby its output voltage chan\
ges suddenly in the vicinity of
the stoichiometric air±fuel ratio. This characteristic is used to det\
ect the oxygen concentration in the ex-
haust gas and provide feedback to the computer for control of the air±\
fuel ratio.
When the air±fuel ratio becomes LEAN, the oxygen concentration in the exh\
aust increases and the oxygen
sensor informs the ECU of the LEAN condition (small electromotive force: O V)\
.
When the air±fuel ratio is RICHER than the stoichiometric air±fuel\
ratio the oxygen concentration in the
exhaust gas is reduced and the oxygen sensor informs the ECU of the RICH condition (large electromotive
force: 1V).
The ECU judges by the electromotive force from the oxygen sensor whether th\
e air±fuel ratio is RICH or
LEAN and controls the injection time accordingly. However, if malfunction of the oxygen sensor causes
output of abnormal electromotive force, the ECU is unable to perform accurate \
air±fuel ratio control.
The main oxygen sensors include a heater which heats the Zirconia element. The heater is controlled by
the ECU. When the intake air volume is low (the temperature of the exhaust\
gas is low) current flows to
the heater to heat the sensor for accurate oxygen concentration detectio\
n.
Code No.Diagnostic Code Detecting ConditionTrouble Area
(1) Open or short in heater circuit of main
oxygen sensor for 0.5 sec. or more.

Open or short in heater circuit of main oxygen sen-
sor.

Main oxygen sensor heater

ECU
21 V
28
(2) Main oxygen sensor signal voltage is reduced to be-
tween 0.35 V and 0.70 V for 60 sec. under conditions
(a) ~ (d). (2 trip detection logic)*
(a) Coolant temp.: Between 70 5C (158 5F) and 95 5C
(203 5F).
(b) Engine speed: 1,500 rpm or more.
(c) Load driving (EX. ECT in 4th speed, A/C ON, Flat road, 50 mph (80km/h)).
(d) Main oxygen sensor signal voltage:
Alternating above and below 0.45 V.

Main oxygen sensor circuit

Main oxygen sensor
*: See page TR±21.
HINT : Diag. code 21 is for the left bank main oxygen sensor circuit. Diag. co\
de 28 is for the right bank main oxy-
gen sensor circuit.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±62
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Diag. Code 22Water Temp. Sensor Circuit
CIRCUIT DESCRIPTION
Water Temp.°C ( °F)Resistance
(k
)Voltage
(V)
±20 (±4)16.04.3
0 (32)5.93.4
20 (68)2.52.4
40 (104)1.21.5
60 (140)0.60.9
80 (176)0.30.5
100 (212)0.20.3
Code No.Diagnostic Code Detecting SystemTrouble Area
22Open or short in water temp. sensor
circuit for 0.5 sec. or more.
Open or short in water temp. sensor circuit

Water temp. sensor

ECU
< Reference >
TR±64±
ENGINE TROUBLESHOOTING Circuit Inspection
The water temperature sensor senses the coolant
temperature. A thermistor built in the sensor
changes the resistance value according to the cool-
ant temperature. The lower the coolant temperature,
the greater the thermistor resistance value, and the
higher the coolant temperature, the lower the therm-
istor resistance value (See Fig. 1.).
The water temperature sensor is connected to the
ECU (See next page). The 5 V power source voltage
in the ECU is applied to the water temperature sen-
sor from the terminal THW via a resistor R. That is,
the resistor R and the water temperature sensor are
connected in series. When the resistance value of
the water temperature sensor changes in accor-
dance with changes in the coolant temperature, the
potential at the terminal THW also changes. Based
on this signal, the ECU increases the fuel injection
volume to improve driveability during cold engine op-
eration. If the ECU records the diagnostic code 22,
it operates the fail safe function, keeping the coolant
temperature at a constant 80 °C (176 °F).
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Diag. Code 22Water Temp. Sensor Circuit
CIRCUIT DESCRIPTION
The water temperature sensor senses the coolant tem-
perature. A thermistor built in the sensor changes the
resistance value according to the coolant temperature.
The lower the coolant temperature, the greater the
thermistor resistance value, and the higher the coolant
temperature, the lower the thermistor resistance value
(See Fig. 1.).
The water temperature sensor is connected to the ECU
(See next page). The 5 V power source voltage in the
ECU is applied to the water temperature sensor from the
terminal THW via a resistor R. That is, the resistor R and
the water temperature sensor are connected in series.
When the resistance value of the water temperature
sensor changes in accordance with changes in the cool-
ant temperature, the potential at the terminal THW also
changes. Based on this signal, the ECU increases the
fuel injection volume to improve driveability during cold
engine operation. If the ECU records the diagnostic
code 22, it operates the fail safe function, keeping the
coolant temperature at a constant 80 5C (176 5F).
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±68
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Diag Code 25 26

Air±Fuel Ratio Lean Malfunction
Diag. Code 25, 26
Air±Fuel Ratio Rich Malfunction
CIRCUIT DESCRIPTION
Refer to page TR±62 for the circuit description.
Code No.
Diagnostic Code Detecting SystemTrouble Area
(1) Main oxygen sensor voltage is 0.45 V or less (lean) for 90 sec. under conditions
(a) and (b).
(2 trip detection logic)
*2
(a) Coolant temp.: 70 °C (158 °F)
or more.
(b) Engine speed: 1,500 rpm or more.
 Open or short in main oxygen sensor
circuit
 Main oxygen sensor
 Ignition system
 ECU
25
(2)*1 Difference of air±fuel ratio feedback
compensation value between front (No.
1 3 cylinders) and rear (No. 4 6
cylinders) is more than 15 percentage
for 20 sec. or more under conditions (a)
and (b).
(2 trip detection logic)
*2
(a) Engine speed: 2,000 rpm or more.
(b) Coolant temp.: Between 60 °C
(140 °F) and 95 °C (203 °F).
 Open and short in injector circuit.
 Fuel line pressure (injector leak, blockage)
 Mechanical system malfunction (skipping
teeth of timing belt)
 Ignition system
 Compression pressure (foreign object
caught in valve)
 Air flow meter (air intake)
 ECU
(3)*1 Engine speed varies by more than 15
rpm over the preceding crank angle period
during a period of 20 sec. or more under
conditions (a) and (b).
(2 trip detection logic)
*2
(a) Engine speed: Idling
(b) Coolant temp.: Between 60 °C
(140 °F) and 95 °C (203 °F).
 Open and short in injector circuit.
 Fuel line pressure (injector leak, blockage)
 Mechanical system malfunction (skipping
teeth of timing belt)
 Ignition system
 Compression pressure (foreign object
caught in valve)
 Air flow meter (air intake)
 ECU
26*1
(1) Difference of air±fuel ratio feedback
compensation value between front (No. 1
3 cylinders) and rear (No.4 6 cylinders) is
more than 15 percentage for 20 sec. or
more under conditions (a) and (b).
(2 trip detection logic)
*2
(a) Engine speed: 2,000 rpm or more.
(b) Coolant temp.: Between 60 °C
(140 °F) and 95 °C (203 °F).
 Open and short in injector circuit.
 Fuel line pressure (injector leak, blockage)
 Mechanical system malfunction (skipping
teeth of timing belt)
 Ignition system
 Compression pressure (foreign object
caught in valve)
 Air flow meter (air intake)
 ECU
261
(2) Engine speed varies by more than 15
rpm over the preceding crank angle peri-
od during a period of 20 sec. or more un-
der conditions (a) and (b).
(2 trip detection logic)
*2
(a) Engine speed: Idling
(b) Coolant temp.: Between 60 °C
(140 °F) and 95 °C (203 °F).
 Open and short in injector circuit.
 Fuel line pressure (injector leak, blockage)
 Mechanical system malfunction (skipping
teeth of timing belt)
 Ignition system
 Compression pressure (foreign object
caught in valve
 Air flow meter (air intake)
 ECU
*1: Only for USA specification vehicles.
*2: See page TR±25.
TR±70
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Diag Code

25Air±Fuel Ratio Lean Malfunction
Diag. Code
26Air±Fuel Ratio Rich Malfunction
CIRCUIT DESCRIPTION
Refer to page TR±62 for the circuit description.
Code No.Diagnostic Code Detecting ConditionTrouble Area
(1) Main oxygen sensor voltage is 0.45 V or less (lean)
for 90 sec. under conditions (a) and (b).
(2 trip detection logic)*
(a) Coolant temp.: 60 5C (140 5F) or more.
(b) Engine speed: 1,500 rpm or more.
Open or short in main oxygen sensor circuit

Main oxygen sensor

Ignition system

Water temp. sensor
25 V
26
(2) Main oxygen sensor voltage is alternating above and below 0.45 V at 5 times par
second or more under conditions (a) and (b).
(2 trip detection logic)*
(a) Engine speed: Idling
(b) Coolant temp.: Between 60 5C (140 5F)
and 95 5C (203 5F)

Open or short in injector circuit.

Fuel line pressure (injector blockage)

Air flow meter (air intake)

Engine ground bolt loose

Foreign object caught in valve
(3) Difference of air±fuel ratio feedback compensation
value between right and left banks is more than 10
percentage for 30 sec. or more under conditions (a)
and (b).
(2 trip detection logic)*
(a) Engine speed : 2,000 rpm or more.
(b) Coolant temp.: Between 60 5C (140 5F)
and 95 5C (203 5F)

Fuel line pressure (injector leak, blockage)

Mechanical system malfunction
(skipping teeth of timing belt)

Ignition system
*: See page TR±21.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±74
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Diag. Code 27Sub±Oxygen Sensor Circuit (Only for
USA spec.)
CIRCUIT DESCRIPTION
The sub±oxygen sensor is installed on the exhaust pipe. Its construction\
and operation is the same
as the main oxygen sensor on page TR±60.
Code No.
Diagnostic Code Detecting ConditionTrouble Area
(1) Open or short in heater circuit of sub± oxygen sensor for 0.5 sec. or more.
Open or short in heater circuit of sub±oxy-
gen sensor.
 Sub±oxygen sensor heater.
 ECU
27
(2) Main oxygen sensor signal is 0.45 V or
more and sub±oxygen sensor signal is
0.45 V or less under conditions (a) (c).
(2 trip detection logic) *
(a) Coolant temp.: 80 °C (176 °F)
or more.
(b) Engine speed: 1,500 rpm or more.
(c) Accel. pedal: Fully depressed for 2 sec. or more.
Open or short in sub±oxygen sensor cir-
cuit.
 Sub±oxygen sensor
 ECU
* : See page TR±25.
TR±76
±
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