Lexus SC300 / Lexus SC400. Service manual — part 867

Diag. Code 22

Water Temp. Sensor Circuit

CIRCUIT DESCRIPTION

Water Temp.

°

C (

°

F)

Resistance

(k

)

Voltage

(V)

–20 (–4)

16.0

4.3

0 (32)

5.9

3.4

20 (68)

2.5

2.4

40 (104)

1.2

1.5

60 (140)

0.6

0.9

80 (176)

0.3

0.5

100 (212)

0.2

0.3

Code No.

Diagnostic Code Detecting System

Trouble Area

22

Open 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).

DIAGNOSTIC CHART

HINT: If diagnostic codes ”22” (water temperature sensor circuit), ”24” (intake air temperature sensor

circuit) and ”41” (throttle position sensor circuit) are output simultaneously, E2 (sensor ground)
may be open.

Check voltage of sensor.

Check for momentary
interruption (See page

TR–27

).

Replace water temp. sensor.

Repair or replace harness or
connector.

Check for open and short in harness and
connector between ECU and water temp.
sensor.

Check resistance of sensor.

Check and replace ECU.

WIRING DIAGRAM

a

a

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–65

NG

OK

OK

NG

OK

NG

INSPECTION PROCEDURE

HINT: If diagnostic codes ”22” (water temperature sensor circuit), ”24” (intake air temperature sensor cir-

cuit) and ”41” (throttle position sensor circuit) are output simultaneously, E2 (sensor ground) may
be open.

1

Check voltage between terminals THW and E2 of engine (&ECT) ECU connector.

C

OK

P

1.

Connect the Check Harness A. (See page TR–34)

2.

Turn ignition switch on.

Measure voltage between terminals THW and E2 of en-
gine (& ECT) ECU connector.

Check for momentary interruption (See page

TR–27

).

2

Check water temp. sensor.

C

OK

P

Disconnect the water temp. sensor connector.

Measure resistance between terminals.

Resistance is within Acceptable Zone on chart.

Replace water temp. sensor.

3

Check for open and short in harness and connector between engine (& ECT) ECU and water
temp. sensor (See page

IN–27

).

Repair or replace harness or connector.

Check and replace engine (& ECT) ECU.

TR–66

–

ENGINE TROUBLESHOOTING

Circuit Inspection

Diag. Code 22

Water 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

°

C (176

°

F).

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–68