Lexus SC300 / Lexus SC400. Service manual - part 670

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

DIAGNOSTIC CHART

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.

WIRING DIAGRAM

Check and replace ECU.

Repair or replace harness or con-
nector.

Replace water temp. sensor.

Check voltage of sensor.

Check resistance of sensor.

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

Check for momentary interruption

 (See page 

TR–23

).

TR–69

–

ENGINE TROUBLESHOOTING

Circuit Inspection

NG

OK

OK

NG

OK

NG

INSPECTION PROCEDURE

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.

1

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

C

OK

P

(2) Connect the Check Harness A.

(See page TR–30)

(2) Turn ignition switch on.

Measure voltage between terminals THW and E2 and
engine & ECT ECU connector.

Check for momentary interruption
(See Page 

TR–23

).

2

Check water temp. sensor.

C

OK

P

Disconnect the water temp. sensor connector.
(See page FI–99).

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.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–70

–MEMO–

TR–71

ENGINE TROUBLESHOOTING