B1203

B1203

Component Location:

General Description
A water temp sensor located at heater unit detects coolant temperature. It contains negative type thermistor whoses resistance value is in inverse proportion to temperature. Resistance of water sensor that varies with temperature makes signal voltage of A/C ECU change. In this way A/C ECU detects temperature. Signal of water sensor is used for cold engine lockout control - When the driver operates the heater before the engine is warmed up enough, to prevent driver from unpleasantness of cold air. the heater control unit reduces blower motor speed until coolant temperature reaches the threshold value -

DTC Description
The ECM sets DTC B1203 if there is a short circuit in water temp sensor signal harness or the measured voltage value of the sensor is less than the threshold value (0.1 V)

DTC Detecting Condition:

Schematic Diagram:

Signal Waveform:

Monitor Scantool Data
1. Connect scantool to Data Link Connector (DLC).
2. Engine "ON".
3. Monitor the "WATER TEMP SENSOR" Parameter on the Scantool.






4. Are the DTC B1203 present and is parameter of "WATER TEMP.SENSOR" fixed?
- Parameter of "WATER TEMP SENSOR" will be fixed at 28.4 °F, if there is any fault in WATER TEMP SENSOR.

YES
a. Go to "Inspection & Repair" procedure.

No
a. Fault is intermittent caused by poor contact in the sensor's and/or A/C control unit's connector or was repaired and A/C control unit memory was not cleared. Thoroughly check connectors for looseness, poor connection, bending, corrosion, contamination, deterioration, or damage. Repair or replace as necessary and then go to "Verification of Vehicle Repair" procedure.

Terminal and Connector Inspection
1. Many malfunctions in the electrical system are caused by poor harness and terminals. Faults can also be caused by interference from other electrical systems, and mechanical or chemical damage.
2. Thoroughly check connectors for looseness, poor connection, bending, corrosion, contamination, deterioration, or damage.
3. Has a problem been found?

YES
a. Repair as necessary and go to "Verification Vehicle Repair" procedure.

No
a. Go to "W/Harness Inspection" procedure.

Signal Circuit Inspection
1. Check for short to ground in harness
1. Ignition "OFF".
2. Disconnect water temp. sensor.
3. Measure resistance between terminal "1" of water temp sensor and chassis ground.

Specification : Approx. Infinite ohms






4. Is the measured resistance within specifications?

YES
a. Go to "Component Inspection" procedure.

No
a. Check for short to ground in harness. Repair as necessary and go to "Verification of Vehicle Repair" procedure.

Component Inspection
1. Check WATER TEMP Sensor
1. Ignition "OFF".
2. Disconnect water temp. sensor.
3. Measure resistance between terminal "1" and "2" of water temp. sensor.

Specification : Refer the specifications in fig 3)









4. Is the measured resistance within specifications in fig 3)? (tolerance limits ± 3%)

YES
a. Go to "Check A/C Control Unit" procedure.

No
a. Substitute with a known-good water temp. sensor and check for proper operation. If the problem is corrected, replace water temp. sensor and then go to "Verification of Vehicle Repair" procedure.

2. Check A/C Control Unit
1. Engine "ON".
2. Disconnect water temp. sensor.
3. Measure Voltage between terminal "14" of A/C Control Unit and chassis ground.

Specification : Approx. 5 V






4. Is the measured voltage within specifications?

YES
a. Check connectors for looseness, poor connection, bending, corrosion, contamination, deterioration, or damage. Repair or replace as necessary and then go to "Verification of Vehicle Repair" procedure.

No
a. Substitute with a known-good A/C Control Unit and check for proper operation. If the problem is corrected, replace A/C Control Unit and then go to "Verification of Vehicle Repair" procedure.

Verification of Vehicle Repair
After a repair, it is essential to verify that the fault has been corrected.
1. Connect scantool and select "Diagnostic Trouble Codes (DTCs)" mode and then clear DTC.
2. Operate the vehicle and monitor the DTC on the scantool.
3. Are any DTCs present?

YES
a. Go to the applicable troubleshooting procedure.

No
a. System is performing to specification at this time.