Thermostat
THERMOSTAT, ELECTRICAL
WARNING: Always vent the exhaust to the outside when carrying out this test. Failure to the follow this instruction may result in personal injury.
NOTE: This procedure is most accurate if carried out indoors at less than 37.8°C (100°F) ambient air. This test may be carried out with or without the hood open and with the engine warm or cold.
1. Check the coolant level in the radiator and the coolant expansion tank. Fill as required.
2. With the engine OFF, disconnect the cylinder head temperature (CHT) sensor electrical connector and attach Service Coolant Temperature Monitor Harness as a jumper between the PCM and the CHT. Attach a digital multimeter to the thermostat monitor harness. Voltage values (0 to 5 V) may now be monitored while the sensor retains its connection to the harness.
NOTE: Running this test with the vehicle in gear or with the A/C compressor clutch engaged (running) will cause incorrect diagnosis.
3. Place transmission in Park (P) or Neutral (N).
4. Start the engine and allow to idle throughout this test. Allow the engine to run for 2 minutes, then record the CHT voltage. Record the CHT voltage every 60 seconds. When the CHT voltage trend changes direction or only changes slightly (0.03 volts or less) from the previous reading, record this as the thermostat opening voltage. Use the voltage and corresponding coolant temperature chart listed below.
NOTE: The CHT sensor has a temperature to voltage overlap zone, within this zone it is possible to have either a 'Cold End' or 'Hot End' voltage at the same temperature.
5. If the thermostat opening voltage is greater than 0.79 volts and less than 80°C (176°F), INSTALL a new thermostat.
6. If the thermostat opening voltage is less than 0.79 volts and greater than 80°C (176°F), the thermostat is OK and should not be changed. Refer to the Symptom Chart for further instructions.
THERMOSTAT TEST
1. Connect the WDS to the data link connector (DLC).
2. Using the WDS datalogger function, SELECT the following sensors (as applicable to the application):
- IAT - intake air temperature (IAT) sensor
- ECT - engine coolant temperature (ECT) sensor
- CHT - cylinder head temperature (CHT) sensor
- LOAD-engine load
- VSS - vehicle speed sensor (VSS)
- RPM- engine speed
- DSRPM - desired engine speed
The IAT sensor output is useful if the engine being tested is cold or after an over-night cold soak. The ECT sensor or CHT sensor and the IAT sensor should either indicate the same value or be within I to 2 degrees Celsius of each other.
The ECT sensor output is important to display as it indicates the engine warm-up and opening temperature for the thermostat. It will initially indicate a slightly higher reading just before the thermostat opens and then drops back before settling to a near flat line output.
CAUTION: If the ECT sensor output reaches the 120°C default line under normal cooling system pressure, internal damage may be caused to the engine and a diagnostic trouble code (DTC) will be set in the PCM. The test should be stopped and the cause located and corrected. If the cooling system does not pressurize, the coolant will boil at 100°C which may also damage the engine. CARRY OUT the Coolant Expansion Tank Cap Pressure Test Component Test in this section.
If the WDS only allows the ECT sensor to be displayed in volts, refer to the table for corresponding Celsius values.
The CHT sensor output is useful to examine the cylinder head temperature rise during the warm-up cycle and later during the normal light throttle cruise test. This sensor output may vary between vehicles with manual transmission and vehicles with automatic transmission and should be used for reference only.
The LOAD display is used for reference as it is necessary to maintain a stable load line during the test. It is necessary to carry out the test under normal light throttle cruise driving conditions and average loads, typically 40% to 70% of the load value.
The VSS output is used for reference but can help to identify misfires and sensors which fail during the warm-up cycle.
The RPM display indicates the engine speed and can be compared with the DSRPM.
The DSRPM is the desired or calculated idle speed which the PCM commands the engine to reach. If the thermostat opens too early (before the correct opening temperature has been reached), the engine will not reach this value.
When using the WDS in data logger mode, the signals recorded should remain within the DEFAULT values set by the WDS.
3. WARNING: Make sure that the WDS is placed in the vehicle so that it does not interfere with the safe operation of the vehicle. Do not place the WDS in the deployment path of any air bag. Failure to follow these instructions may result in personal injury.
NOTE: The road test is best carried out with the aid of another technician in the vehicle to enable the vehicle to be driven safely while the sensor outputs are monitored within datalogger. If there is only one technician available, the WDS can be set up (using the record/capture mode camera icon) before leaving the workshop to record a 16 km (10 mile) test.
NOTE: The results from the test are more conclusive if the engine is cold when the test is started.
Carry out a road test.
4. Drive the vehicle at a constant throttle opening and set speed until the ECT value settles into a shallow rise and fall signal, close to a straight line. This indicates that the thermostat is functioning correctly.
NOTE: Some thermostats indicate the temperature(s) in Celsius and Fahrenheit.
The graphic shows the location and an example of the opening temperature (88°C) and fully open temperature (102°C) of a thermostat.
The graphic shows an alternative method used to show the opening temperature (88°C) and fully open temperature (112°C) of a thermostat.
NOTE: Generally, most thermostats maintain a coolant temperature between 88°C (190°F) and 92°C (198°F) although dual stage electric thermostats may increase the coolant temperature up to 100°C (212°F) under light engine load conditions.
The engine should start cleanly and the ECT value will rise quite quickly with smooth progression. If the ECT signal appears unstable or erratic, the ECT sensor, electrical connector and wiring harness to the PCM need to be visually inspected for damage, chafing or water ingress.
The temperature should rise to approximately 90°C for a thermostat that has an 88°C value. The signal value will then fall as cooler coolant enters the engine.
If the ECT value fails to maintain a constant value and falls back to lower figures, typically between 60°C (140°F) and 70°C (150°F), the thermostat and its sealing function within the thermostat housing must be checked.
5. INSTALL a new thermostat if it does not operate as specified.
6. Carry out another road test from step I using the same criteria to confirm that the concern has been rectified.
7. Using the WDS, clear the PCM keep alive memory (KAM) or electrically erasable programmable read only memory (EEPROM) so that new drive values can be learnt.