Circuit Description
The Engine Coolant Temperature (ECT) sensor is a thermistor which controls the signal voltage to the VCM. The VCM applies a voltage on the 5 volt reference circuit to the sensor. The sensor resistance is high when the engine is cold. Therefore, the VCM senses a high voltage signal and the VCM indicates a low coolant temperature. The sensor resistance becomes less as the engine warms and the voltage drops indicating high coolant temperature. The voltage measures about 1.5 to 2.0 volts at normal engine operating temperature 85°C to 95°C (185°F to 203°F).
This is a type B DTC.
Conditions for Setting the DTC
The following conditions will set the DTC:
| • | The vehicle is in the CNG operation mode. |
| • | The engine has been running longer than 5 seconds at intake air temperatures greater than 0°C (32°F) or longer than 600 seconds at intake air temperature less than 0°C (32°F). |
| • | The ECT circuit voltage is higher than 4.395 volts. |
Action Taken When the DTC Sets
The ECU sets the DTC 14 after 2 consecutive driving trips with the fault active.
Conditions for Clearing the MIL/DTC
The ECU turns the MIL OFF after 3 consecutive driving trips without a fault condition present. A DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°C (72°F) from the start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during that same ignition cycle) or the code clearing procedure has been used.
Diagnostic Aids
Always diagnose ANY VCM codes that are present before beginning this diagnostic procedure.
The temperature should rise steadily to about 90°C (194°F) after starting the engine. The temperature will stabilize when the thermostat opens. A faulty connection or an open in the 5 volt reference circuit or the sensor ground circuit results in a DTC 14.
Use the Temperature vs. Resistance Value Table to test the coolant sensor at various temperature levels in order to evaluate the possibility of a skewed (mis-scaled) sensor. A skewed sensor could result in poor driveability complaints. Refer to Temperature vs. Resistance in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
-
The engine coolant temperature voltage measures greater than 4.395 volts if the fault is present.
-
This test simulates conditions for a DTC 14. The wiring and the ECU are OK if the ECU recognizes the grounded circuit (low voltage) and displays a low voltage message.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Alternative Fuels (AF) Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check |
2 |
Does the scan tool display the ECT sensor voltage greater than the specified value? | 4.9 V | Go to DTC P0118 ECT Sensor Circuit High Voltage in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual | Go to Step 3 |
Backprobe the ECT sensor signal in the ECU connector using a DVOM. Does the DVOM display the ECT sensor signal greater than the specified value? | 4.39 V | Go to Step 4 | Go to Step 5 | |
4 | Disconnect ECU connector C001. Does the DVOM display the ECT sensor signal greater than the specified value? | 4.39 V | Go to Step 7 | Go to Step 6 |
5 |
Is there continuity? | -- | Go to Step 9 | Go to Step 14 |
Replace the ECU. Refer to Engine Control Unit Replacement . Is the action complete? | -- | Go to Step 17 | -- | |
7 |
Does the scan tool display the ECT sensor voltage less than the specified value? | 0.82 V | Go to Step 10 | Go to Step 8 |
8 | Jumper the ECT sensor signal circuit to a known good ground. Does the scan tool display the ECT sensor voltage less than the specified value? | 0.82 V | Go to Step 11 | Go to Step 12 |
9 | The DTC is intermittent. Refer to Diagnostic Aids if no additional DTCs were stored. If any additional DTCs are stored, refer to those tables. Are any additional DTCs set? | -- | Go to the applicable DTC table | -- |
10 | Inspect the ECT sensor connector and the VCM connector for the proper connection. Is a problem found? | -- | Go to Step 14 | Go to Step 15 |
11 | Inspect the ECT sensor ground circuit for an open between the ECT sensor and the VCM. Is a problem found? | -- | Go to Step 14 | Go to Step 16 |
12 | Is DTC P0123 also set? | -- | Go to DTC P0123 TP Sensor Circuit High Voltage in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual | Go to Step 13 |
13 | Inspect the ECT sensor signal circuit for an open between the ECT sensor and the VCM. Was a problem found? | -- | Go to Step 14 | Go to Step 16 |
14 | Repair the circuit as necessary. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 17 | -- |
15 | Replace the ECT sensor. Refer to ECT Sensor Replacement in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 17 | -- |
16 | Replace the VCM. Important:: Reprogram the VCM if it is faulty. Refer to VCM Replacement/Reprogramming in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 17 | -- |
17 |
Does the diagnostic run and pass? | -- | Go to Step 18 | Go to Step 2 |
18 | Are any ECU or VCM diagnostic trouble codes displayed that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
