Schematic Reference: Engine Controls Schematics
Circuit Description
The engine coolant temperature (ECT) sensor is a thermistor (a variable resistor whose conductive value changes with engine coolant temperature) in series with a fixed resistor in the engine control module (ECM). The ECM applies 5 volts to the ECT sensor. The ECM monitors the voltage across the ECT sensor and converts it into a temperature reading. When the engine is cold the ECT sensor resistance is high, and when the engine is warm the ECT sensor resistance is low. Therefore, when the engine is cold the ECM will receive a high voltage input, and when the engine is warm the ECM will receive a low voltage input.
Conditions for Setting the DTC
| • | The ignition is ON. |
| • | ECT sensor signal indicates an engine coolant temperature -40°C (-40°F). |
| • | Above condition is met for at least 5 seconds. |
Action Taken When the DTC Sets
| • | The malfunction indicator lamp (MIL) will illuminate. |
| • | The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer. |
| • | The ECM will enter the Fail-Safe Function and assume the engine coolant temperature is 30°C (86°F). |
Conditions for Clearing the MIL/DTC
| • | The MIL will turn off after three consecutively passing cycles without a fault present. |
| • | A history DTC will clear after 40 consecutive warm-up cycles without a fault. |
| • | DTC(s) can be cleared by using a scan tool or by disconnecting the ECM battery feed. |
Diagnostic Aids
Check for any of the following conditions:
| • | A damaged wiring harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the ECT display on the scan tool while moving connectors and wiring harnesses related to the ECT sensor. A change in the ECT display will indicate the location of the fault. |
| • | When replacing the ECT sensor the Temperature to Resistance Value scale may be used to test the sensor at various temperature levels to evaluate the possibility of a shifted (incorrectly calibrated) sensor. A shifted sensor could result in poor driveability complaints. Refer to Temperature Versus Resistance . |
| • | After the engine is started, the ECT sensor should rise steadily to about 95°C (203°F) and stabilize when thermostat opens. Verify that the engine is not overheating and has not been subjected to conditions which could create an overheating conditions (i.e., overload, trailer towing, hilly terrain, heavy stop and go traffic, etc.). |
| • | A shorted to voltage sensor signal circuit may cause DTC P0118 to set. |
An intermittent malfunction may be caused by a fault in the ETC sensor circuit. Inspect the wiring harness and components for any of the following conditions:
| • | Backed out terminals. |
| • | Improper mating of terminals. |
| • | Broken electrical connector locks. |
| • | Improperly formed or damaged terminals. |
| • | Faulty terminal to wire connections. |
| • | Physical damage to the wiring harness. |
| • | A broken wire inside the insulation. |
| • | Corrosion of electrical connections, splices, or terminals. |
If the DTC P0118 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.
Test Description
The numbers below refer to the step numbers in the Diagnostic Table.
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The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
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Determine if fault is present.
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Review Freeze Frame data to determine when the DTC set. Always record this information.
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This step determines if the fault is caused by an open or shorted signal return circuit, a open in the ground circuit or by a malfunctioning ECT sensor.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | |||
Is the ECT display at or less than the specified value? | -40°C (-40°F) | |||
Is a DTC P0118 set? | -- | Go to Diagnostic Aids | ||
Is the ECT display at the specified value? | 164°C (327°F) | |||
5 |
Is the ECT display at the specified value? | 164°C (327°F) | ||
6 | Check for a poor connection at the ECT sensor and repair as necessary. Was a problem found? | -- | ||
7 |
Was a repair necessary? | -- | ||
8 |
Was a repair necessary? | -- | ||
9 |
Was a repair necessary? | -- | ||
10 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement. Is the action complete? | -- | -- | |
11 | Replace the ECM. Refer to Engine Control Module Replacement. Is the action complete? | -- | -- | |
12 |
Are any DTCs displayed on the scan tool? | -- | Go to the Applicable DTC Table | System OK |
