Circuit Description
The vehicle control module (VCM) requires an accurate indication of fuel level for evaporative emission (EVAP) system diagnosis. The fuel level in the fuel tank changes the rate of vacuum decay for the EVAP system leak diagnostic trouble codes (DTCs).
The fuel level sensor changes resistance based on fuel level. The fuel level sensor has a signal circuit and a ground circuit. The VCM applies about 5 volts on the signal circuit to the sensor. The VCM monitors changes in this voltage caused by changes in the resistance of the sensor to determine fuel level.
When the fuel tank is full, the sensor resistance is high, and the VCM signal voltage is only pulled down a small amount through the sensor to ground. Therefore, the VCM will sense a high signal voltage or a full fuel tank. When the fuel tank is empty, the sensor resistance is low, and the signal voltage is pulled down a greater amount. This causes the VCM to sense a low signal voltage, or an empty fuel tank.
The VCM uses the input from the fuel level sensor in order to calculate the fuel level in the fuel tank. This information is then sent to the instrument panel cluster (IPC) through serial data.
The diagnostic will not run when the tank is more than 85 percent or less than 15 percent full. This sensor signal disables the misfire when the fuel levels are less than 15 percent.
Conditions for Running the DTC
The ignition switch is in the RUN position.
Conditions for Setting the DTC
The fuel level sensor voltage remains above 2.9 volts for more than 20 seconds.
Action Taken When the DTC Sets
| • | The control module stores the DTC in history after the first failure but will not illuminate the malfunction indicator lamp (MIL). |
| • | The control module records the operating conditions at the time the diagnostic fails. The control module stores the failure information in the scan tools Freeze Frame/Failure Records. |
Conditions for Clearing the MIL/DTC
| • | A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. |
| • | A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during the same ignition cycle. |
| • | Use the scan tool Clear Information function. |
Diagnostic Aids
Important: Fuel level is used to determine EVAP system diagnosis. Always diagnose the fuel level sensor before any other EVAP system components or DTCs.
Do not use the fuel gauge as an indication of a shorted or open circuit as the IPC may always default to EMPTY or FULL depending upon the vehicle model.Check for the following conditions:
| • | A poor connection or damaged harness -- Inspect VCM harness connectors for the following: |
| - | Backed out terminals |
| - | Improper mating |
| - | Broken locks |
| - | Poor wire to terminal connections |
| • | Intermittent test -- Monitor a scan tool or digital voltmeter connected between the related circuits while moving the connectors and harnesses. If the failure is induced, the voltage reading will change. This will help locate a circuit problem. |
Test Description
The numbers below refer to the step numbers in the diagnostic table.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
2 |
Is the voltage more than the specified value? | 2.9 V | ||
3 | This DTC is intermittent. Are any other DTC stored? | -- | Go to the applicable DTC table | Go to Diagnostic Aids |
Is the voltage near the specified value? | 5.0 V | |||
5 | Check for an open in the fuel level signal circuit. Was a problem found? | -- | ||
6 | With a test lamp connected to B+, probe the fuel level ground circuit. Is the test lamp ON? | -- | ||
7 | Check for a short to voltage in the fuel level signal circuit. Was a problem found? | -- | ||
8 | Check for an open in the fuel level ground circuit. Was a problem found? | -- | ||
9 | Check the electrical connector at the fuel level sender for corrosion and improper mating. Was a problem found? | -- | ||
10 | Repair the circuit as necessary. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
11 | Replace the fuel level sensor. Refer to Fuel Sender Assembly Replacement . Is action complete? | -- | -- | |
12 |
Is the action complete? | -- | -- | |
13 |
Does the scan tool indicate the diagnostic Passed? | -- | ||
14 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to the applicable DTC table | System OK |
