Test B: Incorrect Fuel Gauge Indication
PINPOINT TEST B: INCORRECT FUEL GAUGE INDICATIONNormal Operation
The instrument cluster (IC) is hardwired to the fuel sending unit, which is a variable resistor controlled by the action of a float arm. The fuel gauge receives its signal from the fuel level sensor, part of the fuel pump module. The fuel level is determined using a variable resistance fuel level sender unit, with an approximate resistance range between 138 ohms ± 4 ohms when full (F) and 7 ohms ± 2 ohms at empty (E) for vehicles equipped with flex-fuel and 160 ohms ± 4 ohms at full (F) and 15 ohms ± 2 ohms at empty (E) for vehicles not equipped with flex-fuel. The fuel level sender unit is hardwired to the instrument cluster (IC) through the signal circuit 29 (YE/WH) and the return circuit 396 (BK/OG). The instrument cluster (IC) monitors the resistance readings that are sent to the instrument cluster (IC) from the fuel pump module and commands the fuel gauge with a corresponding movement of the pointer. This information is also supplied by the instrument cluster (IC) to the PCM through the communication network.
On flex-fuel vehicles, basic fuel level indication is the same as gasoline vehicles, except that the flex-fuel vehicles incorporate the use of a flex-fuel module and converter harness. The flex-fuel module and converter harness is a DC-AC interface, used to prevent chemical reactions created by the interaction of DC current with the fuel. The flex-fuel module and converter harness is part of the wire jumper harness between the vehicle wiring harness and the fuel pump module and is installed as an assembly. The flex-fuel module and converter harness receives voltage on circuit 705 (LG/OG) and is grounded through circuit 396 (BK/OG).
The instrument cluster (IC) uses 4 different operating modes to calculate the fuel level:
- Anti-slosh (default mode)
- Key OFF fueling
- Key ON fueling
- Recovery
After a fuel fill up, the time for the fuel gauge to move from empty (E) to full (F) ranges from 2 seconds to 55 minutes depending on which operating mode the fuel gauge is in.
Anti-Slosh Mode
The default fuel gauge mode is called the anti-slosh mode. To prevent fuel gauge changes from fuel slosh (gauge instability due to changes in fuel sender readings caused by fuel moving around in the tank), the fuel gauge takes approximately 55 minutes to go from empty (E) to full (F).
Key OFF Fueling Mode
The key OFF fueling mode (2 seconds to read empty [E] to full [F]) requires 3 conditions be met:
- The key must be in the OFF position throughout the entire refueling of the vehicle.
- At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
- The instrument cluster (IC) must receive a valid key ON fuel sender reading within 1 second of the key being put into the RUN position. The key ON sample readings are considered valid if the fuel sender reading is between 15 ohms ± 2 ohms and 160 ohms ± 4 ohms.
If these conditions are not met, the fuel gauge stays in the anti-slosh mode, which results in a slow to read full (F) event.
Key ON Fueling Mode
The key ON fueling mode (approximately 90 seconds to read empty [E] to full [F]) requires 3 conditions be met:
- The transmission is in PARK (P) (automatic transmissions), or the parking brake is applied (manual transmissions).
- The key is in the RUN position.
- At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
In key ON fueling mode, a 30-second timer activates after the transmission is put into the PARK (P) position or when the parking brake is applied. When the 30 second time has elapsed and at least 15% of the vehicle's fuel capacity has been added, the fuel gauge response time is 90 seconds to read from empty (E) to full (F). When the transmission is shifted out of PARK (P) or the parking brake is released, the fuel gauge strategy reverts to the anti-slosh mode. This mode prevents slow to read full events from happening if the customer refuels the vehicle with the key in the RUN position.
Recovery Mode
Recovery mode is incorporated into the instrument cluster (IC) strategy to recover from a missing fuel level input after a refueling event. Missing fuel level inputs result from intermittent opens in the fuel sender or its circuits. Recovery mode (empty [E] to full [F] approximately 20 minutes) is initiated when the following 2 conditions are met:
- The instrument cluster (IC) is in the anti-slosh (default) mode.
- The actual fuel level in the tank is greater than what is being displayed by the fuel gauge.
Instrument Cluster (IC) DTCs:
PCM DTCs:
This pinpoint test is intended to diagnose the following:
- Fuse
- Wiring, terminals or connectors
- Fuel pump module assembly
- Flex-fuel module and converter wire harness (if equipped)
- Fuel tank
- Fuel level sensor
- Instrument cluster (IC)
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