Fuel Gauge

WARNING: This page is about a different car, the 2005 Mercury Mountaineer and 2005 Ford Explorer. However, it is still accessible from the selected car via links, so may be relevant.

Fuel level is determined using a variable resistance fuel level sender unit, with an approximate resistance range between 15 ohms +/- 2 ohms at empty (E) and 160 ohms +/- 4 ohms at full (F) on 4.6L engines and between 7 ohms +/- 2 ohms at the lower stop position and 138 ohms +/- 4 ohms at the upper stop position on 4.0L engines. If equipped with flex-fuel (4.0L engines), the basic fuel level indication is the same except that the flex-fuel vehicles incorporate the use of a flex-fuel module. The flex-fuel module is a DC-AC interface. To prevent electrochemical reactions with the alcohol in the fuel, the flex-fuel module applies an AC voltage to the fuel level sender to measure the fuel sender resistance based on current fuel level and returns a modified resistance matching the fuel level to the instrument cluster. The flex-fuel module is part of the fuel pump module jumper harness and is installed as an assembly.

The instrument cluster 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.

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 full (F) to empty (E).

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 when refueling the vehicle.
At least 15 percent of the vehicle's fuel capacity must be added to the fuel tank.
The instrument cluster 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.

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).
The key is in the RUN position.
At least 15 percent 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. When the 30 second time has elapsed and at least 15 percent 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), 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 is incorporated into the instrument cluster strategy to recover from a missing fuel level message during a refueling event. Missing fuel level messages 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 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.