Fuel Level - Reads Low Fuel After Battery Disconnected
911(996)9/99 2015
Binder - 2, Fuel
Carrera 4 - Running Out of Fuel
Vehicle Type:
911 Carrera 4 (996)
Model Year:
As of 1999 (X)
Concern:
Running out of fuel after the battery is disconnected.
Information
In all current models except the 911 Carrera 4 (996), the fuel level warning light (reserve) illuminates when there is approx. 10 liters of fuel remaining in the fuel tank. When the reserve light comes on, the driving-distance-on-remaining-fuel (trip computer) is calculated by monitoring the fuel consumption signal (ti), and continues until the fuel tank is empty (or refilled).
In the case of the 911 Carrera 4 (996), the shape and configuration of the fuel tank prevent the fuel level sensing system from being able to physically read down to a level of 10 liters. Therefore, on the 911 Carrera 4 the driving-distance-on-remaining-fuel calculation begins when the fuel reaches a level of 19 liters (approx. 1/4 tank).
As fuel is further consumed below 19 liters, the amount of fuel remaining in the tank is a calculated value. The calculated value is derived from the fuel consumption signal (ti), and continues until the fuel tank is empty (unless the fuel tank is refilled).
Whenever the fuel level is less than 19 liters and the ignition is switched off, the fuel-level and driving-distance-on-remaining-fuel values are stored in instrument cluster memory.
If battery power is interrupted, these stored values are lost (erased from instrument cluster memory). Once lost, these values will not be automatically recalculated unless the fuel tank is filled with at least 19 liters (crossing the calculation-switchover threshold).
To summarize, if the battery is disconnected with the fuel level less than 1/4 tank, the fuel level and driving-range-on-remaining-fuel values will be incorrect. When the battery is reconnected, the fuel calculation will default to 19 liters, when in fact, the fuel level is most likely less than 19 liters. This can result in the vehicle running out of fuel, with a seemingly good fuel level indication.
If the battery must be disconnected on a 911 Carrera 4 (996), perform one of the following to prevent running out of fuel:
1. Fill the fuel tank with more than 19 liters of fuel (1/4 or more) before disconnecting the battery. This will eliminate the danger of invalid values being stored because the fuel level will already be above the calculation switching point when the battery is reconnected.
or
2. Fill the fuel tank with at least 19 liters of fuel after reconnecting the battery. This will reset the calculation process with valid values.