Low Fuel Telltale Lamp Theory of Operation

The powertrain control module (PCM) monitors the resistance of the fuel level sender (tank sender) as a function of the feedback voltage, and calculates the amount of fuel in the tank. The PCM provides a pulse width modulated (PWM) ground to control the fuel gauge as well as serve as a ground source for the LOW FUEL telltale lamp through Quad Driver "A."

The PCM will turn On the LOW FUEL telltale lamp when the calculated fuel level in the tank reaches approximately 1.5 gallons (5.7 L). The level is calculated by taking a buffered average of the sender resistance. The resistance value used to trigger the LOW FUEL telltale lamp is 519 ohms or more. The sender resistance must vary between 677 and 440 (pad # 1 and # 2) or be stationary at 677 ohms (pad # 1) to trigger the LOW FUEL telltale lamp. The buffer is used to prevent the LOW FUEL telltale lamp from turning On and Off when the level is fluctuating between 677 and 440 ohms (pad # 1 and # 2).

The time required for the buffer average to drop below 519 ohms will vary based on driving habits and fuel level. The buffer will reset when the PCM is powered down for more that 10 seconds.

If the LOW FUEL telltale lamp is inoperative when the vehicle is out of fuel, the resistance is not reaching the threshold of 519 ohms.

To correct the condition it will be necessary to bend the float up, which will increase the float level as measured on the bench. This will cause the sender to indicate a higher resistance value for the same level of fuel. If the float level is increased too far, the LOW FUEL telltale will come On with too great of a reserve and the fuel gauge will read too low for the actual amount of fuel in the tank.