K Yukon/Denali 4WD V8-5.7L VIN R (2000)
Fuel Gauge: Description and Operation
Magnetic fields of two coils moves the pointer of the fuel gauge. The coils are at right angles to each other. Battery voltage is applied to the E-coil
(empty) and the circuit divides at the opposite end of the coil. One path continues to ground through the F-coil (full). Another path goes through CKT 30
(PPL) to ground on single tanks using the variable resistance of the level sensor in the fuel pump and sender (Gasoline) or the fuel gauge sender (Diesel).
On dual tank vehicles, CKT 30 (PPL) passes resistance from either the Vehicle Control Module (VCM) (Gas) or the Fuel Pump Balance Module
(Diesel).
On gasoline vehicles with dual tanks, the VCM sends a 5 volt reference through CKT 474 (GRY) to the Fuel Pump Balance Module. The Fuel Pump
Balance (LT BLU) for the Secondary Sender. Ground for both senders runs through CKT 900 (BLK) to the VCM. The Fuel Pump Balance Module will
send the return voltage to the VCM through CKT 1589 (PPL/WHT) and the VCM will send the signal onto the Fuel Gauge through CKT 30 (PPL). The
fuel pump balance module averages the resistance for vehicles with two fuel tanks. When the tank is low, the resistance of the level sensor is low. A large
current flow passes through the E-coil and the pointer moves toward E on the fuel gauge. When the tank is full, the level sensor resistance is high, and
less current passes through the E-coil, and the pointer moves toward F on the scale. With two coils operating the pointer, the fuel gauge is not affected by
changes in the system's battery voltage.
On diesel vehicles with dual tanks, CKT 30 (PPL) runs directly to the Fuel Pump Balance Module. Grounds for the two fuel tank senders run directly to
G104. The remainder of the operation is the same as gasoline.
For single fuel tank vehicles, the fuel gauge includes the following functions:
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The level sensor in the fuel gauge sender produces a resistance of about 40 ohms (Gas) 0 ohms (Diesel) empty and about 250 ohms (Gas)/ 90
ohms (Diesel) full.
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A short to ground in the sender or the wiring provides a fully empty indication.
^
An open in the circuit provides completely full indication.
For dual fuel tank vehicles, the fuel pump balance module does the following functions:
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Measures the resistance for both of the fuel senders. Resistance is the same as single tank units.
^
Averages the resistance and produces an equivalent resistance that reflects the average level of both fuel tanks.
^
Power to the module, and a change in the input resistance, produces a change in the output resistance to the fuel gauge.
^
The fuel pump balance module responds to a very low resistance (short to ground) in one or both fuel sender circuits by considering the affected
tank(s) to be empty.
^
A very high resistance (open circuit) in either fuel sender circuit causes the fuel pump balance module to produce a zero output (both tanks empty)
signal to the fuel gauge.
^
Equalizes the level in both of the fuel tanks. Fuel is fed to the engine from the front (primary) fuel tank.
^
Energizes the fuel pump balance relay when the level in the front tank decreases to about two gallons less than the level in the rear (secondary) fuel
tank. This starts the secondary fuel pump which transfers fuel from the rear tank to the front tank.
When the level in the front tank increases to about two gallons greater than the rear tank, the secondary fuel pump stops. This then empties the rear tank
first.
