Mercedes Mullins
The GMC Sierra 3500HD is a truck that comes with a dual-tank fuel system. This system includes a primary front tank and a secondary rear tank. The fuel gauge operates by reading the levels of both tanks and displaying the average fuel level. When the primary tank falls below 16.9 gallons, and the secondary tank is above 2.6 gallons, a fuel transfer pump is activated, moving fuel from the secondary to the primary tank. This continues until the primary tank reaches 20.9 gallons, at which point the pump turns off. If the secondary tank drops below 2.6 gallons and the primary tank is also below 20.9 gallons, the pump is activated again for 300 seconds to transfer the remaining fuel. While the rear tank is used for fuel, some users have reported issues with the fuel gauge, indicating that the rear tank is not being used.
| Characteristics | Values |
|---|---|
| Year of GMC model | 2001, 2004, 2008, 2012, 2022 |
| Model | GMC 3500 Cab & Chassis Truck |
| Number of fuel tanks | 2 |
| Fuel tank capacity | 23.5 gallons (front), 40 gallons (rear) |
| Fuel gauge | Combined level info from both tanks |
| Primary fuel tank level | Less than 64 liters (16.9 gallons) |
| Secondary fuel tank level | Greater than 10 liters (2.6 gallons) |
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What You'll Learn
The front tank is the only one that uses fuel
The GMC 3500 is a truck that comes with a dual-tank system. This means that the truck has two fuel tanks: a primary and a secondary tank. The front tank is considered the primary tank and is the only one that uses fuel.
The GMC 3500's dual-tank system works as follows: the computer reads both the front and rear sensors to operate the fuel gauge. If either sensor is disconnected, the fuel gauge will read as empty. When the primary fuel tank level falls below 16.9 gallons, and the secondary fuel tank level is above 2.6 gallons, the fuel transfer pump is activated. This pump transfers fuel from the secondary tank to the primary tank until the primary tank reaches 20.9 gallons, at which point the fuel transfer pump is turned off. If the secondary tank level drops below 2.6 gallons and the primary tank is below 20.9 gallons, the ECM activates the fuel transfer pump for 300 seconds to pump the remaining fuel to the primary tank.
There is no fuel pump in the rear tank, and fuel is only pumped from the rear to the front tank. The rear tank remains full and is not used to power the truck. This means that the front tank is the only one that uses fuel.
Some users have reported issues with the dual-tank system, such as the rear tank not transferring fuel to the front, or the fuel gauge not working properly. These issues can be caused by various factors, including faulty sensors or pumps.
The GMC 3500's dual-tank system offers several benefits, such as increased fuel capacity and the ability to optimise fuel usage. However, it is important to properly maintain and diagnose any issues with the system to ensure it functions correctly.
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The rear tank never depletes
The GMC 3500 is a truck that comes with the option of two fuel tanks. The two tanks are a primary and a secondary tank, with the primary tank being the front tank. The rear tank never depletes because the fuel is transferred from the rear tank to the front tank. This transfer happens when the computer reads both the front and rear sending units and operates the fuel gauge. If there is more than a 20% difference in levels between the two tanks, a lift pump is used to transfer fuel from the rear to the front tank.
The rear tank is never used up because it is the secondary tank and only comes into play when the primary tank reaches a certain level of depletion. The rear tank will then be used to fill the primary tank, ensuring that the engine always has enough fuel to run. This is done automatically, so the driver does not need to worry about switching between tanks or running out of fuel.
Some people have reported issues with the rear tank not transferring fuel to the front, resulting in the front tank running empty while the rear tank remains full. This could be due to a faulty fuel gauge, a problem with the transfer pump, or another issue with the fuel system. In these cases, it is important to have the truck diagnosed and repaired by a qualified technician.
It is worth noting that the procedure for filling up dual tanks is a subject of some debate. Some people recommend filling the rear tank first, while others suggest that it doesn't matter which tank is filled first as long as the keys are in the ignition, allowing the transfer pump to operate.
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The fuel gauge displays the average fuel level of both tanks
The GMC 3500 has two fuel tanks, a primary and a secondary tank. The front tank is the primary tank and is the only one that uses fuel, while the back tank remains full. The fuel gauge displays the average fuel level of both tanks.
The computer reads both the front and rear sending units to operate the fuel gauge. If either one is not working or disconnected, the fuel gauge will read as empty. The fuel transfer pump is activated when the primary tank falls below a certain level and continues to transfer fuel from the secondary tank until the primary tank reaches a set level. The pump is also activated when the secondary tank drops below a certain level, and the remaining fuel is pumped to the primary tank.
There is no switch to manually choose between the tanks, and the engine draws fuel from both tanks simultaneously. The fuel gauge reading is an average of the levels in both tanks. This is achieved through two level sensors that feed information to the PCM (Powertrain Control Module), which then displays the average on the gauge.
Some users have reported issues with the fuel gauge reading accurately, with the gauge reading empty or not moving from empty even when the tank has been filled. This may be due to a faulty sending unit or another issue with the fuel system.
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The fuel transfer pump moves fuel from the rear to the front tank
The 2001 GMC 3500 Cab and Chassis truck has dual fuel tanks. One user reported that their rear fuel tank was not transferring fuel to the front tank, and the fuel gauge was registering empty. This issue may be caused by a faulty fuel transfer pump.
The fuel transfer pump is also designed to ensure that the rear tank is not completely emptied. If the primary tank level falls below the threshold and the rear tank level is low, the pump will transfer the remaining fuel from the rear tank to the primary tank. This process usually takes around 300 seconds.
Some GMC 3500 models have a switchless dual-tank system, which automatically transfers fuel from the rear to the front tank. This system uses a computer to monitor the fuel levels in both the front and rear sending units. If there is more than a 20% difference in fuel levels between the two tanks, the system activates a lift pump to transfer fuel from the rear to the front tank.
It is important to note that the specific fuel transfer pump operation may vary depending on the model year and configuration of the GMC 3500.
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The rear tank has no fuel pump
The 2001 GMC 3500 Cab and Chassis truck has two fuel tanks, but the rear tank does not seem to be working as intended. The rear tank is not transferring fuel to the front tank, and the fuel gauge is registering empty. This is a common issue with GMC 3500 trucks with dual fuel tanks.
The rear tank is not equipped with a fuel pump, which means that fuel cannot be pumped from the rear tank to the front tank. The front tank is the primary fuel tank and is the only one that supplies fuel to the engine. The rear tank is the secondary tank and is intended to extend the range of the vehicle by providing additional fuel capacity.
The fuel transfer pump is located between the two tanks and is responsible for regulating the level of fuel between them. When the primary fuel tank level falls below a certain threshold (less than 64 litres or 16.9 gallons), and the secondary fuel tank level is above a certain level (greater than 10 litres or 2.6 gallons), the fuel transfer pump is activated. The pump transfers fuel from the rear tank to the primary tank until the primary tank reaches a level of 79 litres (20.9 gallons). Once this level is reached, the fuel transfer pump is turned off.
If the fuel transfer pump is not working correctly, it can cause issues with fuel transfer between the tanks and lead to inaccurate fuel gauge readings. In some cases, the fuel gauge may drop to "E" and indicate low fuel even when there is fuel in the rear tank. This can be caused by a faulty sending unit or a problem with the float arm in the rear tank, which can get stuck when going over bumps.
To resolve the issue of fuel not transferring from the rear tank, it is recommended to check and replace the fuel transfer pump if necessary. In some cases, hotwiring the transfer pump may be a temporary solution, but it defeats the automatic fuel transfer feature. It is also important to check for any fault codes that may be causing the issue and clear them to bring the transfer pump and gauge back online.
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Frequently asked questions
The front tank is the only one that uses fuel. The back tank stays full and is never used.
The front tank is the primary tank and should be refuelled first. The rear tank is the auxiliary tank and should be filled after.
The computer reads both the front and rear sending units to operate the fuel gauge. If either one is disconnected, the gauge will read empty. The gauge displays the average amount of fuel across both tanks.
When the primary fuel tank level falls below 16.9 gallons and the secondary fuel tank level is above 2.6 gallons, the fuel transfer pump is activated and transfers fuel to the primary tank. When the secondary tank level falls below 2.6 gallons and the primary tank is below 20.9 gallons, the ECM commands the fuel transfer pump to pump the remaining fuel to the primary tank.
There is a weatherpak-style connector along one of the crossmembers near the pump, just ahead of the rear axle.
