Sylvie Willis
Dual tank fuel gauge systems are designed to work with two tanks, often referred to as the front and rear tanks. The system can be configured in various ways, with some setups utilising a transfer pump to maintain equal fuel levels in both tanks, while others use valves to control fuel flow between the tanks. The fuel gauge operates by taking readings from both tanks and combining them to display the total fuel level. This is achieved through the use of fuel senders, which consist of variable resistors controlled by floats in the tanks. The resistance changes as the fuel level varies, allowing the gauge to indicate the amount of fuel present in both tanks. However, issues with the transfer pump, faulty sending units, or wiring problems can cause the fuel gauge to malfunction and display inaccurate readings.
| Characteristics | Values |
|---|---|
| Dual tank system | Works as one |
| Fuel gauge sender | Variable screened resistor made up of a ceramic substrate |
| Fuel gauge sender controlled by | Float in the fuel tank |
| Fuel gauge sender resistance when tank is low | Low |
| Fuel gauge sender resistance when tank is full | High |
| Fuel gauge sender resistance range | 40 ohms (empty) to 250 ohms (full) |
| Dual tank setup | Runs everything off the front tank |
| Rear tank | Transfer tank |
| Gauge wire shorting to ground | Makes the gauge peg past empty |
| Gauge reading issues | Could be due to faulty sending unit, faulty wiring, etc. |
| Gauge reading issues resolved by | Checking wiring, replacing sending units, fixing plumbing, etc. |
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What You'll Learn
Dual fuel sensors, one gauge
The basic principle behind this system involves using electrically identical fuel senders in both tanks. These senders consist of variable screened resistors controlled by a float in the fuel tank. When the fuel level is low, the resistance in the sender is low, and when the fuel level is high, the resistance is high. By connecting the senders in series and paralleling them with appropriate resistors, the gauge can display the fuel levels of both tanks simultaneously.
For example, let's say Tank 1 has one-third the volume of Tank 2. By paralleling the Tank 2 sender with 750 ohms and the Tank 1 sender with 200 ohms, the gauge will read 'full' when both tanks are full and '1/3' when Tank 2 is empty. It will continue to display 'Empty' when both tanks are empty. The correct divider values would be calculated based on the actual ratio of tank volumes.
However, one challenge with this system is that it assumes the fuel levels in both tanks will be the same if they are connected together. To empty one tank without affecting the other, valves would be required on each tank. Additionally, the meter is likely part of the circuit, and disconnecting it could result in sensor voltage fluctuations.
Overall, while dual fuel sensors, one gauge system provides a simple indication of combined fuel volumes, it may require further modifications and considerations to ensure accurate and reliable fuel level readings in both tanks.
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How to fix a dual tank fuel gauge that reads empty
If your dual tank fuel gauge is reading empty, there are several steps you can take to try and fix the issue.
Firstly, check the wiring between the gauge and the sending unit. Loose, broken, or short-circuited wires due to moisture or heat can cause inaccurate readings. Use a multimeter to test the continuity and voltage of the wires and inspect the connectors and terminals for any signs of damage or corrosion. If issues are found, repair or replace the wiring.
Secondly, ensure your plumbing is configured correctly. Improper plumbing can lead to incorrect fuel gauge readings. Make sure to run the correct cab and chassis fuel segment to enable the transfer pump.
Thirdly, check for a blown fuse to the fuel gauge. A blown fuse can disrupt the signal between the sending unit and the gauge, resulting in an empty reading.
Additionally, a faulty fuel gauge itself could be the issue. Physical damage, water damage, or electrical issues can cause the gauge to malfunction. To test this, remove the gauge from the dashboard and use a multimeter or an external power source to check its functionality. If it fails to respond or shows incorrect readings, replace the gauge.
Finally, a blocked breather in the fuel tank can also lead to inaccurate readings. Locate the breather hose or vent and clear any obstructions. If the hose or vent is damaged, it may need to be replaced.
If these basic troubleshooting steps do not resolve the issue, it is recommended to consult a professional mechanic for further diagnosis and repair.
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How to build a circuit for a dual tank fuel gauge
A dual tank fuel gauge works by combining the resistance of the floats/resistance value in parallel, which can cause a mess in the reading. To build a circuit for a dual tank fuel gauge, you can follow these steps:
Firstly, you need to separate the circuits for each tank to accurately read the fuel level in each. This can be done by adding a switch to select the tank the gauge is monitoring. This will prevent issues such as sucking air from the first empty tank.
Secondly, ensure that the tanks are not connected in series, as this would result in the same fuel level in both tanks. To have different fuel levels, valves would need to be installed on each tank.
Thirdly, the fuel gauge sender unit needs to be set up correctly. The fuel sender unit consists of a variable screened resistor made of a ceramic substrate. It is controlled by a float in the fuel tank, so when the fuel level is low, the resistance in the sender is low, and vice versa.
Fourthly, the circuit should be designed based on accurate information, not assumptions. The sensor voltage needs to be known when the gauge is attached and when the tank is empty and full.
Finally, the circuit can be designed with an indicator to show which tank is being read. This can be done with a bicolor LED, with green and red colours to indicate the tanks.
Overall, building a circuit for a dual tank fuel gauge requires careful consideration of the fuel sender units, tank setup, and accurate information for the circuit design.
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How dual tank systems work on gas trucks
Dual fuel tanks used to be a common feature in American trucks, but they have since been phased out due to engineering advancements and other factors. While dual fuel tanks offered benefits such as increased fuel capacity for strenuous tasks and better weight distribution, they also presented several inconveniences and hazards. One of the most significant issues was the malfunctioning of fuel selector valves, which could lead to one tank overfilling the other. Refuelling dual tanks was also a cumbersome process, requiring gasoline to be pumped into two separate tanks.
In a dual tank system, the tanks can be considered independent, with each tank having its own fuel pump and assembly. However, there have been reports of interactions between the tanks, where using one tank seems to deplete the other. This could be due to a faulty design or other factors. To address this issue, some trucks employ a Fuel Delivery Module (FDM) in each tank, which includes a fuel pump, gauge sender, and a check valve.
The fuel gauge sender in each tank consists of a variable screened resistor made of a ceramic substrate. It is controlled by a float attached to the fuel tank. When the fuel level is low, the resistance in the sender is low, and when the fuel level is high, the resistance is high. As the float moves from empty to full, the resistance gradually increases. A faulty gauge wire or an open circuit between the gauge and the driver's side radiator support can cause the gauge to read incorrectly.
Some dual tank setups utilise a transfer pump controlled by the Engine Control Module (ECM) to manage fuel levels in both tanks. The ECM monitors the levels in each tank and activates the transfer pump when the front tank's level drops below a certain threshold, typically around 75%. The ECM also ensures that fuel is only pulled from the front tank, and the rear tank serves as a transfer tank. If the ECM detects an unexpected level change, it defaults the gauge to empty and disables the transfer pump.
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How to diagnose a dual tank balance module
Diagnosing a dual-tank balance module can be done by following these steps:
Firstly, locate the balance module. It is usually situated by the front (main) lift pump, slightly towards the rear of the vehicle along the frame rail. Once located, you will see a fuel pump balance relay on top of the module. This relay turns the rear pump on and off as needed to transfer fuel.
Next, you will need a digital volt/ohm meter to check the following: Terminal A- The pink wire should show a reading of 12 volts with the key in the run position. The fuse is in the underhood fuse/relay centre, marked IGN E 10 AMP. Terminal B- The light blue wire is the signal wire from the secondary (rear) fuel tank. Using an ohm meter, the resistance from a good chassis ground to this wire should be between 0 and 90 ohms, with 0 indicating an empty tank and 90 a full tank.
You can also test the two pins on the harness with all the wires. Unplug the module and test the pins with a meter. A reading of 35.5 empty is unusual, as most GM FSU's have a 0-90 ohms range.
For diesel vehicles, the dual-tank setup operates with CKT 30 (PPL) running directly to the Fuel Pump Balance Module. The grounds for the two fuel tank senders run directly to G104.
If you are experiencing issues with the fuel gauge, it could be due to a faulty pump assembly or a problem with the instrument panel. Ensure all fuses and relays are checked, and that the wiring is intact.
In some cases, the balance tank module may not be present, as newer vehicles use the PCM (Powertrain Control Module) instead.
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Frequently asked questions
This could be due to a faulty sending unit, faulty wiring, or a faulty transfer pump. Check the wiring and clean the grounds. If the gauge still doesn't work, try replacing the sending unit.
You can try a continuity test on the fuel sender wire to make sure a signal is getting back to the gauge. If not, then the problem is with the sender.
The transfer pump transfers fuel from the rear tank to the front tank. The ECM controls the transfer pump and watches the level in each tank to allow a single gauge to show the combined fuel for both tanks.
The gauge is a voltage-based gauge and reads the voltage drop across the sender resistance relative to 12v.
