Hello and welcome to just answer. The fuel is siphoned from one side to the other internally through plastic tubes connected between the 2 modules. Here is a description of how it works.
OPERATIONThe way the pump works is as follows, fuel enters the reservoir of the driver side module. The fuel pump pumps the fuel through the filter to the passenger side module through a supply line inside tank. The pressure regulator inside the passenger side module regulates the pressure at 58 psi. All unused fuel that is not sent to the engine is fed through a venturi at the bottom of the passenger side module. This creates a low pressure siphoning effect and draws fuel from the passenger side of the tank and transfers it to the drivers side tank via siphon hose inside the tank. While the vehicle is running the fuel in the passenger side of tank is continuously transferred to the drivers side. Fuel will continue to fill the drivers side tank till it reaches the bridge section and then start to spill over to the passenger side. As stated above we have two fuel level senders, the reading of these senders are averaged out to give us the fuel gauge reading. When we are diagnosing a sender concern the passenger side reading should never be higher than the Drivers side reading. It is possible, depending on fuel level and driving habit before diagnosing, to spill fuel over to the passenger side that might indeed show a lower resistance value than the driver side. The fuel gauge gives an indication to the vehicle operator of the level of fuel in the fuel tank. This gauge is controlled by the instrument cluster circuit board based upon cluster programming and a hard wired input received by the cluster from the fuel level sending units on the modules in the fuel tank.
Here is a Star case that talks about replacing the Primary fuel module when there are fuel gauge problems.
Case Number XXXXX Vehicle Issue Fuel gauge is inaccurate.;Customer may complain of running out of fuel when the gauge indicates there is fuel in the tank. Customer may complain of only adding 10 gallons of fuel when gauge reads empty. System or Component Erratic Operation / Fluctuates,Inaccurate;Customer Concern (No DTCs);Fuel Gauge Recommendation Please remember that the secondary module (right side) is just that. There is no pump on that side. There is a venturi action which needs fuel flow from the primary side to operate the syphoning effect to transfer fuel back to the primary side (left). Without the flow from the primary (left side) there is no secondary transfer. The fuel sending unit resistance readings should be about 996 ohms Empty and 52 ohms Full and will only be equal at full tank or empty tank. The voltage readings should be about 0.6 volts Full and 6.7 volts Empty. At a 1/2 of tank, on the right side (Secondary) reading should be around EMPTY, While on the Left side (Primary) it should be nearly full. If reading are not correct then, try a new primary module. NOTE: please check for foreign material or contamination when ever erratic fuel gauge issues are encountered or fuel tank services are performed Modification Date 08/14/06 4:43 PM
Well when you fill it at the gas station, the fuel will "spill over" from the left side to the right side through the middle narrow section of the tank, so both sides will become full. It is the siphoning action while the vehicle is running to keep both sides equal. I hope this makes sense.
Yes maybe try doing that next, and let me know what happens. If I can think of anything else then I will let you know.