96 aux. fuel tank k3500 question?

[kc33]

I have 96 gmc k3500 cab chassis flat bed with an aux tank behind the rear duals. It has a lift pump from it tied into the vent line of the front driver's side tank. So the back aux tank fuel is pump into the front tank. The front tank was completely empty but the back tank was full of fuel . I had to drain the back tank to drop it inorder to replace the sending unit . The aux. tank lift pump works fine. So when does the fuel go into the front tank? Just as needed? Does the aux not completely empty into the front tank. I also seen a computer or module like box attach to the frame rail in front of the front tank. What is it?:confused:

[laidback383]

on the 1985 body style trucks they used a switch they had a by pass valve that was operated in the cab. not sure if that help or not.

[racer55]

The module you refer to is called the balance module.

It monitors the fuel level in both tanks and averages those levels to signal the gauge.

It also decides when the rear LP runs,when the front tank is approx. 2 gallons lower than the rear,the module pumps fuel from the rear tank to the front.
When the front tank has approx. 2 gallons more fuel than the rear tank the module shuts off the rear LP and the cycle repeats untill the rear tank is empty.

If your front tank was empty then you may have a problem with the front sending unit or the back sending unit or the balance module itself.

Troubleshooting time.

[kc33]

The module you refer to is called the balance module.

It monitors the fuel level in both tanks and averages those levels to signal the gauge.

It also decides when the rear LP runs,when the front tank is approx. 2 gallons lower than the rear,the module pumps fuel from the rear tank to the front.
When the front tank has approx. 2 gallons more fuel than the rear tank the module shuts off the rear LP and the cycle repeats untill the rear tank is empty.

If your front tank was empty then you may have a problem with the front sending unit or the back sending unit or the balance module itself.

Troubleshooting time.


Could one drop the aux tank and remove the balance module and just run one tank or................ could this screw the main computer up?

[racer55]

You can run the front tank only by bypasing the balance module completely,just less capacity on one tank.

The schematic is here:
http://www.thedieselpageforums.com/tdpforum/showpost.php?p=273597&postcount=8