Most fuel tanks in the AD trucks were isolated from the cab using either foam rubber, hard rubber or other material. This was to prevent the tank vibrating against other metal parts (safety issue since the tank was inside the vehicle). So it must have the extra ground wire. You've said your sending unit works when it is not inside the tank but doesn't work correctly when installed in the tank. Is there any way you can post an image of your sending unit? Was it a generic unit and did you have to adjust height and length of the float rod? Is there a possibility your float is rubbing against the side of the gas tank? There should be only one way the sender can be installed (so that the float will travel parallel to the tank sides but not touching them), but in 60 years of working on these I have seen situations where the sender was assembled incorrectly and could never work right because of interference with the walls of the tank. If it is working fine when it is not in the tank, then obviously something happens when it is mounted in the tank that changes things. The more information you can give us the better our answers will be.
