I have a 1969 XL 500 that I just replaced the sending unit on. I tested the new sending unit with an Ohm meter before I installed. The range was from 10 ohms to 78 ohms. I purchased an AutoMeter fuel gauge that operates from 0 to 80 Ohms. Anyway, the fuel gauge reads reverse the amount of gas in the tank. The more gas I put in the tank, the more the tank reads empty and visa-versa. The sending unit can not be installed upside down in the tank, so I ruled at least that much out. The gauge is hooked up per the instructions, ground, sending unit, and ignition...So I am at a loss. Anyone that has any ideas,PLEASE let me know. TIA Eric

 

Check wiring at the fuel tank float unit and check for proper grounds. Sounds like one positive and one negative are flip-flopped. I would check the grounds and that the voltage regulator is OK. The ignition goes in and one wire comes out to the tank float. Try to drop the ground wire off the dash gauge - my '64 diagram shows none needed. Should be ignition into guage feeding to tank float unit and grounding near there.