Technical Fuel Sender has me foxed!

[Tin Tin]

Morning all,

The fuel sender unit on my 66 500 is really confusing me as it triggers the Fuel warning light on the dash even when the tank is full.

I have bench tested the sender unit using a continuity meter and it works correctly, completing the circuit when the float is at approx. ¾ drop. Using the test circuit during installation, (fuel tank is full) the circuit is correctly broken as the sender is inserted into the tank, however as soon as the sender unit body flange touched the tank sender mounting threads, somehow a circuit completes that will then trigger the warning light.

Am I missing something or have a fundamentally misunderstood how the sender works?

 

[CaptMonkeyPants]

Could the terminals be swapped on the sending unit? Looking at the circuit diagram, one terminal goes to ground, while the other goes directly to the indicator light. The other end of the light goes to battery positive. The only way it should illuminate is if the circuit is completed from the light to ground. So there must be a short somewhere...

 

[Tin Tin]

Many thanks for the suggestion, I checked the connections and they're fine so I've pursued the "grounding" route.

It turned out the sender unit pick up the pipe was somehow grounding out on the tank hence sending a false flag, I spent a while bending the pick up pipe until I could insert the sender unit without feeling any interference from the inside of the tank. Once I was able to install the sender unit without anything touching (other than the mountain plate) the grounding was sorted and the low fuel indicator lamp now works properly.

Thanks once again for your input.