Compression Ignition Addict
Join Date: Dec 2007
Thanked 262 Times in 253 Posts
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I'm not aware of a common Tail light / Sending unit connection, I'd think you have separate issues....
At 400mi I'd also think you're pretty dry and it might be the perfect time to drop the tank and check the sending units output as well as the wiring at the top of the tank.
I think you should see something like 15 Ohms when the float is down (E) and 160 Ohms when lifted all the way up (F). If that checked-out it's gonna be in the wiring or even the gauge itself maybe.