Interesting conversation here guys
This is my fuel gauge:
It is a Teleflex Sterling Series Fuel Gauge. My 2nd gauge ; the first gauge was the same Teleflex gauge. I figured out the first Teleflex gauge was faulty through the same trial and error.
EDIT: This gauge has the plug-and-play harness style connections (one big plug, lots of wires leading to it). The ground wire for the gauge does not connect to the mounting bracket of the gauge, it is wired internally.
The backlights to my gauges are wired to come on with the NAV lights- I wired them this way simply because I thought that was how they were supposed to be wired. I like the idea of keeping them on all the time to help with moisture build-up.
I was not aware that the backlight grounded to the mounting stud on the back of the gauge. That makes perfect sense because, like FP says, if the backlight shared a ground with the gauge, the gauge would dim as I burned more fuel out of the tank.
I think you guys are on to something... As with almost everything that I've refitted my boat with, this gauge (like the first) came literally out of a grab-bag of parts that my dad had gathered up for me at Key West Boats. Some of the stuff worked, some didn't, some had a scuff on it and was thrown away, and some stuff was missing parts. This gauge was missing a part- the metal (copper?) mounting bracket. I made my own bracket out of some scrap metal and small diameter threaded rod, low carbon zinc and aluminum. I have a habit of spraying everything electronic and metal on my boat with WD40 after every trip- including all the wiring and the back of the gauges.
I'm thinking that some kind of galvanic corrosion has occurred on the back of this gauge, which has created this open circuit in the backlight. I'm not sure why or how this is causing the gauge to read full when the lights are turned on, but its starting to make sense now.
... Still this brings me back to the same point as before, if the fuel gauge pegged when the lights were turned on, that means the meter found a battery (common ground) rather than the float ground. How?? The ground on the back of the gauge is always there regardless of if the lights are on or off?
My guess is that the ground was found through the ground wire that runs directly from the gauge to the negative ground bus, because the ground that was supposed to be present on the back of the gauge has disintegrated. It wasn't the gauge that was trying to find a ground, it was the bulb... ? you tell me.
