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Fuel level gauge, odd problem

CATPart

Well Known Member
The Gauge:
Isspro 3975

It has 5 connections at the back:
Instrument light (+)
Instrument light (-)
Signal (sender)
12V Positive (+)
Ground (-)

I found a dangling wire behind the panel. The wire splits into 2 and connects the instrument light (-) and the Ground (-). If the wire is dangling disconnected and touching nothing the gauge reads correctly at half full. Then when the instrument lights are switched on, the gauge goes up to Full but is not illuminated. Now if I touch the dangling wire to ground, the gauge illuminates and reads correctly. Still holding the wire to ground, as I switch the lights off and on, everything works as it should.

What I find odd is that:
1-The gauge works at all with this wire disconnected
2-The gauge reading changes to full as described above

I assume maybe the gauge is grounded in its mounting, so that is why it may work when this ground is disconnected. But why would it go to Full when the light is switched on? I'm just trying to figure out if there is a larger issue, or is this is normal operation. It seems if I simply connect this ground wire then everything will work as it should.
 
The dangling wire is finding a ground through another means but not good enough for the draw of the gauge and light together.
"Dangling" grounds can do some very weird things.

I could tell you a story about a 1978 GMC pickup that GM (over months) could never find the problem. I finally found the "dangling" ground on my own.
 
Hi Mel,
The dangling wire was not finding a ground, because I was preventing it from doing so, I held it to make sure it touched nothing. So maybe there is enough grounding thru the mounting to allow the gauge to function, but once the light is turned on, this ground path is not sufficient? Reading the info on the gauge, it says if the signal line is grounded then it will read Full. How this could be affected by the instrument light switch is strange. Maybe it is simply the internal circuitry of the gauge causing this. The plane is never flown at night, so this was overlooked probably for many years.
 
But the wire was also tied to the light. The light has a path through the element. With power to the light off, it can find a ground. Trust me, it can happen.
The situation on my GMC pickup was the stop light was finding a ground through the backup switch and to the ignition. In just the right configuration, foot on the brake, shifter in reverse, turn the ignition off and the engine would keep running.
 
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