Fuel Gauge

[boatnamesue]

Installing new KUS Analog Fuel Gauge -

Red is power
Black is signal/sender
Blue is ground
Yellow is backlight

Red wire is spliced to ignition key 12 volts.
Black wire spliced to fuel sender/signal.
Blue wire spliced to ignition key ground.
Yellow wire spliced to console switch panel. 

When ignition is OFF and switch panel turned ON, fuel gauge back light operates and fuel needle to full.
When ignition is ON and switch panel turned OFF, no backlight and no signal. 
When red wire is disconnected from ignition 12 volts and switch panel turned ON, fuel gauge back light operates and fuel needle to full.

How is this possible?  Why is the fuel gauge operating when the red wire is completely disconnected from the circuit and why isn't the fuel gauge operating when the ignition key is turned ON and switch panel turned OFF.  I'm completely puzzled here...this simple circuitry doesn't require the need to be electrician, yet I must be have something wrong.  Many thanks.

[GoneFission]

Check the ground on the tank to the ground terminal block and put the ground wire from the gauge on the same block instead of going through the switch.  Seems like you have a grounding problem.  The gauge reads resistance through the sender - it's a pretty simple system.  Here is a nice article on troubleshooting gas gauge issues:  https://handtoolsforfun.com/how-to-test-a-fuel-gauge-with-a-multimeter/#:~:text=%20Measuring%20resistance%20on%20a%20fuel%20gauge%20,is%20either%20a%20loop%20or%20overload.%20More%20 

[boatnamesue]

Check the ground on the tank to the ground terminal block and put the ground wire from the gauge on the same block instead of going through the switch.  Seems like you have a grounding problem.  The gauge reads resistance through the sender - it's a pretty simple system.  Here is a nice article on troubleshooting gas gauge issues:  https://handtoolsforfun.com/how-to-test-a-fuel-gauge-with-a-multimeter/#:~:text=%20Measuring%20resistance%20on%20a%20fuel%20gauge%20,is%20either%20a%20loop%20or%20overload.%20More%20 

Will do. Many thanks.

[boatnamesue]

Not a ground issue....turns out the issue is with the tach harness accessory pigtail.  The fuel gauge power wire connects to this pigtail and is fed 12v (from cranking battery) when ignition is turned on.  Well this pigtail is receiving only 11.8v, yet the tachometer is receiving 12v.  So this volt drop is enough to not power on the fuel gauge.

I removed the fuel gauge from the tach harness loop and applied direct power/ground/signal to the fuel gauge and there is no issue.  These tach harnesses are fairly cheap new ($30), so not gonna burn time trying to locate where the resistance is within the harness and splice it out, just gonna buy new and be done w/it.