Re: best time to read gas gauge
Ok...to answer the question at hand. Reading the gauge with the ignition off tells you NOTHING... There has to be current running through the sender wire to the level sender in the tank in order for the gauge to read. If you shut the car off with a full tank, and the gauge still says full, and let the car sit for 10 years, and turn the car on, it will start off at full, but will quickly drop to where it needs to be due to evaporation. Thats the way sending units work. Its a measure of the resistance to ground. If there isnt voltage (usually less than 12 volts) going through the sender wire, the gauge wont read tank level. If you were to cut the sender wire, then turn your ignition on...the gauge would peg due to reading infinite resistance to ground. Not sure off hand which direction it would peg tho...
Now...as to why the fuel gauge doesnt go to zero when the car's shut off. I have a thought about that. Gauges in newer cars have buffers built in to them so that the sloshing fuel doesnt make them jump up and down. If they didnt have this, they would be all over the place (i.e. my 67 bonneville. no idea how much fuel was in that thing at any given time haha). I THINK...THINK...fuel gauges have an "air core" to dampen the movement. When you turn the gauge off, it doesnt "free drop" like the other gauges. The needle doesnt just go back and forth freely like the others. If you take a cluster, you can move the otehr gauges around all you want and they sort of spring back. But the fuel gauge is kind of stiff.
Im pretty sure thats how my autometer gauge works... Some cars might have the buffer thing in the tank, but Im 98% sure thirdgens dont.
Someone can correct me if Im wrong on the not going to zero thing...but I think thats what it is.
Last edited by ghettocruiser; 07-23-2008 at 12:18 PM.