Low gas warning light - based on needle position or fuel tank sensor? - Scion xB Forum
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post #1 of 6 Old 03-16-2008, 10:50 AM Thread Starter
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Low gas warning light - based on needle position or fuel tank sensor?

I just installed an autotechnic gauge face overlay in my first gen xB.

Ended up having to take the fuel gauge needle off to install it and I thinkI didn't get it back on in the same position; I believe I was at slightly below half a tank when I took the gauge off and now it's registering 3/4 of a tank.

Two ways to recalibrate the needle occur to me - fill the tank up and then adjust the needle to full, or wait until the tank is near empty, and then adjust needle to empty.

Since I prefer to fill up with whole tanks at a time, I'm leaning towards waiting until the fuel warning light comes on, then adjust the needle to read empty, fill up, and then make sure it goes to full.

Question though - does the low fuel warning light come on based on the position of the fuel gauge needle (in which case I'm worried I'll run out of gas before it comes on), or does the low fuel warning light come on based on a sensor in the tank, regardless of what the position of the fuel gauge needle is?
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post #2 of 6 Old 03-16-2008, 11:15 AM
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Re: Low gas warning light - based on needle position or fuel tank sensor?

This is strictly a guess...but I'd have to say it's activated by a sensor/sending unit in the tank. The reason I say this is because I've had my low light come on almost immediately after making a quick turn or after accelerating from a red light.
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post #3 of 6 Old 03-16-2008, 11:52 AM
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Re: Low gas warning light - based on needle position or fuel tank sensor?

All fuel level gauges are controlled by a sender float mounted in the gas tank. The signal is sent from the float to the gauge.

You're exactly right Kevin, if you are really low on fuel and you accelerate quickly you may see the light come on for a second or two also. If it's already on and you stop suddenly it may go off for a second.

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post #4 of 6 Old 03-16-2008, 11:52 AM
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Re: Low gas warning light - based on needle position or fuel tank sensor?

Well, I can tell you that i've had your same problem Thumps. I do recall when the light would go on and it came on just about half way between the second to the last line and the empty line. Now, my light doesnt come on until I actually hit the empty line, and when I fill up, it says I have about another 2 gal to go. I will have to try the fill up, put needle on the full line and see what happens. So I guess what i'm trying to say is, I think the light comes on when you hit about 2 more gal until empty. But thats just me
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post #5 of 6 Old 03-16-2008, 12:26 PM
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Re: Low gas warning light - based on needle position or fuel tank sensor?

too reset it,
why dont you just full up to full?,
and set it at full...
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post #6 of 6 Old 03-16-2008, 12:36 PM Thread Starter
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Re: Low gas warning light - based on needle position or fuel tank sensor?

Quote:
Originally Posted by buckbee23 View Post
too reset it,
why dont you just full up to full?,
and set it at full...
Because then I'd have to put in half a tank and screw up my nice series of obsessively-compulsively logged full tank fill ups along with mileage calculations. (Yes, I actually write down mileage, gallons purchased and money spent in a little notebook every time I fill it up)

Actually, I think when the warning light goes on is the more accurate way to set the needle. Different gas pumps can click off at different full levels, depending on how long the nozzles are.

Quote:
Originally Posted by surfcitylocal View Post
All fuel level gauges are controlled by a sender float mounted in the gas tank. The signal is sent from the float to the gauge.
...
Thanks, that helps my thinking.

So it's probably either the fluctuating resistance or voltage level of the sender that both the warning light and the gauge are reading, which means the warning light is independent of the gauge position, but they're both dependent on the same sensor. The gauge measures the fluctuation; the warning light just lights when a certain signal level is hit.

Last edited by Thumps; 03-16-2008 at 12:36 PM. Reason: Automerged Doublepost
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