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Old 07-02-2008, 09:31 AM   #11 (permalink)
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Originally Posted by pcp View Post
Hello Gregte, Yes you can read how much fuel is in the tank with the a multimeter....
Yes, I already know you can do this. My question is concerning the resolution of the pot used in the sending unit. In a couple posts above Red has indicated that current day cars are similar to older cars in that regard and therefore the low res component is not suitable for my purpose.

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Old 07-02-2008, 01:20 PM   #12 (permalink)
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Maybe you could go to the parts store and ask to see some from newer cars. You might be able to tell by looking at them. I know on mine you can clearly see the conductive lines on the sender. Maybe some newer ones might have a solid resistive sheet.
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Old 07-02-2008, 01:44 PM   #13 (permalink)
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I know that Ford tend to use segmented senders as part of the fuel delivery module that have something like 50 segments to read from the top of the tank to *near* the bottom. The readout is dependant on which segments the float places the sender (as there are I believe 2 or 3 contacts on the wiper arm that sends the signal), so the output is an average voltage across the segments. It also works on a voltage, not resistance input. The instrument cluster control module then interprets this voltage and applies logic filtering to damp out changes in level when cornering (slow fill constant). If the ignition is turned off and then on again and it sees a different tank level between off and on, it then applies a fast fill constant and the guage goes directly to the new reading (as it determines that the tank has been filled up).

New systems are pretty complex!!
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Old 07-02-2008, 02:15 PM   #14 (permalink)
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I just want a gas gauge that sits on the top line when full and the bottom line when empty.

Right now at full, it's somewhere around halfway through F and at really empty (but not totally) its somewhere around halfway through E. There are 2 ticks between top line and 3/4, 4 ticks between 3/4 and 1/2, 4 ticks between 1/2 and 1/4, and 2 ticks between 1/4 and bottom line.

How much is left in the tank? I can usually guess to within a 1/2 gallon, but I've gone over 60 miles after the "low fuel" light turns on. If I sit in the car with the power on but engine off, I can watch the fuel gauge slowly drop and slowly rise again when I start driving. You'd think it'd be easy to read on level ground with the car at a stand-still.
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Old 07-02-2008, 03:29 PM   #15 (permalink)
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Originally Posted by i_am_socket View Post
I just want a gas gauge that sits on the top line when full and the bottom line when empty.
I once read that the automotive industry tried a gage that read full when full, not 1/8 inch over full on the meter, and of course started dropping right away as you consumed gas, instead of staying over the full mark for the first 50 miles. But, people didn't like it. They liked the 'feeling' of still having a full tank even after driving a good amount.

This can be seen everywhere you look, the idea of us people trying to fool our brains. It is a reality that we human beings like to pretend things are different than they really are, even though we know better. It does not stop at childhood.
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Old 07-03-2008, 09:08 AM   #16 (permalink)
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Quote:
Originally Posted by Gregte View Post
I once read that the automotive industry tried a gage that read full when full, not 1/8 inch over full on the meter, and of course started dropping right away as you consumed gas, instead of staying over the full mark for the first 50 miles. But, people didn't like it. They liked the 'feeling' of still having a full tank even after driving a good amount.

This can be seen everywhere you look, the idea of us people trying to fool our brains. It is a reality that we human beings like to pretend things are different than they really are, even though we know better. It does not stop at childhood.
I read the same thing once before. That car designers design the gas gauge to make you feel like you can drive forever before it comes off F, then when it gets low, they want it to make you feel lucky that you could make it to the gas station when it was on E.
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Old 07-04-2008, 08:14 PM   #17 (permalink)
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I finally decided to bit the bullet and spend the time locating and tapping into my fuel gage wire and in the end it does indeed appear to have a wire wound pot for the sender. I put a jack under the vehicle to slowly tilt it to change the level in the fuel tank and thus the reading on my voltmeter and it was quite coarse and a bit irregular. So the question is answered for certain.
Thanks for all the suggestions!

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