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Discussion Starter · #1 ·
I noticed when I filled the tank yesterday that it took about 0.3 miles before the MFD reset the Consumption display. What triggers the reset, actually? I know it needs to register a full tank, plus a moving car... but I'm pretty sure the gauge registerd full and I had been driving for a while before it reset. There is always some delay, but this was longest lag I've noticed, and I'm just curious. I know there is a reset button if I feel like pushing it....
 

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Mine always resets in about .1 to .2 miles, but I'm not sure what triggers it. I know if you don't put in enough gas, it won't reset. Too lazy to go get my manual to tell you what that threshold is...
 

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Discussion Starter · #4 ·
What I'm really curious about is technically what signals have to go in before the computer resets it. I'm being a real geek here...
 

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2 things are necessary:
1)At least 3-4 gallon fill. I suspect it's actually a certain % change (i.e. 30% increase in fuel tank level from one shut off to start up cycle) in the tank level that's required but that it equals around 3-4 gallons.

2)The wheels need to complete one full rotation AFTER the fuel tank level has reset from requirement #1.

So, for a normal fill up.....You fill the tank with 8 gallons of gas (requirement 1 met). You drive away, but then have to wait at a stop light for a bit.... The fuel guage slowly shows an increase in the level from your pre-fill up one to all the bars showing.

At this point the MFD will not have reset even though the fuel guage reads full. The light turns green and you begin to roll forward one full revolution of the wheels...then the MFD switchs over and resets.
 

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Hi, efusco,

I've seen that bit about "one full wheel rotation" for a long time & personally I think it's bunk. After I fill my '05 & power up I let the fuel guage come all the way up too (while I'm record'g my gallons, date, odometer miles , & MFD's previous tank miles) before I even move an inch. And I'll tell you that once the guage reaches full, it's many more rotations of the wheels than just one. Although I've never actually flipped over to a trip mileage display, it must be at least one or two tenths of a mile before the MFD re-sets itself (maybe even 3/10's).

So I don't think what firepa63 & Astrowoman are experienc'g is abnormal, but as Astrowoman says, if we want we can just do the manual re-set & be done with it.

Ken (in Bolton,Ct)
 

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Ct. Ken V said:
Hi, efusco,

I've seen that bit about "one full wheel rotation" for a long time & personally I think it's bunk. After I fill my '05 & power up I let the fuel guage come all the way up too (while I'm record'g my gallons, date, odometer miles , & MFD's previous tank miles) before I even move an inch. And I'll tell you that once the guage reaches full, it's many more rotations of the wheels than just one. Although I've never actually flipped over to a trip mileage display, it must be at least one or two tenths of a mile before the MFD re-sets itself (maybe even 3/10's).

So I don't think what firepa63 & Astrowoman are experienc'g is abnormal, but as Astrowoman says, if we want we can just do the manual re-set & be done with it.

Ken (in Bolton,Ct)
Well, I certainly don't have any inside information that I can throw your way to independently confirm this. But my personal experience with 115 tanks of gas in about 50k miles, most at the exact same station and pump, is that my wheel rotation statement seems an accurate reflection of my personal observations.

As my usual station I fill up, I put the car into EV mode (usually) and drive to the stop light where I invariably have to wait. The tank usually gets to the 1/2 full mark as I come to a stop. Before the light changes it gets to the completely full mark, but the MFD will not reset. As soon as the light changes and I start to roll the readout for mileage and mpg go blank, then a moment or two later show zero, then a few moments later they start showing the mpg...of course the miles traveled doesn't show anything until I've gone at least one mile.

Now, I have a very early '04 (got it Oct. 21, 2003) so it's possible that the sequence of events requirement has changed in later models, but I've not heard any other theories about what it takes to get the change over than the one I presented.
 

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I do not have the 2004's repair manuals. I do have the 2004 New Car Features manual, though.

page BE-59 said:
Instant Fuel Consumption
• Displays the value that has been calculated by the meter ECU, which is based on the driven distance and the fuel consumption volume (fuel injection signal from No. 1 injector), provided that the power switch is turned ON.
• The meter ECU displays the instant fuel consumption value on a bar graph.
• The display updates every 0.5 seconds.

Average Fuel Consumption After Refueling
• Displays the value that has been calculated by the meter ECU, which is based on the driven distance after refueling and the fuel consumption volume (fuel injection signal from No. 1 injector).
• The meter ECU determines that the vehicle has been refueled in accordance with the signal from the fuel sender gauge, or the Reset button has been pressed.
• The display updates every 10 seconds.

Driven Distance After Refueling
• Displays the driven distance that is output by the meter ECU after the refueling or the Reset button has been pressed.
• The meter ECU determines that the vehicle has been refueled in accordance with the signal from the fuel sender gauge.
• The display updates every mile.
 

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Discussion Starter · #12 ·
Sniff. I really wanted to know what the code logic is... what the electronic inputs are (if it is a wheel rotation, how is that communicated to the computer) and none of the above actually addresses it. I don't think I could drive close to half a mile without rotating the wheels once! I guess I will just have to live in (admittedly probably unimportant) ignorance.

How does the fuel sender guage work? Is it the only input?
 

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The reset isn't very reliable. I added 3.5 gallons yesterday ($2.399, or nearly 20 cents cheaper than near home, was too cheap to skip) and later noticed that the mileage was still over 200. Manual reset annoys me, but is probably a good habit...
 

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I don't recall seeing specific trigger information in the repair manual, but I'll take a closer look.

From personal experience, and experience reported by others, plus what is mentioned in the owner's manual, a certain number of gallons has to be registered by the guage, then a certain amount of motion detected from the wheels. If you turn on the car right after a fillup, you'll notice the guage creaps up until it shows full, or whatever the actual level of fuel is, which can take a literal minute or two. I spend that time recording my miles and what's on the display for logging later, and do a calculation of my miles per gallon. One segment of my calculator is broken, so I also have to subtract several digits to the right of the decimal place so that the entire display moves right to avoid the bad segment.
 

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About the speed/wheel turn sensing. Remember those purple wires that we cut to allow functions while moving? They send a pulse when the wheels turn. The pulse comes from the speedometer, and the speedometer gets its information from the brake system which uses the wheel speed sensors.

I was fooling around with the NAV service screen to get the correct time from the satellites to adjust the clock, and discovered a display indicating the calculated distance between pulses. I forget exactly, but it was between 300 and 400mm.
 

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Astrowoman said:
Why does the fuel guage creep up like that? WHAT is is "sensing"?

I'm all curious this week.
There are multiple sensors within the fuel tank that work in conjunction with an inclinometer to try to accurately determine the fuel level. I have no idea why it takes so long to creap up unless it starts sensing at the bottom of the tank and then creaps its way up.
 

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First, I believe there is a separate 'measuring chamber' with a small orifice to the main tank so that the level read won't shift so much while driving. Almost like the way coffee urns show you how much coffee is stil in them.

Second, the guage has a dampening effect like most guages, except this one is done electronically compared to mechanical dampening done in many fuel guages. Again this is so bumps and short term changes in pitch don't affect the reading much.
 
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