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Gas Gauge Reset

Discussion in 'Gen 2 Prius Main Forum' started by sloftin, Aug 1, 2020.

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  1. sloftin

    sloftin Junior Member

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    OK - I have a 2008 Gen2 Prius with 380K miles and original everything except brakes (@ 300K!) and bearings (@375K!) and am still on the original hybrid battery pack. Still get around 45-46 mpg. Car has been a dream.

    However, the normally bizarre fuel gauge has gone off the deep end for the past few months, almost like it is 50% "out of whack". Example - from a full tank we would get about 150-175 miles, and have 1 flashing pip with "Add Fuel" screaming at us. We "might" have been able to add 4-4.5 gallons of gas into the tank when this happened. I tried every gauge re-calibration I could read about with no luck. Fast forward to a few thousand miles ago when I had the battery disconnected for about 48 hours to replace all 4 wheel bearings (thanks for that lovely front bearing configuration Toyota!). Anyway, powering the car back up after the bearing replacement (when it just happened to be full of gas), wiped out the fuel gauge misreading for 2 entire tanks (got almost 400 miles per fill-up before one flashing pip and "Add Fuel") and was able to add 9-10 gallons of gas! Now, the next tank of gas reverts back to 180 miles, 1 flashing pip, and allowed us to again add only 4.5 gallons of gas. I know the fuel tank is holding 10-ish gallons of gas, but when the fuel gauge goes wonky, it only seems to believe it can hold about 5 gallons.

    I assume the long power down cleared something electronic. Now, it's fried again. Is this a faulty combo meter or something similar? Anyone seen this bizarre behavior?
     
    #1 sloftin, Aug 1, 2020
  2. ydpplqbd

    ydpplqbd Active Member

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    I am having same problem with my 2006 Gen2. On my last tank of fuel, the last pip started flashing at 215 miles (since most recent fill-up). I drove to almost 500 miles before filling up. IOW, the last pip flashed for about 265 miles AFTER it started flashing. I have been unable to find a reset of the fuel gauge that works to solve this problem.

    Additionally, in the hot summer months, I sometimes use my Gen2 as my field office when out of the office (just imagine me with my phone, clipboard and cell with the AC going full blast). After using the Gen2 as my field office for extended periods (say an hour or two), the fuel gauge becomes even more wonky. After such a field office episode, the last pip will start flashing at about 160 miles (since last fillup). This implies to me that the Gen2's MFD mpg estimate is using excessively pessimistic assumptions about fuel consumption while using the Gen2 as my field office (with AC blasting).
     
    #2 ydpplqbd, Aug 2, 2020
  3. PriusCamper

    PriusCamper Senior Member

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    The guess gauge is reset by disconnecting the 12v battery for a few minutes, but it will take a while for it to read accurately again... Also in both people posting above, it doesn't seemed you've eliminated the rubber bladder in the tank as the source of the problem, but they discontinued this design because it is a problem even though it lowers tank emissions when you fill the tank. Main issue is bladder in tank sometimes doesn't fully expand in cold weather, or if you never fill the tank all the way. For best results, keep your tank as full as possible.
     
    #3 PriusCamper, Aug 2, 2020
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  4. sloftin

    sloftin Junior Member

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    Yes - I've determined the bladder isn't the problem. I've read everything I could get my hands on for that bladder and gas tank. My question is, "is this behavior a sign of a failing combo meter/circuit board"? If it is, I'll swap it out, but since removing half the dash to do that isn't something I want to do, but I will if that'll solve the problem. Thanks for the reply!
     
    #4 sloftin, Aug 3, 2020
  5. sloftin

    sloftin Junior Member

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    As a bit of added information, I came across an excerpt from the Toyota Technical Training course on the fuel and evap system a while back that went into gory details. Here is a diagram from that excerpt:

    upload_2020-8-3_7-52-4.png

    So, if this figure is accurate, it looks like the inclination sensors and meter ECU are onboard that combination meter circuit board buried in the dash. Given ydpplqbd's description of possible heat-related additional wonkiness and the fact that it's hotter than Hades in NC right now, it seems like this circuit board is likely the culprit. I'd just like to be more certain of that before I tear into the dash to replace it.
     
    #5 sloftin, Aug 3, 2020
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  6. ydpplqbd

    ydpplqbd Active Member

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    Very nice graphic of Gen2 fuel tank system!!!
     
    #6 ydpplqbd, Aug 3, 2020
    Last edited: Aug 3, 2020
  7. PriusCamper

    PriusCamper Senior Member

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    Maybe @Elektroingenieur can offer advice... What I can say is the combination meter would be the least likely place for inclination sensors, it would not make sense to have them there.
     
    #7 PriusCamper, Aug 3, 2020
  8. Elektroingenieur

    Elektroingenieur Senior Member

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    Do the “Centered Value Setting (Inclination Sensor)” procedure in the Repair Manual (more info), under Vehicle Interior: Meter: Meter/Gauge System: On-Vehicle Inspection.

    If that doesn’t fix the problem, do the “Fuel Receiver Gauge Malfunction” procedure in the same section, which has you check the combination meter, sender gauge assembly, and wire harness; if none of those checks reveals the problem, the combination meter assembly is replaced as a last resort.
    It made sense to someone. New Car Features for the 2004 Prius—which also applies to later second-generation models, for which Toyota didn’t publish comprehensive New Car Features books—says on page BE-89:

    “This sensor consists of a pendulum, 2 magnets provided for the pendulum and hall IC. Hall IC converts the magnetic flux density change caused by a pendulum inclination into the voltage value and output it to the meter ECU. The meter ECU judges the inclination condition of the vehicle based on this signal and corrects the fuel gauge.”​

    An illustration, labeled “Combination Meter Cross Section,” shows the position of the inclination sensors; they are in the lower part of the combination meter assembly, not on the circuit board itself. The sensors are not sold separately as service parts.
     
    #8 Elektroingenieur, Aug 5, 2020
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