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MG Hanging Up???

Discussion in 'Gen II Prius Care, Maintenance and Troubleshooting' started by ibmindless, Dec 7, 2011.

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  1. ibmindless

    ibmindless Member

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    I have a major problem and my dealer doesn't see any problem. Normally, once I reach a particular cruising speed, I let up on the accelerator and then gingerly apply just enough pressure on the pedal to maintain forward momentum. This always results in a feeling as though the the motor/generator(s) have released their grip on the drive-train and the car is basically coasting or freewheeling. If I am headed down a hill, I can completely release my foot from the accelerator and watch as the car speeds up thanks to gravity.

    When I am coming to a stop, I gently apply braking power and I can feel the MG's slowing the car until I am almost stopped, then I feel the actual brakes taking over the stopping of the car. And of course, I can hear the MG's whine as the car nears its stop.

    Now, I don't notice the MG-whine and it feels like I am braking only with the actual brakes. And when I release the gas pedal going down hill, I quickly slow down. In fact, to make progress going uphill, I have to push the gas pedal down much harder than ever before and it feels as though an MG (or two?) is dragging the car down.

    I described this to the dealer's mechanic and he said that the car is supposed to lose speed going downhill - that it is normal. I argued that it has never done this in the past 30K miles since I bought the car. Further investigation reveals no codes or histories, according to the mechanic.

    When I first experienced these symptoms, I was on the highway and noticed that I had to keep pushing the gas pedal further & harder. It got so bad that I pulled over and I felt the wheels & tires to see if a brake was dragging. When I was by the right rear wheel, I noticed the smell of hot oil (transaxle over heating?). Then I noticed as I was going down a very steep grade (with truck emergency turn-outs) that the car was losing speed rapidly and I had to apply more gas pedal to maintain momentum.

    Any advice for evaluating/remedying this situation?
  2. GrumpyCabbie

    GrumpyCabbie Senior Member

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    There are many posts in here about the Prius not having engine braking, at least not in the traditional sense.

    As you have posted this in the gen3 section I will assume you've replaced the 2007 shown on your profile with a 2010+ model, which will have a B option on the gear shift. This is for engine braking when heading down steep hills.

    Next time give it a try.


    (It's all in the instruction manual ;))
  3. seilerts

    seilerts Battery Curmudgeon

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    How many miles on the car?

    How is the MPG, taking into account the seasonal variation?

    The right rear wheel is not connected to the drivetrain. That is not a place to be smelling hot oil, unless you have a bad wheel bearing, which could explain things. If you are wanting to check it yourself, you need to jack up that wheel, grasp the tire at 3 and 9 o'clock, and see if you can make it wiggle. If it feels loose at all, it is the wheel bearing. Otherwise, check the rear brakes. There are special precautions involved with the brake system, but basically if you disconnect the 12V negative, it takes most of the risk away. If you have a leaking wheel cylinder on the right rear, it could cause that to drag and would also explain the sensation of hot oil. Remove the wheel and brake drum (should slide right off) and take a look.
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  4. justlurkin

    justlurkin Señor Member

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    I bolded that part because it sounds like your parking brake is dragging. That would cause the symptoms you describe-- slowing down more than before when you should be coasting, and the the hot oil smell.

    And no, the transaxle is in the front of the car. The rear wheels on the Prius are not powered.

    Get your parking brake looked at.
  5. ibmindless

    ibmindless Member

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    Thanks, Grumpy, but I am well aware of the B option and I'm very familiar with the instruction manual and the fact that there is no engine braking. That's what seems to be the problem, suddenly I am now getting "engine" braking.
  6. ibmindless

    ibmindless Member

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    Thanks, but it's not the wheel bearing (I did check). The MPG is down 20-25%.
  7. ibmindless

    ibmindless Member

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    Thanks justlurkin, but it's not the parking brake. I think I smelled the hot oil odor because the gentle breeze blew it in that direction. Also, it was not the odor of a hot brake.

    What I'm saying is, it seems like a motor-generator is staying engaged at highway speeds when normally, I would be able to glide/freewheel/coast downhill when I take my foot off the gas.

    Anyone else???
  8. seilerts

    seilerts Battery Curmudgeon

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    Jack that son of a bitch up and see if the rear wheels turn freely.
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  9. 2009Prius

    2009Prius A Wimpy DIYer

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    +1. Always do the simple test first before going into esoteric theories. :rolleyes: But if the OP insists on analyzing the MGs he/she is more than welcome to go through the trouble to set up my PriiDash(TM) program and log the data. :D
  10. qbee42

    qbee42 My other car is a boat

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    We understand what you are saying, but it's not possible. It would take a software problem to cause that, and you don't have a software problem.

    Most likely you have a bad bearing or a dragging brake. Stop fixating on the MGs and look for a more basic problem.

    Tom
  11. richard schumacher

    richard schumacher shortbus driver

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    Software problems do often smell like hot oil :_>
  12. GaryKarenC

    GaryKarenC Junior Member

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    YES. You are not alone! You are not crazy. These other posting helpers are on the wrong track - there is a very real problem that you are describing. I strongly suspect a software/hybrid and battery system interaction problem. Another way of describing the problem we have had intermittently for a year now is that the system enters "B mode" when it should not be. For instance yesterday, for only a part of one trip... flat road, up to speed, let off accel., car will slow quickly and would come to a stop if I let it. MFD indicates regenerative braking, engine off, battery 1 bar short of full. The sound coming from the drive system is the same as when I have put the car in B mode (which I almost never do, but have done so to test this theory). The "feel" of the car "pulling back" is dramatic. We've had the car since new, put 85K miles on it and know how it normally drives. Something dramatic has changed. 2 trips to 2 different dealers have yielded no solution, nor even an acknowledgment of the issue (they cannot replicate the problem because it is so intermittent). Turning the car off usually causes the problem to disappear, but we've found no predictable pattern to help in diagnosing. It's such a strange problem with such unpredictable and changeable symptoms that it is really hard to concisely describe and even harder to convince others of the "realness" of it (Just ask my wife, took her months to convince me of the problem because it had not yet happened to me when I was in the car).

    I would love to hear if you are still having this problem or if anyone else is experiencing this anomalous behavior in the hopes of finding a solution. I only found one other thread with someone describing roughly this problem and that thread devolved into a discussion of the improper use of B mode. No help there either.

    I must say that in all other cases, PriusChat has always been a great boon to me as a DIYer. I have diagnosed and repaired several problems with my 2 Prii with the help of the people who take the time to answer and post here. A much delayed thank you!
  13. qbee42

    qbee42 My other car is a boat

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    From the above description, it sounds like the Prius is entering some sort of B mode, which is why previous threads devolved into a discussion of how to correctly use B mode. Prius are so reliable that problems like this are most often caused by the driver.

    Assuming it isn't the driver, then we can ask ourselves what could cause a Prius to act like it is in B mode. Some options:

    1) Actually in B mode. A bad shifter, wiring, or ECU could cause random shifts into B. With wiring or the shifter the car should show that it is in B mode. With a bad ECU there might be no indication.

    2) A software glitch causing unexpected ECU operation.

    3) A mechanical failure causing drag.

    4) Power problems simulating 1 & 2 above.

    #2 is not likely. If there is a failure mode in the control software, more drivers should be reporting it. Software problems are not unheard of, but there is nothing special about these two drivers where only they should have this failure mode.

    #1 is possible.

    #3 is possible.

    #4 is possible.

    Of the three possible causes, #4 is the one that gets my attention. The two posters swear up and down that it's not a mechanical problem or driver induced problem. I'll give them the benefit of the doubt, although previous experience suggests otherwise.

    It is far more common to have a failing 12V system than a failing ECU, so I would focus on a 12V glitch.

    Here is what I would try:

    1) Test your 12V battery. Failing 12V batteries have been known to cause all sorts of freakish behavior.

    2) Disconnect your 12V battery for 30 minutes. This will reset the ECUs. It is possible that one of your ECUs has gotten into a disallowed state. Resetting will fix this, at least at first.

    3) Report back with anything you find.

    Tom
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  14. seilerts

    seilerts Battery Curmudgeon

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    Behavior is different when the battery is at 7-8 bars. If the HV ECU feels that the HV battery has taken on too much charge, it will do B-mode-like things, such as using MG1 to spin the motor. It may be a new behavior in an older car due to HV battery wear -- HV battery temperature begins climbing at high SoC when it didn't before. It is a software feature, not a bug.
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  15. GaryKarenC

    GaryKarenC Junior Member

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    Tom, thanks for your thoughtful response.
    This issue has only happened during our cold winter months. (I have no idea if this is coincidence or not) Last winter when it first occurred the 12V was original. I replaced the 12V last summer, so these new occurrences are with a fresh 12V installed.
    Of your ideas, based on my analysis I would rule out #3. I can see why #2 seems less likely; leaving #1 and #4.

    I did not mention it, but my wife and I noticed a concurrent change in MFD reported HV battery behavior; i.e. to summarize, it appears that the battery tends toward an overcharged state much more than it used to. One example reported to me by my wife was [FONT=&quot]"while accelerating up a long hill, the battery indication started out in the blue (partially full), and cycled into the green zone, finally showing a totally full state by the top of the hill"

    Truth be told, our 04 has not been that reliable. We've had our share of glitches and issues, big and small. Understandable IMHO considering the complexities designed into it. Our big concern is that this IS an HV battery issue, an early symptom perhaps.

    Regards - Gary
    [/FONT]
  16. GaryKarenC

    GaryKarenC Junior Member

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    Forgive my ignorance, but what is SoC?
    Interesting point, and as I noted in my other post, I am concerned about HV degradation. Perhaps it was designed into the software, but I would definitely NOT consider this behavior a "feature". Thanks - Gary
  17. seilerts

    seilerts Battery Curmudgeon

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    Gary, I read your response to Tom's post. I do think your HV battery may be nearing end of life. But it is hard to know if failure will occur in days, weeks, months, or even years. A weak HV battery can hang on for a long time. How many miles are on the car? How is your fuel economy? Have you ever heard the HV battery fan running (back seat passenger side) or had it cleaned? Starting out in blue territory and winding up with 8 green bars when going uphill implies that there are errors in the SoC (State of Charge) calculation by the HV battery ECU. These errors occur because of poor battery health, not because of the ECU problems.
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  18. GaryKarenC

    GaryKarenC Junior Member

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    Scary news. Approx. 85K. Our fuel economy remains essentially unchanged - we average about 42 mpg. I HAVE heard the battery fan run, recently and it took me by surprise as I do not recall hearing it often. I'm thinking I've heard it only a handful of times over the life of our ownership and most of them have been in the last year. No, I have not cleaned the battery fan.

    I would imagine that "health" of the HV batteries can be evaluated. As far as I can tell this was not done during either visit to the dealer's service shop for this problem. Would it be worth specifically asking for this evaluation?
    Again, thanks for your help!
  19. qbee42

    qbee42 My other car is a boat

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    Cold weather will cause the HV battery to run at a higher State of Charge (SoC), so it is more common to see green bars in the winter. Several factors contribute to this, the biggest being that the ICE (Internal Combustion Engine) runs more to make heat.

    Likewise, as a battery gets older and wears down it has less capacity. Lower capacity will also make the SoC swing more quickly.

    All that said, it's not yet time to panic. SoC swings are normal, and the feel of your Prius will vary with the SoC. This could be just normal seasonal variation. If the battery is really failing the symptoms will get worse. If that happens you can get a salvage battery or a rebuilt unit. It's not a big deal.

    Tom
  20. uart

    uart Senior Member

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    SoC = State of charge (of the HV battery).

    Gary, do you also notice more frequent excursion into very low SoC, like 2 or less bars. That is also a indicator of a battery in poor condition. Especially frequent and rapid excursions in SoC in either direction.

    Re the vent in the back seat. This draws air which cools the HV battery and it frequently gets clogged, especially in older Priuses and especially if you carry a dog (or anything else dusty or hairy) in the back. There are a few guides here on how to check the fan and clean it if it's clogged.
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