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uh-oh - it hums....

Discussion in 'Generation 1 Prius Discussion' started by ChapmanF, Oct 15, 2010.

  1. ChapmanF

    ChapmanF Senior Member

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    At about 160,000 miles, my Classic now has a hum. The pitch of the hum is strictly determined by road speed. It is unaffected by engine speed or by shifting into neutral. It *sounds* loudest in the right rear.

    From past threads it looks like a hum with exactly that behavior has turned out in the past to be a wheel bearing (ok, no sweat) or a shorted MG2 winding (yipes!).

    I'm currently piling on about 600 miles a week helping out my parents for a while so I've already had about 1500 miles of hum without much time to think about it. It is not getting rapidly worse, but it is definitely not going away. Naturally I'm anxious as to whether it's a wheel bearing (or CV joint or other relatively simple problem) or an imminent transaxle failure that I'm looking at. I still don't have my hands on a scangauge capable of reading HVECU data like MG2 temp.

    Using a tuner app on my PDA, if I tweak the reference from a=440 to a=433 then at 60 MPH the hum is the e below middle c. That makes it about 162 Hz. As you'd expect, 45 MPH gives the b below that, and 67-ish is the f# just above.

    The tires (Harmonies ... who knew that would be so apt?) are spec'd at 908 revs per mile, or (conveniently) 908 RPM at 60 MPH. So the wheels (and everything that turns with them) are doing about 15 revs per second at 60 MPH. The hum is about 10.8 times that frequency.

    The transaxle's reduction ratio is (75/26)(44/30)(36/39)=3.905, so MG2 does about 59 revs/sec at 60 MPH. The hum's about 2.75 times that.

    Does anybody happen to know the number of poles in MG2's rotor, or balls in the wheel bearings? (For that matter, is there a nice formula for noise pitch from a ball bearing? I assume there is, but I get dizzy trying to work it out since the balls are moving too.)

    Not to leave any clues out, I did hit the mother of all potholes a few weeks ago (it's hard to say exactly when I first started noticing the hum, but that's roughly in the ballpark). That left a big sidewall bulge in the left front tire. I replaced that and one other tire, rotating the new pair to the rear. When the bad tire was demounted, we saw the wheel also got a flat spot. I haven't had time to buy another wheel yet. The unround wheel (with new tire mounted) wound up in the right rear, which *is* where the hum seems loudest. But there's only one flat spot on the wheel, so from that I'd expect 15 or maybe 30 Hz, not 162.

    What would be your next diagnostic ideas?

    Thanks,
    -Chap
     
  2. SageBrush

    SageBrush Senior Member

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    A hill ?

    I'm just guessing -- your post was a fascinating read, and good luck!
     
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  3. jk450

    jk450 New Member

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    Reduction ratios reduce revolutions in a given period of time, rather than increasing them. In other words, your motor speed should be higher than your wheel speed. Try reversing your calculation.

    What tuner app are you using?
     
  4. ChapmanF

    ChapmanF Senior Member

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    I think we're agreeing ... I had wheel speed of 15 revs per second, giving MG2 speed of 59 revs per second (15 * 3.905) - is that what you get?

    So the sound (162 Hz) is either produced by something vibrating about 10.8 cycles per wheel rev, or something vibrating about 2.75 times per MG2 rev. Since my first post, I found a photo of a classic MG2 rotor that makes me think it has 8 magnets, so I would naively guess that MG2 noise would be 4 or 8 times the MG2 speed (say, 236 or 472 Hz at 60 MPH) rather than what I am hearing. But also since my first post, I also found a thread where Bob Wilson was seemed able to analyze a recorded hum waveform to see if MG2 is the culprit, so he's probably well beyond my guesswork as to what the sound should be.

    You may not believe this, but I'm still carrying around an ancient Handspring Visor onto which I loaded Ondrej Palkovsky's "TuneIt" app years ago. It doesn't quite act like a real tuner (using the microphone and identifying your pitch); it just plays any pitch you select so you can match it by ear. It's ok if it doesn't drive you too crazy trying to match the pitch of a square wave. It has a metronome feature too.

    -Chap
     
  5. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Excellent write up! Rotten problem!

    The MGs are three phase but the frequency will be proportional to rpm. You are right, I was measuring the voltage to and from MG1 and MG2 using a magnetic pickup and have a recording link of MG1 in this web page:

    Do you have a ScanGauge or interest in renting a Graham miniscanner ($150 deposit, $15/mo. rental.) The best signature of a failing transaxle is:
    MG2 temperature > MG1 temperature
    MG2 is located on the end of the transaxle and has air on the end and sides. In contrast, MG1 is sandwiched between the ICE and MG2 and typically runs close to the ICE coolant temperature. When MG2 has an internal short, the excess heat causes it to run warmer, significantly warmer.

    Has there been an unusual drop in MPG? This is not a firm symptom but comes about because of the energy lost IF there is a stator short.

    One good thing is the hum seems to come from the right rear. You might try on the next drive to feel the wheel hub. If you have a bad wheel bearing, it is likely that hub will run significantly warmer than the other. It is a 'cheap' but not definitive test.

    You might also consider finding a stretch of road with a wall or buildings that you can drive by with windows down (say the backside of a shopping center.) Noise from the car will echo off of the buildings or wall and you can quickly confirm which side is louder.

    FYI, reading over my pages, nearly five years old, I'm struck by how much my understanding has changed over time. I'll try to clean them up as I have time. Back then, I was under the impression that Pulse and Glide was critical to high Prius mileage.

    Bob Wilson
     
  6. blamy

    blamy Member

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    All that math is really impressive but before you drive yourself nuts with what might be wrong try rotating your tires and see if the hum goes with it???
     
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  7. Patrick Wong

    Patrick Wong DIY Enthusiast

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    I would suggest replacing the tire that has the bad wheel, with the spare tire, to see whether the sound changes or goes away. Let's eliminate the easy issues first.
     
  8. justkyle

    justkyle New Member

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    That's a good point. It is so much EASIER to get to the prius spare than it is in other vehicles. I'm convinced Government Motors deliberately made the spare in the Rendezvous inaccessible by design.
     
  9. jk450

    jk450 New Member

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    Yes, you're right. I was thinking of RPM, not RPS. MG2 runs at a little less than 3600 revolutions per minute at 60 MPH, and that obviously correlates nicely with 59 revs per second.

    Yes, the Gen I MG2 has eight magnets. Current frequency is four times MG2 speed. It takes two adjacent magnets - a north and a south - to produce a sine wave, which is one cycle.

    You can't diagnose MG2 from frequency alone. There is too much of a chance that another source could create a similar frequency. Frequency can only be used to help narrow the problem.

    Drive your vehicle on a level, smooth road at around 20-30 MPH, with a light throttle. If MG2 is the source, the noise will increase in volume significantly as you brake.

    I am not sure if your noise cropped up at exactly the same time that you swapped the tires, but as blamy and Patrick Wong have said, the first order of business would be to swap tires. Start by returning them to their original locations.
     
  10. ChapmanF

    ChapmanF Senior Member

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    While I know the stator is wound for 3-phase, I've been assuming (optimistically?) that in the case of the sort of shorted-winding defect you've described elsewhere, the fault current we care about is just in the unintended loop created by the defect (since the sound isn't affected much by shifting to N and open-circuiting the intended paths) and so the vibration would come from the interaction of the rotor magnets with one stationary defect in the stator, at a frequency like (MG RPM)(n rotor magnets/2).

    You know, I haven't been around these forums for a while, and last I knew, it was hard to find a miniscanner and I couldn't do much with a ScanGauge in an NHW11, but I've been catching up a little and it looks like vincent1449p with you and w2co have rocked that reality. Wow, that's great. I bought a usb-programmable Elm OBD interface a couple years ago with great intentions of trying to figure the same stuff out, but right now I'm tempted to just buy an SGII and use vincent's tables. Maybe later I can play with a laptop and the Elm box using the same tables. Thanks for working all that out!

    It would be hard for me to say, since my driving patterns are all different now dealing with my folks (short in-town trips replaced with 300 mile jaunts). But it doesn't seem obviously impaired.

    That may be one of the next things I try. The math doesn't really drive me nuts, it's kind of fun and one of the few things I can actually do while driving six hours one way or the other, which is the most time I've got that isn't dealing with other things.
    Thanks! I guess the 8 magnets are arranged like NS SN NS SN NS SN NS SN to get the 4xRPM frequency? (I think there's an illustration in the NHW20 features manual showing the NHW11 magnets arranged sort of tangentially to the rotor rather than radially.)

    You caught me, I should have said that more carefully. I guess I'm seeing it as kind of analogous to DNA evidence, which is really useful for exonerating people (it's pretty obvious when it doesn't match), but not so much for building databases and hauling people in because a computer search matched them at 500 gene loci or whatever with some hair sample.

    In this case, because my hum is not 4xMG2 rpm (not even any integer multiple or divisor of that), right now I'm feeling a lot better about continuing to make long trips in the car, and just trying to pin down a tire/wheel issue as I can. I'll probably try to get a reading of MG1/2 temps too, just for comfort. Maybe this hasn't completely ruled out the tranny either (reality could turn out to be more complicated than the model in my head, which happens a lot really) ... but if my hum had turned out to be exactly 4xMG2, I might still not be certain MG2 was the culprit but I might think seriously about renting a car for all this travel, because there's not enough slop in my schedule to be possibly looking at a red ! on the dash 200 miles from home some night at 2 am.

    Maybe I'll see if there's time this weekend to move a tire around. (Looks for wood to knock on.)

    Thanks for all the suggestions,
    -Chap
     
  11. ChapmanF

    ChapmanF Senior Member

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    Lady luck seems to be smiling on me this time. My sense that the hum was loudest in the right rear seems to be borne out. I put the spare on in that corner, as Patrick suggested, and noticed the hub felt a bit notchy when rotated by hand (schezbzflat!) and noisy when spinning the tire by hand.

    I went for a test drive and the hum was still there. I had hoped that the spare tire's revs per mile would be different enough for me to notice the hum was still there but at a different pitch, which would have been *really* strong evidence it was that bearing. As it turns out, they matched the revs per mile so well that I could scarcely hear a difference in pitch. But I think I still have enough evidence to convict the bearing.

    Looks like I'll be ordering a hub/bearing assembly (wow, a bit pricier than I expected). I think I'll look for a round wheel too, before changing the hub, in case the flattened wheel contributed to the bearing condition.

    Thanks for all the suggestions!
    -Chap
     
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  12. ChapmanF

    ChapmanF Senior Member

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    That did it, no more hum. Picked up a rounder wheel on Monday and took it to Discount Tire, where they didn't even charge me the usual $12 to demount the tire from the square wheel and mount it on the round one.

    The hub/bearing assembly came in at my Toyota dealer on Wednesday for about $262. I had not seen the earlier thread on here about an under-$200 Timken alternative. Does anybody know, does the one from Timken include the ABS sensor, or does it have to be moved over from the old assembly? The Toyota assembly comes with the ABS sensor in place.

    Replacement took me about 4 hours, but that includes a break for dinner and a short drive to retrieve my slide hammer.

    The bulk of the time was spent on the second half of step 5(b) on page SA-45:

    5(b): Remove the 4 bolts and axle hub assembly.

    Sure sounds nice. And the bolts came out with no trouble at all. It's just that that the hub assembly has more than a flat flange for the bolts; its cylindrical body protrudes past the flange, right through a big center hole in the brake backplate, sitting right in the big central hole of the mounting bracket, and it did not want to come out.

    Putting the bolts back in (just a few threads worth) and tapping them didn't budge it, maybe just because it was awkward to get a good angle with a hammer. That might be easier on a lift.

    This is where I sprayed penetrant all around where the hub comes through the bracket, and took a dinner break.

    What did the trick was to torque a slide hammer down onto the wheel studs and ring the chimes for about five minutes straight. That gets the hub to part from the mounting bracket, but not from the brake backplate, which still comes along with the hub and firmly believes it's permanently attached.

    Having the bolts still in place by a few threads (from the earlier tap with a hammer attempt) is still helpful so when the hub/backplate finally pull loose from the bracket they don't just dangle by the brake line.

    To separate the backplate from the hub, I then took the bolts (through the bracket, backplate and into the hub) out, and put them back only through the backplate and into the hub. For that I had to pull the hub/backplate gently away from the bracket about as far as I wanted to bend the brake line, to make room to sneak the bolts in. Then I sprayed more penetrant behind the hub/backplate where I couldn't reach the first time, pushed it all back so the bolt heads were against the bracket, and tapped the backplate in that direction using a hammer and a big blunt drift (end of a ratchet handle really) around the edges of the hub flange. I wound up having to remove the brake shoes to get good angles for that.

    The new hub fit easily right through the backplate and into the bracket bore. It's not even a tight fit, minus the corrosion. For future convenience I used some anti-seize (not on the bolts, only where the hub goes through the backplate and bracket).

    Installation step 6 (confirm the ABS sensor signal) is kind of fun. You ground Ts, watch for the ABS light to start rapid blinking, leave the brake unpressed for a sec, press the brake hard for a few secs, then make a straight-line run fast enough and far enough for the light to stop blinking. Then you stop, ground Tc, and watch for the light to blink a normal code (or a code saying what wasn't normal). It tested out just fine but I had to do the test twice; the first time I didn't accelerate hard enough, and didn't reach enough speed for enough distance to make the blinking stop before I ran out of street.

    -Chap
     
  13. ChapmanF

    ChapmanF Senior Member

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    Followup: three years later, the saga continues with a front bearing this time. The differences between the fronts and rears are interesting.

    -Chap
     
  14. BrokenWrench

    BrokenWrench Junior Member

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    And what did you find with the front bearings?
     
  15. ChapmanF

    ChapmanF Senior Member

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    If you don't follow the blue link, maybe you'll never know!
     
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