transmission hum gets louder as speed increases

Doing the mail route the other day, I was coming in too fast at the end of an ice covered Dead End street. I could see that braking alone was not going to stop me from hitting the huge frozen snow bank that was there. Therefore, I shifted into reverse while sliding and slammed the accelerator pedal to floor. Spinning the wheels backwards saved me from hitting the snowbank and destroying the bumper cover (again). I heard a clunk from the drivetrain, but ever since... my Prius is once again as quiet as a mouse.
Your thoughts are welcome. :welcome:

 

I've been busy this past week and wanted to offer some background about a transaxle failure hypothesis.

If MG2, the larger motor, develops a short, this becomes a progressive failure associated with a speed related hum. The permanent magnet rotor couples with the short to continue heating the stator coils and MG2 temperature climbs significantly. However, this is a difficult thing to measure.

One clue is the humming does not change when you shift into "N" because the current flow is coming from the rotor motion. The shorted windings are trying to electronically brake your car to a stop.

There is a linear relationship between speed and frequency given by the gearing between MG2 and the wheel. I don't have the gear ratio handy but if you can verify the slope vs gear ratio that would be a strong symptom towards a burnt, MG2 stator coil.

If you can get your Toyota technician to go on a test drive and monitor both MG1 and MG2 temperature, we can confirm the diagnosis. At ordinary speeds, say 40-45 mph, MG1 will warm up to close to coolant temperature and MG2 will remain relatively cool. However, if MG2 has a shorted coil, it will get warmer, significantly warmer.

Another approach is to have the transaxle oil changed and inspected by an oil testing service. If MG2 has a shorted set of stator coils, you should see evidence in the tested oil sample. It may even smell 'burnt.'

Eventually the shorted coils expand to make a short to ground. This throws a vehicle code and leaves it immobile until the transaxle is replaced.

By all means, try to rule out all, less expensive hypothesis: bearings, CVT joints, and defective tires. These are all much more affordable than a failed transaxle.

This URL may help you understand what we mean.

Bob Wilson

 

I think one of the front wheels is about to lock up and/or fall off. Get it inspected thoroughly at a dealer.

 

Thanks for weighing in Bob. I remembered your excellent post but not enough to quote the salient points. His hum sounds ominous. If he cannot find any suspension issues might be worth it to him to pop the Inverter cover and check for MG2 motor shorts.

 

Toldja. Apparently the experience of "25 yrs of working on front line military jet fighters" is not entirely applicable to front wheel drive cars after all.

 

I recently had this very same problem with my 2001 Prius --- an almost deafening roaring sound that kept getting louder with time. It was obviously wheel or road noise, since it did not modulate with the engines or even the CVT. I played with the tire inflation, but that didn't make any difference to the sound. After finally reading through this posting and what vince31 finally found out, I took my car in to the dealer (sorry, I still take my car to the Toyota dealer, even at 115000 miles) and mentioned to the service advisor that it could well be a wheel bearing. Sure enough, it was the right rear one; the bearings were melted. I had it replaced (not cheap -- on the order of $600 with labor), and the roar has finally gone away. Ah, the silence once again.

 

Hello,

My symptom is very similar, I will look into the sensor temperatures, perform shift to neutral test and ask the dealer to go out with me. I have taken it to the dealer and they told it had been fixed both times, nothing changed.

The sound is a rotational hum, which is noticeable from 35 and on.
At very high speed it's frequency increases/low hum increases to a higher pitch sound, that's where I get the rotational description for my issue.

I am still under warranty(87K), but I need to convince dealer to take a drive and identify what I hear.

The reason I am posting here and searching to see if others with this mileage or higher have experience something similar.
THanks,

 

It is most likely that the sound is from a bad wheel bearing, or else tire noise.

If the transaxle had failed, it would not be covered under warranty unless you had purchased an extended warranty. The transaxle is covered under the 5 year / 60K mile powertrain warranty.

 

Did you ever find out exactly what the problem was related to the humming sound, I hate people who comes up with their theory and not have an answer to the actual problem . I have a 2006 prius with 200,000 miles on it and the humming has been there for over 100k miles, I am actually thinking about just replacing the whole transmission now, I believe the problem is in the transaxle outer axle bearings. But with 200k miles, it would not hurt to just replace it with a transmission out of another prius with only 50k miles on it for the equivalent of 300 quid. It is not the tires, it is not the wheel bearings, no engine lights, no nothing, not in neutral at 70 mph, just the humming only on acceleration, as soon as you decelerate the humming stops immediately. I believe most prius owners are not aware of this humming thinking that it is normal so they just live with it.

 

It's hard to test a wheel bearing with front wheels off the ground since there is no load on them. If you are not familiar with the "bearing hum noise" find someone with a "Chassis Ear" set of microphones, put one microphone near each wheel and perhaps one on the tranny and go for a drive and switch between the microphones. Amazon.com: Steelman (STE97202) Wireless Chassis Ear: Home Improvement

 

I recently started experiencing a growl / hum which increases in volume and frequency (tone) with road speed. It sounded like it could be a hybrid drive transmission gear, bearing or wheel bearing noise - sort of like offroad tires on a truck - no squealing. It seemed to diminish in a hard left turn, but not 'til I was turning hard enough to have a fair bit of noise from scrubbing the right front. The noise was the same in drive, neutral and braking mode; just varying with speed.

Tonight, I ran the car in diagnostic mode ( can you disable the tration controll? | PriusChat ) to disable the traction control and raised one front wheel at a time, putting the car in drive and spinning each wheel slowly. The noise was clearly evident with the only left wheel spinning and disappeared with only the right wheel spinning. I was careful to keep the speed low and to do this only briefly to avoid excess stress on the bearings in the differential. I then had my wife repeat this with the left front wheel in the air while I slid underneath and listed to the differential housing and the wheel hub through a long screwdriver. The noise was clearly coming from the hub. I'll swap the bearing / hub and report back on the results in this post.

If you choose to do this, make sure the traction control is off, the parking brake is set, the wheels on the ground are chocked, and be very aware that you are working around moving parts and the wrong move could injure or kill you.

The set up that 3prongpaul suggests with wireless microphones is way, way safer!

UPDATE 2013-07-13:
I picked up a wheel hub (NSK, made in USA - 3yr / 45,000 mi warranty) for under $130 from World Auto Parts yesterday. All told, it took me 8 hours.

An hour of that was wasted due to me having a brain fart and touching the brake pedal and getting starting the car to get into neutral instead of just pushing the power button twice. Of course, the piston got pushed out. I knew what happened right away so I positioned the caliper carefully to avoid getting air in the line and reassembled it with teflon tape on the bleed screw threads, the bleed screw cracked open and a one way bleed tool installed on the screw. I was able to reassemble it and bleed out the remaining bit of air without an issue, but I did trigger the DTC and had the ABS & traction control lights on. Thanks to this site, the lights are out and it is driving nicely once again.
I'm in it now - Front wheel bearing/hub replacement | Page 2 | PriusChat

The rest of the job was pretty straight forward. I had a bunch of corrosion preventing me from removing the hub from the knuckle so I removed the knuckle completely. The corrosion cost me 4 hours. The dust shield came off easily, but the bub didn't. It started off with alternating heat, PB Blaster and persuasion with a hammer to finally break it free. Then it took a fair bit of time with the wire wheel to clean everything up. In retrospect the hour round trip to go to work and use the hydraulic press would have been time well spent.

Before winter, I'll be doing the other side even though it sounds and feels fine at the moment.

My Prius is a 2008 with 108k on the odometer. The NE Ohio road salt did some damage.