regenerative braking , e-assist acceleration much louder than usual

155k miles, 2001 prius, original battery

Today while driving, I noticed that regenerative braking seemed much louder than usual. When just taking my foot off the gas at around 30 or 40 mph, there was a noticeable harshness to the usual gentle hum. If I press on the brake lightly to engage more regen braking, it's louder.

I took the car in the freeway for a short drive, and the on-ramp acceleration was louder than usual. I had full power, no check engine light, just a noise that tells me something is about to break.

If I start to coast and I hear the loud hum, putting it in N makes it go away. Or at least makes it quier than the road noise on my junky city streets. I need to find a quiet freshly paved road, and a passenger with better hearing

Braking from 30 mph to 0 while in N (no regen) sounds like it does in any other car.

When I got home, I scanned for codes, and came up with a pending P0300, which is a misfire. Not sure if that's from today. See, a couple weeks ago the engine started with a rough idle and vibration that cleared up within a couple minutes of idling. (The roughness was similar to that occasional condition where when at a stop light the engine cuts out, but sort of chugs along rough for a couple seconds. I'm under the impression this is a common occurance among Prius owners. I've had this car for 3 years, and its only done it 2 or 3 times.)


Any idea what could be wrong? Or should I just drive around until I get some horrible code?

I also have a scanguage II, so if there's any xguages I should program to get extra data, let me know.

Thanks!

 

I've never change the transaxle fluid. Toyota says don't bother. I should have probably known better.

I put in the SG2 xguages for mg1 and mg2 temperature. At was 55F outside, and in the driveway MG1 crept up to 70F, and MG2 stayed at 55. Make sense, MG2 isn't moving when the car is still. After driving a bit (30 min drive to drop off my kid, and then to work) MG1 was at 110, and MG2 was at 180F. I noticed the peak was around 190 or so. So, based on everything I've read online so far, MG2 probably has half-burnt motor windings.

I reset the odometer when I left home, and at the end of my drive I have 46 mpg, which is a little above average for my usual driving style. Maybe I was babying the car a little bit. But if I am wasting energy to heat, its not too much.

Does anyone know what the ECU is looking for before it throws a code for this? MG2 temp is elevated, but everything still more or less works. Are there any preventative action that can be taken to stop the damage from progressing further, or am I just waiting for an imminent catastrophic failure?

To make it worse, I still have the original battery, and at 155k, it probably doesn't have much life left in it. I won't know until that fails too. The two repairs together are probably way more than the car is worth.

 

I scheduled an appointment at Art's Automotive in Berkeley. They fixed my air conditioner a couple years ago when the refrigerant leaked out. I talked to them on the phone, and they said it sounds like the stator. I asked, and they do offer the stator-only swap if the rest of the motor and PSD are in good shape. Hopefully I can get out of it for about $2500 bucks or so, which is what I saw someone said the same thing cost at Luscious. Much better than Toyota's $5000 replace-everything procedure.

After this is fixed, I might sell it while the battery is still good. Or not.. a battery swap I've heard can be in the low $2000's, so that's still cheaper that a down payment on something else. The way I see it if I can keep it going for less than what it costs to get something else, I might as well keep it going. I have no idea how long the battery will last; maybe I will be lucky and that'll be next year's expense. I imagine (and hope) once the battery and MG2 are in good shape, the rest is back to regular maintenance.

 

I've looked into a little bit, and if it is the MG2 stator (which I'm pretty sure it is. I found a youtube video of someone who had a fried mg2, and it was exactly the same noise), I know the low cost option is stator-only replacement. The source of stators is a new MG2, from which a stator is pulled out, and the rest of mg2 is scrapped. This is to save the labor of shimming in a new MG2, which I've found online is terrible work.

Has anyone used Dorman parts? I knew they sold reconditioned prius batteries, but they also have stators on their site. I have no idea how 'used' this is, but would any one recommend / warn against this : Dorman Products - 587-990

 

Well, if I need a stator, I'll ask Art's if they've seen this yet. I'm willing to be the trial run of one of these if they're willing to try installing it. Maybe my car can be their next article. Anyway I've done all I can until my appointment on Wednesday.

 

This is what Art's had to say:

DIAGNOSIS PERFORMED / FINDINGS:
Test drove car. Confirmed excessive wheel speed whine/growl when braking or coasting.
The noise goes away when in neutral (MG2 & inverter aren't active in neutral). This eliminates wheel bearing or brake noise.
Drove in reverse. Confirmed abnormal noise (but at a lower frequency due to slower speed).
This leaves the MG2, differential carrier bearing, drive chain, and inverter. I don't think the noise sounds like inverter noise (this could be confirmed by swapping a known good inverter). All of the other components are in the transaxle.
As far as the MG2 temperature goes. 200F would be way too high for the MG2 inverter temperature, but *may* be normal for the winding
temperature. Toyota views this as proprietary information and does not publish a normal temperature range for the MG2. However, since there are no codes, 200F is most likely within normal range.

RECOMMENDATION:
Probably best to just replace the transaxle. If customer wants to be 100% sure the inverter is not at fault, recommend either a swap or a chassis
ear check of inverter and transaxle case.​

I talked with the mechanic over the phone for a while, and we talked about ways to rule out stator vs the rest of the transaxle. It may be the case that something else is going in the transaxle, and that it doesn't make any noise in neutral because the motor isn't really under load.

He is also the instructor for a local community college hybrid tech class. So he invited me to the college that night, and my car was the class demonstration. They disconnected the inverter, and tested the motor stator directly. The results were inconclusive. They did three tests: 1) test with a milliohm meter. All three windings (U to V, W to V, U to W) read about the same: motor OK. 2) test with the 'all test pro' which measures more than just plain resistance. The highest winding reading was 10% higher than the lowest one, which suggests a bad stator. The third test, which is graphing the current rise time for a 12v load across the windings (a crude measure of inductane), puts all three windings within 5% or so of each other, which is acceptable.

The mechanic thinks that while the stator may be starting to go bad, he doesn't think it would make such a loud noise, and thinks the noise may be something else in the transaxle. Usually the short in the motor eventually makes it way to ground and triggers a code. Leaving me with a few options.

1. Ditch the car.

2. Replace the transaxle. This will solve the problem, since everything it could be is being thrown out. But costs as much as a gen 1 that doesn't have this problem.

3. Keep driving, until a) the short gets worse and a code confirms that stator is burnt, then replace stator or b) whatever else in the transmission is going out eventually breaks and the car dies.

For now, I'm going with '3', since the car still has full power, and if it is the stator, the car should be drivable even with the code.

My daily commute is 20 miles, and my office is near Art's, and we have a second car, so I'm sort of in a OK position to say drive until it dies.

 

As I've driven it around the last couple days, I now notice that at 25-30 mph, when in N you can hear the noise (but it is much much quieter than D) and at 60+ mph the noise is present in N (quieter than in D).

The noise is the loudest when the electric motor is under load: either accelerating or decelerating. When at about 20-30, when I'm doing maximum regen braking (just enough brake to not engage the hydraulic brake), or I'm accelerating with electricity only, the hum seems to be made up of a pulses.

Maybe I can grab a recording of the sound next time I'm driving. I wonder if the frequency of the pulses can be reconciled with the mg2 rpm given my driving speed. I would expect that if the noise was related to the chain or the differential bearing, the pulses would be at some multiple of mg2 rpm, depending on what gear ratios those parts turn relative to each other.