Resonant hum

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willmhull

Member
Joined
May 2, 2021
Messages
13
Hi guys and gals,

I have a 2017 i3 (no range extender), I just bought it, and I have noticed that when I am coming to a stop without braking, there is a loud resonant humming coming from the front end of the vehicle. However, it doesn't happen every time, and sometimes it is louder (really quite loud) than others. I would imagine it has to do with a problem with the auto regen braking system.

Wondering if there is a known issue or what this might be?

Thanks,
Will
 
willmhull said:
I have a 2017 i3 (no range extender), I just bought it, and I have noticed that when I am coming to a stop without braking, there is a loud resonant humming coming from the front end of the vehicle. However, it doesn't happen every time, and sometimes it is louder (really quite loud) than others. I would imagine it has to do with a problem with the auto regen braking system.
Regen braking occurs only on the rear wheels, so it's unlikely that the sound would originate from the front if regen braking were the source. If the battery pack charge level is so high that the battery pack can't absorb the full regen braking power, the force of regen braking would be mimicked by the automatic application of the friction brakes on all 4 wheels. If the friction brakes were the source of the sound, you should hear the sound when pressing the brake pedal.

So I doubt that the sound is related to regen braking, but I don't have a clue what might be causing it.
 
Thanks for the reply Art, these are some good points. I will try to recreate it with some braking.

It definitely could be the friction break. I remember hearing a similar (but harsher) sound when I had a small rock lodged between a pad and rotor on a car a while ago; although, I rarely fill the battery to capacity. Is the friction brake engaged only when the entire pack is full or with each individual full cell in the battery?

Will
 
willmhull said:
Is the friction brake engaged only when the entire pack is full or with each individual full cell in the battery?
The battery management system attempts to keep the charge levels of all cells close to equal at the high charge level end, so a full pack would also imply each cell being full. However, according to some British i3 owners who attended a briefing by BMW engineers early on, cell balancing doesn't occur when the displayed charge level is less than ~80%, or maybe it occurs only very slowly, and occurs only when no current is flowing in or out of the battery pack; i.e., when the battery pack is idle. So if you haven't allowed the battery pack to rest very long above an 80% charge level, some cell imbalance could exist.

When charging, the charging power tapers off as the charge level approaches full. I assume that regen braking power would also taper off to avoid damage to the battery pack. So at higher charge levels, an i3 might be mimicking pure regen braking power at lower charge levels by blending lower regen braking power with friction brakes. The brake rotors on our i3 have rusty surfaces which allows me to hear when the friction brakes are being used. However, I don't typically charge above 90%, so I probably haven't experienced much regen braking mimicking.
 
After driving using the brakes to stop (some downhill neutral braking and braking while stopping short), I do not think that it is the breaks. Unfortunately, I cannot recreate the sound.

I was wondering what else is in the front of the vehicle? For example, is there a compressor that runs the active battery cooling? I wonder if there is a leak somewhere? I have heard that compressors can create a similar sound when there is a leak in the system?
 
The brakes sound very different if the car has sat for a few days and the rotors have accumulated some rust on the surface. When the pads scrape that rust the sound is very different and much louder. So, it still could be (could be) the regen activating the brakes after the car has sat and the rotors have rusted.
 
willmhull said:
I was wondering what else is in the front of the vehicle? For example, is there a compressor that runs the active battery cooling? I wonder if there is a leak somewhere? I have heard that compressors can create a similar sound when there is a leak in the system?
The A/C compressor is adjacent to the electric drive motor in the left rear of an i3.

A cooling fan for the A/C condenser and the motor and motor electronics heat exchanger is low in the front. A small vacuum pump for the power brakes is in the left front. Both operate only when needed. Can't think of much else that's in the front and that might cause such a resonant hum.
 
Since the "hum" only occurs when you are decelerating, is it possible there is some loose wheel well cowling that, when moving forward is pushed back, but when braking and weight shifted forward, somehow contacts the tire or rotor? Perhaps the infamous deteriorating strut boots are falling apart and rubbing on the back of the tire?
I would start looking in the wheel wells for something slightly out of place.
 
Evanston,

You may be on to something. I was over at my friend's house today, and he noticed that the tires are misplaced: the 175R19 is on the front passenger, and one of the 155R19 is on the rear on the opposite side.

I just purchased this car, so I imagine somebody made a mistake at some point when they rotated the tires... I wonder if this might be the source of the problem. What is even more annoying is that 1. I had a preinspection at a BMW dealership, 2. I took it to another BMW dealership for an airbag recall and asked them to troubleshoot the sound, 3. The dealership I bought it from did a "24 point inspection," and they never realized that the tires were misplaced. Ridiculous, to say the least.

I hope it hasn't done any damage to the car. ..
 
willmhull said:
I hope it hasn't done any damage to the car. ..

Please do a thorough inspection of the inner sidewall of the tire that was rubbing — that could have created a full circumference "easy open" weak spot that could result in a catastrophic blow out, if it's gone on for long enough.

I hope your dealership acknowledges their error and takes care of you.
 
alohart said:
... at higher charge levels, an i3 might be mimicking pure regen braking power at lower charge levels by blending lower regen braking power with friction brakes.

I'm not aware of the friction brakes being applied in the absence of regen. If my 2017 can't regen (usually at 100% charge or on a chopy road or pothole) it gives me nothing when I lift off, and just coasts.
 
eNate said:
I'm not aware of the friction brakes being applied in the absence of regen. If my 2017 can't regen (usually at 100% charge or on a chopy road or pothole) it gives me nothing when I lift off, and just coasts.
Those who live at the top of hills have reported that friction brakes mimic regen when the power pedal is lifted at a very high charge level. I don't live at the top of a hill and don't usually charge to 100%, so I don't have much experience with this behavior. However, when I have charged to 100%, I've never been surprised by the lack of regen which I'm pretty certain I'd notice living in the suburbs with lots of traffic lights.
 
I'm at a disadvantage too, because I rarely charge to 100%, and when I do, it's at work on the flatlands. But on those rare instances, I have noted (with alarm!) an unmistakable lack of regen approaching intersections.
 
I think I can confirm that the regen is inhibited when the BEV is fully charged and going downhill.
After a full charge, 99% and 173 miles range, my 2019 I3 BEV had (unusually) very little regen going down a steep hill at my location. Brakes seem to work normally.
The next time down the hill, later that day, at 94% and 151 miles range, the regen was back to normal.
 
Yes, I can also confirm. I live at the top of a steep, 4 block long, hill with a stop sign at the bottom. With full charge the i3 slows a bit when I lift off, but I still need to use the brakes at the bottom. With a lesser charge (seems to be in the 92%-95% and below range) the regen slows me to a full stop.
 
UPDATE

Tried to move the front to back - back to front at home and a few of the lugs are cross-threaded. I sent an online reply to a customer service survey request and I had not heard anything, so I called BMW. A technician came to my house and tried to remove the tires and could not. Now, I am scheduled for a service visit tomorrow.

Although the tech is skeptical about the tire causing the sound, I am pretty sure the misplaced tire is the problem. The space between the suspension coil and tire is tight (you can hardly fit a finger in there). Further, I noticed that the sound was much worse and frequent when my overweight friend was riding shotgun. It also commonly occurs when I am stopping on a hill--I imagine the coil is compressed in this situation as well probably expanding and rubbing on the tire. Although, it does sound more metallic...

All told, it should be taken care of tomorrow.
 
I hope this is all covered. Since BMW was last to touch it, and the inspection should include a look at the brake pads, the cross threaded lugs at minimum should have been caught. Whether the tech was responsible, or responsible for noticing, that the tires were crossed, well, that's almost a secondary issue to damaged threads. But if the cross - up was preexisting, the tech might have noticed unusual wear on the inside sidewall.

This, of course, implies the wheels were actually removed to inspect the pads, and the checkbox wasn't just pencil whipped.
 
They brought it in for service last week, moved the tires, and inspected it. Unfortunately, the technician I have been in touch with wasn't there, so I have to call on Monday to see what he found in the inspection.

It hasn't been humming since, so it was probably the issue.
 
What about your cross threaded lugs?

Did they find any wear on the inner sidewall?
 
eNate said:
What about your cross threaded lugs?

Did they find any wear on the inner sidewall?

They didn't report any inner sidewall wear when I asked them about it, which I thought was odd. They fixed the lugs, and relocated the tire. I am happy with that. Still going without the noise.
 
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