Odd noise/vibration from brake pedal

[GTRX3]

Hello,

I’m new to owning the nx, a 2015 model. Drove it for the first time after 2 days and it’s developed an odd braking sound. There is no noise during the process of braking but when I’m sat at the lights with my foot on the brake (regardless of being in drive or in neutral) there a weird electrical vibration noise which pulses. Almost sounds like a farting noise. I can also feel light pulsating in the brakes for a few seconds. It then is usually gone until the next time I come to a stop. Is anyone familiar with such problems? Thanks

[Herbie]

The answer may just be in the second line where you mention "neutral".

When you come to a stop at the lights, always keep your foot on the brake and keep it in D(rive) or if you thing you'll be a while, put it in P(ark) - the traction Battery will not charge if the car is in neutral.

If you've not been using it much, or only for short journeys, the traction abttery may be low on charge and what you're feeling is the generator trying to charge it, which would be more noticable with the car stationary.

[GTRX3]

What is the traction Battery? That’s news to me, so there’s no benefit in keeping the car in neutral at the lights?  My journeys are on the short side, so that be very plausible.

[Herbie]

In a hybrid there are two batteries - a 12V Battery that basically 'starts' the hybrid system and gets the car into READY mode (which is our equivalent of the engine running) and also a traction Battery that drives the electric motors and is around 288 to 300V depending on the car.

Conventional cars have a starter motor that, when you turn the key, will crank the engine to get it running. The starter motor can draw more than 300A from the Battery to start the car and when the engine is running, the alternator will replace the electrical energy used and keep the 12V Battery charged up but it's main job is to run all the car's electrical systems - the 12V Battery is only used for starting.

Our hybrids don't have a starter motor as such and they don't have an alternator either. Our 12V batteries only have to boot the hybrid computers and one or two other things and it takes less than 20A to do that job, which is a lot different to the 300A a conventional starter motor would need. Because of that, Toyota/Lexus thought they could save a bit of money by only installing small-capacity 12V batteries, which is fair enough, except that the downside is that the 12V Battery goes flat quicker than a larger Battery would. For instance, it's not unusual for the 12V Battery to go flat if parked up for a couple of weeks at the airport while you're away on holiday. Quite a few of us carry something like this in the car just in case it happens.

So, that's the 12V side. Remember I said that our cars don't have alternators? Well, it's because the petrol engine doesn't run all the time - the car sometimes switches off the engine and runs on electric motors, which are powered by the traction Battery. Anything that would normally be run by a belt from the engine, such as the alternator, the aircon compressor or the power steering pump, are all driven by electric motors so that they can still be used when the petrol engine is off. In fact, once the car is READY then the traction Battery feeds all of the car's electrical needs, just like the alternator does in a conventional car.

Taking the alternator as an example, the 12V Battery starts the car and gets it into READY mode. Once READY, some clever electronics in what's known as a DC/DC Converter (our equivalent of the alternator), steps down the 300V from the traction Battery to around 14.5V to keep the 12V Battery charged. Different boxes of electronics do the same (albeit at different voltages/currents) for the aircon compressor and other stuff, all run by the traction Battery.

The other main job of the traction Battery is to provide power for the electric motors that drive the wheels when the petrol engine is off. This Battery is mainly charged by regenerative braking. The regen braking system takes the kinetic energy that would normally be wasted in heat by friction of the brake pads against the discs and turns it into electricity to charge the traction Battery. Every time you lift your foot off the accelerator you'll see the power meter needle drop into the 'Charge' zone of the meter, but maybe not so far. When you actively press the brake pedal the needle will drop right down to the bottom of the 'Charge' scale, indicating maximum rate of charge.

If the traction Battery is dropping low on charge but no braking is taking place to charge it, the hybrid system will run the petrol engne, which in turn runs one of the motor/generators (MG1 and MG2) in a backward rotation to act as a generator and charge the Battery. I often get MG1 and MG2 mixed up so that may not be strictly correct but you get the idea - it's one or the other. It can be quite noticable if the car isn't moving at the time, which is why I think that this may be what you're feeling.

However, as I said above, if the car is in neutral then this won't happen, which is why you need to keep it in drive.

Something else to bear in mind is that these are hybrid cars, not electric cars. If you run out of petrol, the most you'll get on Battery alone is maybe a couple of miles, no more, and it also opens up a whole host of other problems.

To be a fully-functional car, three things are needed as they all work together - the 12V Battery, the traction Battery and the petrol engine.

Hope that helps,
Herbie

[GTRX3]

Yes that was great. Thank you for the very detailed explanation!

I did always wonder how a starter motor would cope with the hybrid system but that makes sense. And did not occur to me about the alternator wouldn't be able to function normally either. Cheers

[Herbie]

You're welcome Imran.

I forgot to mention it above but the way the system actually starts the petrol engine is that MG1 spins the engine up to 1,000rpm before applying fuel and a spark.

[Britprius]

I could not have put it better myself Herbs. To describe the hybrid functions rely needs a book, but the big no no that is not self evident is avoid neutral if at all possible.

John.

[Herbie]

You've probably forgotten more about this than I'll ever know John so thank you sir

[GTRX3]

Here's a video of the sound. The Battery appears to have charge in it.

https://youtu.be/9QNk58vRk2k

[Herbie]

2 hours ago, GTRX3 said:

Here's a video of the sound. The battery appears to have charge in it.

https://youtu.be/9QNk58vRk2k

Absolutely nothing like I was thinking and I have to confess that I haven't got a clue as to what that could be.

The car is obviously stationary there so just to confirm, this only happens when you press the brake pedal does it?

[GTRX3]

Yes, seems like there are more pulses when the brake pedal is pressed lightly or one bigger one when it's pressed more firmly. Tried doing some emergency stops as well just because it had been raining heavily the past few days. I wonder whether its an issue with the brake actuator?

[Herbie]

It does sound like something to do with the brake actuator. Have you looked for any codes with an OBD2 code reader or scanner?

[GTRX3]

No left the obd scanner at the parents home.  I may have to go to a mechanic/lexus garage. The car has had full lexus service history so far, but its out of warranty at over 80k miles and is 5 years old. I'd imagine thery would charge a diagnostics fee.

[Herbie]

2 hours ago, GTRX3 said:

but its out of warranty at over 80k miles and is 5 years old. I'd imagine thery would charge a diagnostics fee.

How long have you had it? It sounds quite new to you so you should still have rights under the Sale of Goods Act to get the seller to put things right, unless it was a private sale.

[GTRX3]

I've had it since January, it was bought from Arnold Clark. They said it had 2 months warranty from them. Didn't get the extended full year warranty 🤦🏾‍♂️

[Britprius]

7 hours ago, GTRX3 said:

Here's a video of the sound. The battery appears to have charge in it.

https://youtu.be/9QNk58vRk2k

Imran. That noise is very much like a noise in the brake system the Prius can suffer with. The cure is to bleed the brakes, but for this you will need Techstream to be able to bleed the brake actuator properly.
It is caused by tiny air bubbles in the brake actuator.

John.

[Herbie]

See, I knew John would get it sussed 

So, if you're going to do it yourself then as John says, you'll need Techstream and a Windows laptop to install it on. The good thing is that you don't have to bother about an OBD2 code reader now because Techstream does so much more.

In order to connect the laptop to the car you'll need a miniVCI cable, a selection of which can be found here on eBay These cables almost invariably come with a pirated copy of Techstream, which is the actual diagnostic software Lexus techs use in the dealer workshops. Of course, I can't condone the use of pirated software, that's a matter for your own conscience. The proper way would be to freely and legitimately download it from here and then pay for a block of time to use it.

Whichever way you decide on, you'll need that miniVCI cable to connect the laptop to the car anyway.

[GTRX3]

If it is that would be great because brake actuators are b****y expensive!

[Herbie]

3 minutes ago, GTRX3 said:

If it is that would be great because brake actuators are b****y expensive!

It's a Lexus - everything is b****y expensive!

[GTRX3]

Coming from a Subaru Impreza, it makes that thing look like a bargain.

[Herbie]

23 minutes ago, Don C said:

Why?

Why what? Why avoid putting the car in neutral?

Because the hybrid system doesn't work in neutral.

The following text is taken from https://en.wikipedia.org/wiki/Hybrid_Synergy_Drive

Neutral gear (equivalent): Most jurisdictions require automotive transmissions to have a neutral gear that decouples the engine and transmission. The HSD "neutral gear" is achieved by turning the electric motors off. Under this condition, the planetary gear is stationary (if the vehicle wheels are not turning); if the vehicle wheels are turning, the ring gear will rotate, causing the sun gear to rotate as well (the engine inertia will keep the carrier gear stationary unless the speed is high), while MG1 is free to rotate while the batteries do not charge. The owners manual^[22] warns that Neutral gear will eventually drain the Battery, resulting in "unnecessary" engine power to recharge batteries; a discharged Battery will render the vehicle inoperable.

[NemesisUK]

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