Rx400h "b" Position

[fabben]

I've always used the "B" position to to help with my braking.

What I've found over the last six months is that this is having less and less a affect. So now it's almost of no use, there's just a slight sensation the the car is slowing.

My question is how does the "B" position work in practice, can it loosen up over time and just require some adjustment.

Or is it suggesting something more serious is happening in the automatic transmission.

The RX400h is almost two years old and is due it's second service at the end of May.

[lfoggy]

I use the 'B' position much less frequently - only when descending early steep hills. When I engage 'B' it is noticeable but not dramatic. Certainly not like changing down a gear in a manual car. I suspect you are just getting used to it rather than there being a problem with the transmission itself......

[ihpj]

'B' position is only for added engine breaking and IIRC to help charge the Battery due to the resistance it creates.

Only really to be used to encourage Battery charging (you will use more fuel due to the added resistance) or - as has been indicated - to use to slow your descent on a steep hill.

I too would suggest you are used to it - one way to tell is the increased revs with it engaged than without.

[BGLister]

I could be miles off the mark, but the way I imagined the B setting to work is that it slides the CVT into giving a lower gear ratio, to create more drag and more charging. If this is/was the case, I wondered if putting it into B would also give faster acceleration by continuing to force a 'lower gear'? (at the expense of economy). Of course...this may not be the case either!!

[jonthebarber]

I have never used the "B" as i never knew what it did , until i have read this thread ! Going off course a bit , does it make any difference ? My rx400 runs fine in the year i have had it , without it , so why bother using it !

Jon

[JBPRX400h]

I could be miles off the mark, but the way I imagined the B setting to work is that it slides the CVT into giving a lower gear ratio, to create more drag and more charging. If this is/was the case, I wondered if putting it into B would also give faster acceleration by continuing to force a 'lower gear'? (at the expense of economy). Of course...this may not be the case either!!

The "B" position simply couples the transmission directly to the engine to provide "normal" engine braking when descending hills (or for slowing the car when approaching junctions etc but beware of the following drivers as your brake lights won't be illminated!) It won't provide any extra charging over and above normal slowing down through regenerative braking and it certainly won't give you a "lower gear" for faster acceleration.

One point to look out for though is, if you use the cruise control, it (cruise control that is) will not operate whilst you are in the "B" position and will also cancel as soon as you select "B". (It has caught me out on a few occasions when I have tried to select cruise control after descending a hill and the human brain being what it is naturally assumes that the cruise control has packed up! Of course as soon as you select "D" cruise control becomes available and all those mental pound signs fade away again!)

Hope that helps

JBP

[BGLister]

Thanks for that. I wish more general info like this was made more accessible. B gear looks like an enigma to most of us!

[Mr Pants!]

I could be miles off the mark, but the way I imagined the B setting to work is that it slides the CVT into giving a lower gear ratio, to create more drag and more charging. If this is/was the case, I wondered if putting it into B would also give faster acceleration by continuing to force a 'lower gear'? (at the expense of economy). Of course...this may not be the case either!!

The "B" position simply couples the transmission directly to the engine to provide "normal" engine braking when descending hills (or for slowing the car when approaching junctions etc but beware of the following drivers as your brake lights won't be illminated!) It won't provide any extra charging over and above normal slowing down through regenerative braking and it certainly won't give you a "lower gear" for faster acceleration.

One point to look out for though is, if you use the cruise control, it (cruise control that is) will not operate whilst you are in the "B" position and will also cancel as soon as you select "B". (It has caught me out on a few occasions when I have tried to select cruise control after descending a hill and the human brain being what it is naturally assumes that the cruise control has packed up! Of course as soon as you select "D" cruise control becomes available and all those mental pound signs fade away again!)

Hope that helps

JBP

JBP,

Interesting as my dealer told me it does provide extra charging when in B mode.

I too only use it down steep hills or if I am going through some exceptionally twisty roads and use the engine braking rather than keep tapping the brakes.

I was under the impression the resistance came from the electric motors, hence the extra charging and would also think this could wear if used too much?

Perhaps a dealer would be best placed to answer this for you.

:)

[ColinBarber]

The extra resistance comes from the engine being kept running during braking, rather than shutting down. Fuel delivery is cut so you don't use any petrol but one of the electric motors actually maintains the rpm of the engine to give the required amount of engine braking. This uses electricity and is therefore less efficient than normal D mode.

[sando63]

Hello, I have a RX 400h, and recently just returned from a caravan break. The Lexus tows the caravan great, but on my way off the caravan park (wet and muddy) my Lexus began to slip and slide, I did manage to get off the field, but wondered after, I thought I heard, that if I had selected B gear it would have made the removal of car and van a whole lot easier, and I kicked myself for not trying it ! Would it have helped ?

Regards,

Dave.

[Herbie]

58 minutes ago, sando63 said:

Hello, I have a RX 400h, and recently just returned from a caravan break. The Lexus tows the caravan great, but on my way off the caravan park (wet and muddy) my Lexus began to slip and slide, I did manage to get off the field, but wondered after, I thought I heard, that if I had selected B gear it would have made the removal of car and van a whole lot easier, and I kicked myself for not trying it ! Would it have helped ?

Regards,

Dave.

No, it wouldn't.

You would normally use a lower gear to give more traction and control in the mud but that's not what the B setting does.

As mentioned above, when going on a long descent the traction Battery can reach full charge and it won't be able to accept any more electricity, so if you move the gear lever to B that excess electricity is fed to MG1 (Motor/Generator 1). The motor is used to spin the engine without any fuel, so the whole thing acts as a big load or a big resistor to a) use the excess electricity and b) to slow the car down by using the compression of the engine (engine braking).

[sando63]

Thanks Herbie, I also use the b gear to slow me down, but also is it ok to select b gear at higher speeds to assist in braking ?

[Herbie]

1 minute ago, sando63 said:

Thanks Herbie, I also use the b gear to slow me down, but also is it ok to select b gear at higher speeds to assist in braking ?

Not sure why anyone would want to do that Dave - brakes are for slowing down/stopping, not gearboxes, especially on a hybrid 

The whole idea of these hybrids is that regenerative braking uses the kinetic energy that would normally be wasted in friction heat, to turn a generator to produce electricity to charge the traction Battery. I doubt that using B would help either generation or braking processes in normal usage.