Hi

 

I have a 1.4 petrol engine, auto gearbox.

 

Driving down a steep hill earlier I noticed the gearbox was in D4 at 60 mph. I was braking gently to control my speed. The cruise control was off. As soon as the road leveled out the gearbox changed up to D7 OK.

 

I assume this is normal?  How does the gearbox 'know' that it needs a lower gear to help control speed ?

 

Thanks all

 

K

There is an "inclination" sensor to detect the gradient so it know you are on a downward slope, there is no fuel being injected & you are applying the brakes.

Edited 17 June, 20169 yr by Gabbo

It's clever. Got a big powerful microcontroller chip in there 

I like this feature. In all other auto cars that I have owned the gearbox had an ambition to get into top gear as soon as possible and stay there for as long as possible, so down a long hill the cars would tend to run away with you with very little ,if any, engine braking.

Thanks all this has been helpful.

 

Personally I prefer to use the brakes to slow the car, as brake linings are cheaper to replace than clutches etc. However, I know I can override the auto gearbox so not a problem

 

K

There is no wear on the clutches from engine braking, unless your clutches are already defective/slipping.

Agree with Gerrycan.

 

On my way home I have a 3km decent with a gradient around 5%.

Braking a long distance like this is not good for the life of the brakes & I prefer to leave it in 5th gear which will hold a constant speed down the decent with only an occaisonal tap of the brakes as required.

Thanks all this has been helpful.

 

Personally I prefer to use the brakes to slow the car, as brake linings are cheaper to replace than clutches etc. However, I know I can override the auto gearbox so not a problem

 

K

As said, there is no wear on your clutch in this scenario, clutch is fully engaged so wearing no more than when driving up a hill. There certainly would be wear on your brakes though if using them regularly on long descents, in addition you may overheat them such that if you need to stop at the bottom you could struggle, you may even warp the disks.