Is it safe to slow down an automatic car by changing gears?

Question

In manual transmission cars, the driver can change to a lower gear as the car travels downhill to save using the brakes (known as engine braking I believe).

Just wondering what would happen in an AUTOMATIC transmission car that's travelling downhill, and the driver changes gears from "D" to "2" or even "L" to save using their brakes. Does it damage the gear box or engine at all?

Answer 1

if going downhill and u downshift from drive to 2nd gear and your not running the speed that you would need to run in 2nd gear then all your doing is trying to watch the transmission fall outta the *** end of the car or just have it go completey out on you!

Answer 2

on a long downhill run it is better to select a lower gear to utilise engine braking and keep your brakes cool. it makes no more wear changing up a gear than down a gear. caution not to over rev the engine

Answer 3

That's what its for.  When travelling down a steep grade you are supposed to use D3 or D2 or whatever gear is appropriate to LIMIT your speed with engine braking.  That way you don't overheat your brakes, causing them to fade.

For normal driving on level roads DO NOT use the transmission to stop the car.  That is what brakes are for.

Answer 4

If it's an older style auto, you'll wear out the bands inside the gearbox that operate the clutch packs that channel drive between the different gears. If it's one of the newer boxes that are actually automated manuals, then it'll just wear out the gear synchros a little quicker.

Some cruise controls will gently apply the brakes for you if you're too d**n lazy to use the brakes yourself, and as far as 'saving' the brakes, if you know how to drive and don't 'ride' the brakes, going down hills doesn't wear your brakes out any more than normal driving on flat roads.

Answer 5

the only time you want to do that is if your at the top of the hill at a dead stop choose the gear you want to go down the hill in and use that gear.DO NOT shift down while going down the hill.cheaper to replace the brake pads than the trans.you only want to shift to a lower gear while going uphill.all transmissions will down shift themselves when needed going up hill.

Answer 6

Providing you are not travelling too fast downhill you can use an automatic just like a manual.

Regards.

Answer 7

EDIT: I don't know what is bugging hruth, he sure seems to have a bee in his bonnet.  Answers cross in being submitted so answerers don't always have the opportunity to read the answer before.  Now hruth says he is right because he is a mechanic.  Now as motorists know you can't trust all mechanics, some are good, some bad. Now he  doesn't say he is an auto transmission repairer and WHAT HE SAYS IS CONTRARY TO ADVICE GIVEN BY  TRUE RECOGNIZED EXPERTS, including manufacturers, car manuals, lecturers, training institutes etc. Mark F ' A shop manager', not just one of the mechanics under his supervision disagrees with hruth. See his comment below. If you are on a long steep down grade like going down a mountain it is SAFER to use your gearbox than to be braking all the time.  Not only does it reduce brake fade and brake overheating but you have more control of your car.  Your brakes will then more efficient if you need to use them in an emergency.  SAFETY is the main factor not saving a couple of dollars, life is worth more than that.

I was always told that the gearbox itself is not harmed by this. This information came from the owner operator of the major  auto transmission center in a city of over 1.5 million. What you have to avoid is over revving the engine by changing down at too higher speed. Transmissions have a recommended maximum speed for each gear.

I researched this thoroughly after hruth's outburst. Here is advice with sources.

Downhill grades won't overheat transmissions but they are hard on brakes. Placing an automatic transmission in a lower gear will engage additional clutches in the transmission so that the engine can supply some compression braking to assist the wheel brakes. Using the engine and transmission to slow the vehicle increases vehicle safety by reducing heat in the regular brakes.

Going uphill, downhill, or into a strong wind, if you are loaded or towing a trailer, selecting a lower transmission gear will make the drive more enjoyable, economical and safer.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC)

Engine braking does not consume clutch packs in the automatic any more

than any other shifting. in fact, it's less wearing than a full power

acceleration shift. From Honda website

Periodically, we have tourists use their brakes exclusively to descend Pikes Peak (instead of using a lower gear that would provide the engine braking you're talking about). The sneaky ones manage to bypass the mandatory brake checkpoint a little over halfway down (you have to have the temperature of your brakes checked by a forest service technician).

The reaction of a driver when their brake fluid boils on the way down a mountain can be poignantly pathetic, especially when the car is filled with the wife and kids of the driver. It's almost a haunting effect to be driving up the mountain and have a car come rolling silently down the mountain and to stare at the terror filled faces peering desperately out the window. Obviously, a driver couldn't possibly slam their car into park as it rolled down the hill at 45 - 50 mph. That would ruin their transmission. Better to shut off the ignition, lock up the steering wheel, and hope the car can withstand plunging off the side of a cliff once the first turn arrives.

The general rule of thumb for going downhill is to use whatever gear your car needs to climb that hill. You will follow many untrained drivers down hills that have their brake lights always on. Chances are they are either driving a auto in D from dream (drive) or in too higher gear in a manual. The car wants to race away and they ride the brakes excessively. Select a lower gear so the gears help r****d the car, if you need to slow, brake firmly and then come off the brakes to let them cool. Don't ride the brakes! (safe driver training manual).

My car owner’s manual advises changing down gears in the automatic going down long downhill grades. Why would the manufacturers advise this if it was harmful to the transmission?  It is a safety issue, see the Pikes Peak item above.

Driving downhill

If the EGS control unit detects downhill driving, it will automatically shift down a gear if the vehicle's speed picks up. This enhances the engine's braking effect. A downshift is only effected if the engine speed is below the maximum speed of the lower gear. BMW 6 speed auto specs.

A. You can shift gears according to the road condition. For normal in-town and highway driving, use D4. The car will start with first gear and shift automatically to second, third and fourth. Use D3 when driving in heavy traffic. Use D2 for increase engine brake when driving down hill or for increase engine power when driving up hill. Use the first gear to provide maximum engine braking effect or when driving down a steep gradient. For more details, refer to your vehicle owner's manual. (Renault car owners club adice)

Answer 8

DON'T DO IT!!!!   AUTOMATIC TRANSMISSIONS GO OUT QUICK ANYWAY.    GET A STANDARD THEN YOU CAN DO THAT

Answer 9

You can do it, however it is considerably more expensive to replace a transmission than it is a set of brakes.

Answer 10

Not a good practice at all ?  Sooner or later the trans will be

affected or you are  going to shift back to drive and wind up

revving in neutral or popping it to reverse...

Just work those brakes properly....and you'll be fine...