Downshifting an Automatic Transmission

I have seen a book about it - I think it was called "How to push start an automatic car"

Down shifting is an integral function of the automatic transmission and if articulation of the manual shifting mechanism is used with understanding of this functional characteristic damage should not occur.

To use for engine braking some observation should be held that the vehicle approach the start of the down grade slowly and gradually gain speed while engine braking is occurring.

Engine braking is the engine's ability to slow the car when the transmission is in a low gear.

When you take your foot off the accelerator, decreasing the flow of fuel, the engine's speed slows. Logically, the vehicle slows along with it, but the amount and rate of deceleration depends on what gear the transmission is in. The lower the gear, the greater the engine braking.

The main reason you'd want to employ engine braking is to descend a steep hill. In such a situation, riding the brakes might overheat them and cause them to fade, possibly with disastrous consequences. That said, downshifting by too much for your current speed can be damaging or dangerous as well, so be sure to read your owner's manual to find out the maximum speed at which you can run in lower gears or with overdrive turned off.

This isn't OT.

I agree with you. And, as someone with zero transmission design experience, my opinion is that brakes are cheaper than transmissions.

Even without ABS, most cars built over the last 30 years or so, all fail to one front brake and the opposite rear......ABS has nothing to do with it except that when the brakes failed, that car happened to also have ABS.....

I have not had it happen to me, but I would expect that with ABS might be a tick better than without!!!

For anyone else, if you don't learn how to keep a car from over speeding when going downhill by using the transmission, then you might be living dangerously, as a previous poster pointed out very adequately....and correctly...

Its nothing to do with brakes being cheaper than transmissions.....anyone who thinks like that will surely get his "come-uppence" one day.....most probably with a trailer...but even without could be still be dangerous......

I personally feel (and I would welcome the opinions of others here) that the ability to go down a hill, without dangerously overheating the brakes, is part and parcel of good driving practises.....

By the way, done correctly it does no harm to the transmission.....or engine.....

Down shifting is only a problem if the revs are way more then the engine can handle.

But most automatic transmissions have a setting for inclines?

On my newer car (the previous was 17 years old)

The car Manual states that "On long down hill stretches, the auto transmission will automatically downshift to save brake wear and heating for extended periods of time"

MY8.0 2008 Holden VE Calais

There is one down side to this that I have noticed that hasn't been mentioned. If your engine has some milage on it, engine braking can increase oil usage because of the vacuum put on the compression chamber increases the ingress of engine oil that is then sent out the exhaust.

Only if the engine seals are already worn.....

But even huge clouds of blue smoke will be a lot less than a teaspoon full....if you can afford to drive, surely you can afford to arrive in one piece and use a tiny amount of oil at the same time?

Think of it being a tiny touch of extra lubrication.....

So, far I still haven't heard from the design side as to what specifications or outcomes are possible structually by down shifting, however, I appreciate the input on the brake rotors and the possible effect of long term negative pressure on pulling in oil. I guess the best advice was to check the owner's manual and then contact the dealer.

On another blog, one responder suggested braking to a lower speed before downshifting, which is probably good advice as well, but doesn't answer my question.

Generally the automatic transmission is the strongest link in the set up of drive train components. With exception of applications in heavy vehicles. So being that it is the other drive train components that suffer damage instead; for example universal joints, axles etc..

jimlilly3,

Actually the general nature of the question as stated eliminates any technically accurate answer. Narrow the scope to specific applications.

You've found us out. We're just a bunch of opinionated non-design side people. (And a cat, an alien and a gremlin)

There are automotive forums out there where you might find the level of technical expertise you desire. Try Google.

Have you gone to the SAE website to look for technical help?

www.sae.org

Use the transmission for controlling your speed on down hills, where otherwise continuous braking would be required.

One should not let brakes overheat as this could cause hydraulic oils to overheat, especially if it was not recently replaced, and completely negate the brakes....

Learn how to do it in a proper manner, slowing down with the brakes before down shifting....so as not to over speed the engine.

On an engine and transmission in good reasonable condition it does no discernable harm, but could save your like....

Describe what type you're statement principally focuses upon:

I think you are implying that downshifting some types of automatic transmissions would be damaging and fot others it would not. Is that correct? If so, can you group the ones that fall into the different classes, ie, those you can safely downshift and those you can't?

No I am implying that application or the vehicle the transmission is placed in will determine to some extent the probabilities of a damage occurring.

Sure brake pads are fairly inexpensive, but brake rotors are not, and they are what will suffer the most damage from continuous braking on downhill grades.

It's the extenuating circumstances incurred when they fail that ratchet up the expenses.

That Allison 1000 tranny is superb indeed.

GA for a great post.

You show knowledge & understanding of the problem....

If you really want a smart arse reply, I'd say don't drive an automatic for towwing. Then you actually have control over what's going on, and you can go down the gearbox as much as you like.
Del

There is a cost problem here ;o(

Personally I'd drive a Manual any day over a Auto (Slush-box)

But to purchase a new car these days, if your towing anything substantial, you have no real choice than to purchase a Auto.

The current lineup of Holdens,

1500 & 2500kgs tow packs are available (close enough)

The Clutch in a Manual is only rated to the lower tow pack, you cannot get a car off the showroom floor with the higher rated clutch, the reason being is that people don't want to drive a manual with a heavy clutch (a heavy clutch is needed for the added towing capacity)

Auto trannies are capable for either tow pack option.

So, if your like me and need to tow with the higher capacity tow pack, you need to purchase a Auto, but also, only 1% of cars manufactured are manuals (yes, 1 in a 100)

The automatic is much stronger compared to the manual. The clutch is a weak link in a power train heavy or not; that's why they're not using them. If were only to do with the clutch pedal pressure the manufacturers would provide an hydraulic assist.

Hydraulic/vacuum assist = more money to assemble/manufacture

Actually the clutch isn't that weak, once you get the thing to engage, you have no loss of power thru the transmission from Engine to GB, in a slush-a-matic, you have power losses thru the torque converter.

I run ceramic pucker clutch in my racing buggy, there are no moving parts, just a rotor with ceramic puckers (standard clutches have multiple parts, the pucker/clutch surface is coupled to the output shaft (input shaft of the GB) by springs to soften the engagement)

In the GB itself, I'd have said that both should be about the same strength, the gears would all be to the same construction/hardening and the bearing areas would also be to the same strength, though, there are many more galleries and tubes in a Auto for hydraulic control, I don't know if a auto runs synchro's, most manuals do (unless they are the crash box ones, or the RoadRanger type on trucks)

Auto's can also be thought of "Easier to drive" or "less to think about" once your in a forward gear, you don't need to think about changing up/down at anytime, so they are more suitable for a majority of people on the road (and many of those shouldn't be on the road in the 1st place)

Yeah, I don't dispute that brake fade can also be the fluid boiling (generally because over time the fluid has absorbed moisture), but what I tried to convey (but maybe not very well), was that as long as the brake fluid is in reasonable condition, you can get pad fade long before the system gets hot enough to cause any boiling in the hydraulics. You can simply try 3 or 4 crash stops (max braking) from say 100kmh, & by the 3rd or 4th stop you will feel the brakes "go off".

Your explanation here is better than the previous attempt!!

With regard to fading, it definitely depends on the quality of the disks......some really fast German cars have had really poor disks.......stop 5 times from 60 MPH and the brakes were basically useless.....some very red faces after AutoBild did its testing......

Some other quite "normal" cars, could stop 10 times and the brakes actually got better after each stop!!!

A auto gearbox will allow gearshifting to a lower gear if the speed is too great,

This seems to be contrary to other responses, unless you have a modified transmission. Is this what you were driving? This may get to the earlier request to specifiy the transmission type.

How many fan belts have you thrown from downshifting? It seems like one or two would be sufficient, unless you are an over the road trucker and are driving a heavy rig without Jake brakes.

I actually agree that you can force a down shift on an auto box, but unless the brakes have already failed, one should first brake to a reasonable speed for the next lower gear before shifting so as to not over rev.....

This is also true of manual gearboxes.....

Many here seem to forget this vital point!!!

an automatic transmission generally provides much less engine braking than a manual gearbox.

Untrue, absolutely.

Good answer. The breaking affect of a transmission is going to be related to the change in gear ratio.

If you are driving an 18 speed Eaton, there is going to be a gear ratio that suites most driving conditions. If you have an old Power-glide, it is only high, or low. And the torque converter is going to soften the harshness of the downshift.

Good post generally, but completely wrong on the point about heat from pads getting into the brake fluid. It happened rarely with Drum brakes as the contact between hot brake and fluid was not so close and happened rarely....

This is the reason that Dot 3 was originally developed, specially for DISKS!!! Dot 2 is only OK for drum brakes.....

See the following article for full details of why Dot 2 (drum brakes) was simply not good enough for disks....Dot 3 and Dot 4 have far higher boiling points, important for Disks.....

www.shotimes.com/SHO3brakefluid.html

Due to the fact that brake fluid takes on water from the atmosphere, it needs to be changed every 2 years or so as the extra water reduces the boiling point and makes the possibility of losing braking power in this manner an even more serious problem......

By the way, it is not the pad that cannot take the heat, its the disks themselves.....when hot, resistance is reduced.

Afterwards, once "Burnt", they lose some of the maximum braking power and then need to be replaced, though some makes are far better than others in this area..... Ceramic disks for example do not suffer in this way at all.......but the fluid still does!!

You should also read the infos at the following link:-

http://www.stoptech.com/tech_info/wp_brakefluid_1a.shtml

They mention just how dangerous this fluid boiling can be......live and learn, or on this case "learn to live!"

Have a great day.

GA.

A bit, whats on your mind?

Thanks for your answer. It does a nice job of addressing my question. In summary what I have put together from all of the responses is for unmodified automatic transmissions, generally:

• they are designed to not overspeed through a combination of valve body and electronic controls (this can be verified by downshifting and noting that nothing happens until the vehicle slows),
• they are not a weak design point, ie, there should be no problem downshifting.
• when downshifted on long grades, engine overheating, not transmission problems are the main concern.
• downshifting on long declines is the proper thing to do to prevent brake (and hydraulic fluid) overheating and fading.
• break-rotor warpage can be an additional and expensive problem caused by break overheating.
• checking the owner's manual for remarks is recommended.

I don't know if anyone has anything else to add, but I think that this is a pretty good list.

Thanks to all of you who have responded.

Just one thing:

if one is intending to use an automatic transmission to accomplish engine braking. The vehicle should crest the hill in the correct gear. Waiting until the brakes are hot and or when the brakes can not hold the vehicle speed down is fool hardy.

Do not downshift automatics or manual shift transmissions while engine braking down hill.

You can go down hill too slow a thousand times but once too fast is too many.

GA from me for bringing some common sense into this discussion.

The "Penny Pinchers" who finally lose their brakes completely, before losing the whole vehicle at best, should ponder your words before ever getting in a car again!!

Well put and thanks from me.

As you have now put the case well, I am staying quiet for the rest of this "Scottish" financial discussion of what costs more - brake pads or transmission......they who discuss costs during an emergency, have most forgotten the third alternative that could happen to the the lives of themselves and their loved ones!!!! Typical Engineers......

--------------------------------------------------------------------

Joke (or why I mentioned my folk above!)

How was copper wire invented?

Two Scots fighting over a Penny.........

This is a basically quick description of this discussion and some of the people discussing it!!!

Remember, fair wear and tear is normal when using something like a car, keeping your brakes cool (as well as yourself!) for 100% effectiveness when descending a hill, is part of good driving sense.....and not really a subject for such an inane discussion over costs....

Remember:-

"Casu Consulto"

Great answer. You truly have your crap together on auto trannys.

I hate to say this, but: It depends! engine braking is somewhat effective for light weight vehicles with big engines and slow moving vehicles (trucks) with 'Jake Brakes' which convert the engine into a big air compressor.

Downshifting and using engine braking does cause more wear to the transmission planetary gear sets, and adds to the heat load. MOST automotible and light truck transmissions are not designed for extended operation in the intermediate speeds. Operating in for instance 3rd of 5 or 2nd of 4 down a long mountain grade for 10 minutes puts more operating time and wear on that gear set than probably 10,000 miles of nomal driving. Those tiny planetary gear sets will also generate a lot of heat the transmission fluid , and cooler has to dissipate-(presuming the gear set gets enough oil to adequately cool it)-and the oil may get too hot and cause serious damage to some internal plastic parts.

If I were driving in mountainous territory, I WOUOD intall the largest transmission oil cooler that would fit, ensuring it had thermostatic control to avoid over-cooling. I would also install AMSOIL synthetic transmission fluid AND a oil filter in the cool oil return line. Ford part No. XC3Z-7B155-CA Trans Fluid Filter Kit about $15- is an excellent choice. The filter only (no fittings,hose, or hose clamps) is part # XC3Z-7B155-BA

Brake pads are cheap compared to transmission hard parts. Brakes are designed to absorb energy and dissipate it as heat. Automatic transmissions are not.

I agree to some extent in your response to what would be a "specific load force for a given duration of time" that details would need to be initiated to quantify specifics justifying your answer.

There will always be circumstances or variables that will create problems outside the range of the question asked. My answer above is under the impression of normal vehicle weight with "normal" towing limits respected for the given vehicles intended limits. Larger vehicles designed with Jake brakes are obviously capable of much larger load control under specific conditions as well. Those limits can also be broken as we see many run off areas in our mountain roads here.

In my own studies with plenty of long descending roads with temperature readings, pressure readings, tear down of specific units under specific load usages I've found it necessary to really see for my self for the utilization of other transmission designs I incorporate now. Isolating readings within the transmission helps to identify and quantify what components generate the most frictional heat under what conditions and what specific wear factors can be changed accordingly. Thus for the sake of implementing more efficiency in design. Yes planetary gear sets create heat but not as much as you may think under normal engine breaking conditions and fluid temperatures rise much higher with in line radiator cooling in today's modern transmission designs.

For sake of the question asked here, and for most consumers and mountain roads driven, they have little to concern themselves with when utilizing both engine and wheel breaking in most modern automobiles. As always there will be limits.

As usual there are all sorts of sites discussing this with answers from contradicting to complete agreement. I judge results based on much of my own study and research or from sources I find that their testing methods have sound principles, can compensate for instrumentation variables and where results or testing can maintain consistency.

If I were driving in mountainous territory, I WOUOD intall the largest transmission oil cooler that would fit, ensuring it had thermostatic control to avoid over-cooling. I would also install AMSOIL synthetic transmission fluid AND a oil filter in the cool oil return line. Ford part No. XC3Z-7B155-CA Trans Fluid Filter Kit about $15- is an excellent choice. The filter only (no fittings,hose, or hose clamps) is part # XC3Z-7B155-BA"

I totally agree!

Some one asked for specifics. There is only one transmission that I feel comfortable quoting specifics. The transmission is an Allison 750 DRD with input retarder.

It was installed in 1986. in a 66,000 lb all wheel drive airport fire engine. It was rated for engines up to 540 Hp ( possibly slightly higher, but only less than 5 %. Memory fails me.) The torque rating was for 1850 lb-ft. The input retarder was rated at a whopping 500hp. In it's day that was the top of the hill for retarders.

This is a truck that will top 68 MPH. The retarder was controlled by an additional pedal to the left of the brake pedal. The procedure for full use of the retarder was to pull the shifter into first gear, and stand on the retarder. The truck would stop* from full speed with only the retarder. It would brake hard at higher engine speeds, and less aggressively at lower engine speeds. But the entire stopping process would never bring the engine above it's governed engine speed of 2300rpm. During these extreme braking procedures, Allison would allow the temperature of the transmission fluid on it's way th the cooler to reach 330 degrees. Under all other conditions, the temperature in the oil pan was limited to 225 degrees.

Two of these transmissions lived for 13 years each with no repairs to either of them. We used C4 motor oil in 15w 40 weight, with Pall synthetic media filters.

If the manufacturers of the automatic transmissions thought they were going to have reliability problems with people downshifting them, they would delete that function, or modify it so that it would not do damage.

The additional wear on the planetary components should not a major concern. it will be on the opposite wear side of the gear when retarding or breaking.

*Sorry, I forgot my foot note. An output retarder will not stop the truck. It will only bring the truck to engine idle speed in first gear. In this case, less than 2 MPH.

On the weekend, I tried down shifting to a lower gear in my car, this has electronic shifting, 75km/h was the fastest I could downshift

Heavy Accel, and it would upshift about 80kmh into 2nd

I have yet to get a open enough stretch of road to test the higher gears

I can only add a few personal experiance notes. First dont wait until you deside to start pulling trailers with any vehical before you install an transmission cooler. Every vehical I have ever owned got the physicaly largest transmission cooler I could fit put on it as soon as I bought it.

Every modern transmisson I have read about or have taken apart myself has had an internal thermostat valve to regulate the fluid temurature. It gets to -40 below zero F in our winter an I have never had over cooling issues! These are the Ford transmissons I have personaly owned and ran super sized coolers on in our cololdest winter days with out problems. AXOD, AOD, AODE, E4OD, C6.

I had a 1989 ford taurus with the stock 3.8L V6 and the AXOD transmission. I put a transmisision cooler on it two weeks after I got the car. The transmission cooler was for a 22000 pound RV. I regularily pulled 5000# loads with that car and still the stock factory transmission never gave me trouble. I used downshift dynamic braking whenever I pulled the trailer. I changed transmission fluid every 20000 miles and that transmission lasted to 255000 miles. The car itself just wore out. The transmission was still good the day it was scrapped.

Every transmission repair tech I know says those AXOD transmissions were 100K mile at best life expectancy with the factory cooling systems. With the super sized cooler I had every one of them said they would not doubt they could go 300K - 400K miles in normal driving. I have often been told by them its the poor cooling system the factory puts on transmissions thats responsible for 90% of the transmission failures. Not just Ford but all manufactures. Regualrily schedualed fluid changes and a $100 cooler unit will double or triple your transmission life on any vehical!

Sorry for the bad spelling. The forum spell check would not function when I wrote this.