Downhill in lower gear with automatict ransmission

Joined
Dec 2, 2002
Messages
6,388
Location
Washington St.
Originally Posted By: jaj
If you're doing a long, steep descent (we have a bunch within about four hours of where I live) you should disengage overdrive or drop the transmission by one gear - RPM should maybe rise to 2500RPM or so. You want additional braking; it's there to help, not to do the entire job. Then use the brakes to keep the speed under control: pick a max speed slightly faster than traffic and a slow speed slightly slower. When the car coasts to the max speed, slow it down to the minimum speed with a firm brake application, then release the brakes and allow it to coast back up the to max again. This will keep your brakes from overheating. What you're avoiding is brake fade or failure. If you just hold your foot on the brake to control the speed, heat just keeps building up and up and up until the pads fade or the fluid boils. This is because the pads stay pressed on the disks, and heat transfers from the disk through the pad to the caliper. The longer you hold the pedal, the hotter the whole assembly gets. However, if you brake and release, the rotor temps spike up more but they're cast iron, so they don't care. What matters is that between applications, the calipers aren't pressing the pads against the rotors. Heat transfer stops and they get a chance to cool down between applications; they're less prone to overheat.
Excellent points. One more thing---if the brake fluid isn't fresh and very low in water content, the heat from hard braking can cause any moisture in the fluid to flash to steam. Your pedal goes to the floor and you have no braking. Keep your brakes cool by downshifting on long steep downgrades.
 

MolaKule

Staff member
Joined
Jun 5, 2002
Messages
22,363
Location
Iowegia - USA
Originally Posted By: supton
Why would the clutches run hotter? ...
I am not sure what you are questioning here, but a gear translation is "engineering speak" for a gear shift either up or down. Anytime a clutch pack is activated or deactivated, the temperature of the clutch pack (or band and drum) rises. This heat is carried away by the ATF. A temperature spike in the fluid would be the result.
 
Joined
Nov 9, 2008
Messages
18,718
Location
NH
That temp spike, is it from slippage at the shift, or from heat from something else? Is it a spike or a continous elevation in temperature?
 
Joined
Aug 21, 2012
Messages
144
Location
Camden, MI, USA
spike.... and the temperature of the entire sump of fluid is likely raised by a degree or two. once the shift is done, there is no more slip at the clutches(or at least, you don't want any more slipping going on), so no more heat generation from that event. when it comes to climbing mountains.... i would much rather go up in 3rd with the TCC locked than 4th with it unlocked. the torque converter slipping when it could be coupled will generate a LOT of heat. going back down.... well, everyone else has already discussed this.
 
Joined
Dec 24, 2013
Messages
3,748
Location
Massachusetts
There should be very little heat generated if the shift is performed properly. The old mechanical shifting transmissions would apply 2 bands at once to get a smooth shift for Grandpa and Grandma. It puts the tranny in a bind and creates lots of heat. That's why on the older stuff we used to see piles of clutch material in the pan. That's why hotrodders invented shift kits. It deapplys a band then applies the next gear's band allowing the engine to freewheel for a fraction of a second. It gives those neck snapping shifts gearheads love. My vette with a t400 and shift kit used to grab second so hard it'd break both rear tires loose and the rear would kick out a few inches to the right. A modern transmission with an efficient shift program should not generate much heat at all. In engine braking mode the torque converter should lock. Going up the hill the trans has to shift and you need enough torque to get the car up the hill. Going down you are using a fraction of that torque to slow the car and making up the rest with the brakes. So you're really generating a lot less heat going down than you did going up. I just don't get what all the teeth gnashing is about. Bottom line if you are that particular about your stuff, don't drive in the mountains at all. It puts more stress on stuff. yes.
 
Last edited:

MolaKule

Staff member
Joined
Jun 5, 2002
Messages
22,363
Location
Iowegia - USA
Originally Posted By: supton
That temp spike, is it from slippage at the shift, or from heat from something else? Is it a spike or a continous elevation in temperature?
It is from the short period slippage at gear change (translation). It takes a finite amount of time for one clutch pack to disengage and a finite amount of time to reengage another clutch pack.
Quote:
In addition to the oil flow description given in the previous paragraph, gear translation is further impacted by a rise in the interface temperature of the on-coming band and drum and/or the interface temperature rise of the on-coming clutch and plate, which increases the likelihood of a torque reversal during the gear shift. If interface temperatures rise, so does the ATF temperature.
During a coasting downshift, the temperature rises about 5-10F for about ten seconds and then drops within about 10-12 seconds, depending on when the coasting downshift occurs, and severity of grade. Your readings may vary depending on speed, vehicle weight, transmission design, and programming.
 
Last edited:
Joined
Dec 12, 2002
Messages
43,672
Location
'Stralia
Originally Posted By: MolaKule
Originally Posted By: supton
That temp spike, is it from slippage at the shift, or from heat from something else? Is it a spike or a continous elevation in temperature?
It is from the short period slippage at gear change (translation). It takes a finite amount of time for one clutch pack to disengage and a finite amount of time to reengage another clutch pack.
That, I very much doubt, or there'd be similar, and far worse spikes on every upwards change, where there is significant power flow taking place...time after time after time, versus a single movement into a lower gear, at low power flows. if a tranny holds 7 quarts, 1.8F rise in fluid temperature is equivalent to 20 hp total slippage for a second...Reduce the shift time to something real, like a fraction of a second, and the momentary clutch engagement/disengagement time can't be measurable in the thermal inertia of the system. Acceleration up through the gears, you are probably going to have at least twice the powerflow as a downchange for a hill...on the gearchange...
 
Joined
May 27, 2002
Messages
11,005
Location
Canberra ACT Australia
Originally Posted By: FutureDoc
If you ride your brakes down a mountain, they might not be there is a few miles. It is not about wear... it is about safety. As someone who lives in the mountains and commutes over a range regularly: downshift (just not into 1 or 2nd at highway speed). Folks need to remember to keep their tranny fluid in good condition and change it every so often. If you do that, you should be golden. First, if you are not normally driving through the mountains, the once in a blue moon downshifting is not going to kill a tranny unless you neglected your fluid change. If you drive up and down constantly, keep your fluid fresh. Downshift for your brakes are cool and ready in the case you need to use them. People (especially those from flat areas) get stupid in the mountains and think the speed limit dropping from 70 to 50 is a joke. It is easy not to have high RPM and be in a lower gear without a substantial load. Plus, not only is there fading issues, if the hill is bad enough, you can seize your calipers too...
The only intelligent post on this thread so far.
 

tqh

Thread starter
Joined
Oct 17, 2013
Messages
9
Location
Maryland
Originally Posted By: MolaKule
Originally Posted By: supton
That temp spike, is it from slippage at the shift, or from heat from something else? Is it a spike or a continous elevation in temperature?
It is from the short period slippage at gear change (translation). It takes a finite amount of time for one clutch pack to disengage and a finite amount of time to reengage another clutch pack.
Quote:
In addition to the oil flow description given in the previous paragraph, gear translation is further impacted by a rise in the interface temperature of the on-coming band and drum and/or the interface temperature rise of the on-coming clutch and plate, which increases the likelihood of a torque reversal during the gear shift. If interface temperatures rise, so does the ATF temperature.
During a coasting downshift, the temperature rises about 5-10F for about ten seconds and then drops within about 10-12 seconds, depending on when the coasting downshift occurs, and severity of grade. Your readings may vary depending on speed, vehicle weight, transmission design, and programming.
So when I charge up mountains, the temp goes 140->200F, is it because the transmission tries to shift gears so many times, each time adding a couple of degrees? If I just limit the transmission to go to 3 only, no 4 & 5, it'll just stay at 3, won't do gear translation => very little temp change => good strategy?
 
Last edited:

MolaKule

Staff member
Joined
Jun 5, 2002
Messages
22,363
Location
Iowegia - USA
Quote:
So when I charge up mountains, the temp goes 140->200F, is it because the transmission tries to shift gears so many times, each time adding a couple of degrees? If I just limit the transmission to go to 3 only, no 4 & 5, it'll just stay at 3, won't do gear translation => very little temp change => good strategy?
That and the fact that more fuel is being consumed, which raises engine temp, which raises coolant temp, and finally the transmission fluid temp because your trans cooler is in the radiator. If you can shift down to three before the hill climb and this does not lug the engine then I think you will do well.
 
Joined
Dec 29, 2012
Messages
673
Location
Ohio
^^ This is a good strategy most of all because you will have power available quickly, not have to wait for transmission shifting lag. For my 4-speed Automatic, leaving it in overdrive meant the transmission would shift to 4th when I was not accelerating, and then required shifting to 3rd when I pressed the accelerator. Removing overdrive means the transmission stays in 3rd through, and is able to have better throttle response.
 

MolaKule

Staff member
Joined
Jun 5, 2002
Messages
22,363
Location
Iowegia - USA
Originally Posted By: Hyde244
^^ This is a good strategy most of all because you will have power available quickly, not have to wait for transmission shifting lag. Removing overdrive means the transmission stays in 3rd through, and is able to have better throttle response.
That's the way I anticipate hills. On the flat and just before I approach the hill, I force an "Overdrive Lockout." Just before the bottom of the hill, I turn off the Overdrive Lockout and let things start to cool.
 
Joined
Jan 31, 2010
Messages
183
Location
Vancouver, Canada
Interesting topic indeed. I have a question. My previous car (Buick Century) had 4-speed transmission with D,3,2,1 available. Going down the mountain, I used "2", speed about 80-90 km/h (55-60 mph), engine revving but that's fine. Now, I bought Ford minivan (also 4-speed auto tranny), and it doesn't have "2"! Just D,3,1. So it looks like "3" doesn't provide much engine brake, and I don't think "1" is appropriate for highway speeds. Why did Ford engineers do this - skipped the most usable selector mode? Kinda defeats the purpose.
 

CT8

Joined
Oct 9, 2014
Messages
15,387
Location
Idaho
Originally Posted By: tqh
Originally Posted By: MolaKule
Originally Posted By: supton
That temp spike, is it from slippage at the shift, or from heat from something else? Is it a spike or a continous elevation in temperature?
It is from the short period slippage at gear change (translation). It takes a finite amount of time for one clutch pack to disengage and a finite amount of time to reengage another clutch pack.
Quote:
In addition to the oil flow description given in the previous paragraph, gear translation is further impacted by a rise in the interface temperature of the on-coming band and drum and/or the interface temperature rise of the on-coming clutch and plate, which increases the likelihood of a torque reversal during the gear shift. If interface temperatures rise, so does the ATF temperature.
During a coasting downshift, the temperature rises about 5-10F for about ten seconds and then drops within about 10-12 seconds, depending on when the coasting downshift occurs, and severity of grade. Your readings may vary depending on speed, vehicle weight, transmission design, and programming.
So when I charge up mountains, the temp goes 140->200F, is it because the transmission tries to shift gears so many times, each time adding a couple of degrees? If I just limit the transmission to go to 3 only, no 4 & 5, it'll just stay at 3, won't do gear translation => very little temp change => good strategy?
Maybe the torque converter clutch is disengaged allowing the stator to work causing the increased heat? That is where most of the heat is generated with an auto tranny. The oil goes from torque convertor to the trans oil cooler unless there is a thermostat device to speed up the trans oil heating to operating temps.
 
Joined
Jul 3, 2005
Messages
36,339
Location
NY
Originally Posted By: HTSS_TR
I rather replace brake pads (and rotors if needed) every 20-30k miles than rebuild transmission every 80-100k miles.
Me too.
 
Joined
Nov 9, 2008
Messages
18,718
Location
NH
I would too, but if the trans is going to shift anyway... I like to lock out upper gears around town, prevent stupid shifting. Top of the hill, downshift before going down. Or bottom of the hill, if I know it won't stay in gear as I go up. My wife has not figured out shifting on an auto. I'm about to forbid her from driving my truck. Not that I blame her, I find driving an automatic to be more work than stick.
 
Joined
Dec 11, 2013
Messages
4,597
Location
Manchester, England
If your brakes overheat you need better components. If your transmission overheats you need a thermostatic cooler. Or, if you want better control of what gear you're in, you need a manual.
 
Joined
Oct 8, 2014
Messages
1,648
Location
SANTOS, BR
Same rpms considered, engine braking is about 1/6 of the force as compared to throttling up uphill. The proof is that: IF YOU DON'T KEEP BRAKING THE SPEED WILL KEEP BUILDING UP AND UP AND YOUR ENGINE WILL OVERREVV TO A POINT, in a steep enough descent path. What cause fluid heat is transmision of Force and a normal engine braking (below redline), the force is way down, compared to WOT up hill. Coasting make almost no counter wirl effect since parts are going mostly at same speed or in a state of aproximate simultaneity. The engine gives up and raise its rpms. That wouldn't heat the trans fluid and in most instances, if you're going downhill just after climbing it up, it would make the fluid decrease temperature quickier just for the more rapid moviment into the tranny radiator/exchanger. <span style="font-weight: bold">The main reason for the fluid to increase temperature is the coupling effect made by the impeller/turbine counterwirl with endoviscosity increase by pressure, building more heat at every passage between the impeller, vanes and turbine, uncontable times for minute.</span> This discussion as going downhill on the brakes to avoid tranny fluid damage has no room to logic.
 
Last edited:
Joined
Dec 12, 2002
Messages
43,672
Location
'Stralia
When I was teaching my wife to drive, I was trying to get her into the habit of using "2" on the 3 speed auto down the biggest hill into town. Coming down on the brakes, then doing an "emergency" stop as soon as the town limits sign was hit (e.g. kid chasing a ball) was 3-4 car lengths longer on the brakes only descent. Yes it was '70s designed disk/drum, but we still use gears manual or auto....twisty hilly stuff the 4L60E in "2" means never having to wait for it, we don't row it, just leave it where it needs to be.
 
Top