SaniDel

We've noticed that the GX 460 tends to "run away" on a long downhill run, which requires either lots of braking or a manual downshift into sport 'S' mode to keep it under control, especially if there is a nasty surprise at the bottom of the hill like a traffic light that just changed.

Well, the GX 460 is smarter than that, but it needs a little encouragement. On a long downhill run with the transmission in full automatic 'D' we tap the brake pedal once or twice ... just enough to flash the brake lights, but not enough to engage the brakes. That seems to signal the transmission that we don't want to go that fast and it automatically downshifts, which is usually enough to keep the vehicle from accelerating, but not enough to slow it. The effect appears to be the same as sliding the lever left into 'S' as the tachometer climbs to about 3,000 RPM, but there is no gear indicator in 'D' mode to know for sure.

So, what's actually going on when this happens? The GX 460 must know that the vehicle is accelerating, the driver's foot is off the gas pedal and it may know that the vehicle is on a downhill run ... does the GX 460 include an 'inclinometer'? Why does it need a 'nudge' from the driver by tapping the brake pedal to downshift? Or, is it designed that way ... unless the driver taps the brakes, the GX will keep accelerating as it rolls downhill?
Regards,

SaniDel

Koz

5300 lbs!

The trans has intelligence. If you rest your foot on the brakes it should also down shift. You will also see that the trans will not up shift if you let off the gas going downhill (in lower gears). Also, if the trans has a sport mode it will down shift way sooner if you are braking from high speeds, to help slow the vehicle.

Koz

cannysage

why not just use the downhill mode thingy? I'm going to guess that our tranny's got shifting points on the rpm. so let's say you go 60mph @ 3.5k and it drops to 2k within x amount of time, during your momentary braking, then it'll downshift.. same with upshift.

SaniDel

I'll try simply resting my foot on the brake pedal to see if that works the same as tapping the pedal. We haven't noticed that the transmission will not upshift if we lift off the gas, but that would be consistent. We'll try it tomorrow on another "downhill run" ... they are few and far between in Delaware!

You are certainly correct that the sport mode would do all of this and more, but that means an extra step or three ... move the shift lever to the left, then push forward or backward to shift up or down, but don't forget to move it back to the right! Seems a bit ... well, archaic? More to the point, the GX 460 runs a lot smoother if it isn't forced to shift ... if we wanted to manually shift gears we would be in a different vehicle.
Regards,

SaniDel

SaniDel

I'm not sure what that is. The Downhill Assist Control (DAC) system described on page 328 of the Owners Manual is only for steep downhill slopes and under 15 MPH and when the four-wheel drive control switch is in the L4 position. The situation I'm describing is moderate, but long downhill runs at highway speeds ... how to keep the vehicle from overspeed without riding the brakes or forcing a manual downshift using the sport 'S' mode.

Is there another system that is more suitable for controlling downhill speeds in typical driving situations?
Regards,

SaniDel

cannysage

^ that's good to know. I didn't know that. I really gotta dig in to the manual more

akfd

For some odd reason my GX knows when to downshift when I want it to. I also think the transmission has some sort of intelligence; if you repeat doing the same thing downhill a few times, the computer will pick up your habit and downshift automatically.

Craig B

The transmisson worked much better on my BMW X5 as far as downshifting. It was much better than Lexus in that regard. The reliability and durability sucked on my BMW.

SaniDel

A heuristic transmission! I like that, but let's hope it doesn't gain sentience and become "self-aware". ;-)
Regards,

SaniDel

gxgrappler

I've noticed exactly the same thing. I moved to the GX from my '06 Land Cruiser, and the six speed transmission is new to me. I'm thinking that the GX 460 was engineered in a way to get the top mileage number as high as possible.....no roofrail bars, no hitch, and a very high top gear in the transmission.

As far as the downshifting, I've determined that applying the foot brake to the point where the brake light might come on, but before any actual braking occurs must be tied into the computer somehow. You can lift off the gas and there is no downshift at all going downhill at 60+,but if you slightly apply the brake, the downshift occurs every time.

SaniDel

I tried the following experiment:

1. Come to a full stop at the top of a hill with the engine idling.

2. Hold position with the foot brake.

3. Keep the transmission in full automatic 'D', which presumably shifts the transmission into first gear although there is no way to tell as there is no gear indicator.

4. Release the foot brake and keep your foot off both brake and gas.

The GX 460 continues to accelerate as it rolls downhill AND the transmission upshifts repeatedly as the vehicle gains speed. It is hard to tell as the shifts are so smooth, but the tachometer needle 'bounces' slightly as the engine revs above idle, shifts and revs again. I counted three shifts before I ran out of hill and had to apply the brakes.

So, it seems the GX 460 tries very hard to get into the highest gear for the conditions, even upshifting as vehicle speed increases without touching the gas pedal. It downshifts reluctantly when conditions require or when the driver forces a downshift by tapping the brakes ... or stomping the gas, but with the opposite effect ... the GX 460 takes off like a scalded cat!

It seems the GX is always trying to improve fuel efficiency and a good way to do that is to stay in the highest gear, which explains its surprisingly high mileage on sustained highway driving. The downside is that it tends to "get away from you" on downhill runs!
Regards,

SaniDel

SaniDel

I'm convinced ... the GX 460 not only downshifts on demand when conditions are right, but also will do it repeatedly ... if conditions continue to demand it.

On a steep downhill run to the Brandywine River the GX 460 responded to brake pedal 'blips' with downshifts ... twice! The first occurred at about 45 MPH on a moderate downhill and presumably shifted into 4th gear. The vehicle slowed slightly, but re-accelerated when the grade got steeper. A second stab at the brake pedal forced a second downshift, presumably into 3rd, but without a gear indicator it is hard to know for sure.

So, for routine driving we can deal with downhill runs in automatic 'D' by a quick tap on the brake pedal and if the hill is steep enough, repeat as necessary. That certainly is an improvement over having to manually pull the shift lever left for 4th gear, then pull it back for 3rd gear and finally remember to push it back to the right for automatic 'D'. Also, as the brake pedal is used rather than the shift lever, both hands remain on the steering wheel and if something drastic happens the brake pedal is already 'covered' ... just push it harder!

What does NOT seem to force a downshift is "riding the brakes" all the way down a hill. The vehicle slows due to braking, but there is no apparent downshifting until the brakes reduce the speed. Either technique works and you tradeoff brake wear against transmission wear, but my preference is to get the vehicle to downshift sooner rather than later.
Regards,

SaniDel

GB2011GX

I know I'm bumping an old thread here, but...
I dislike when a vehicle tranny holds a vehicle back on a downhill grade.
My '06 Ford F-150 Lariat Crew Cab (251,000 mi and counting!) accelerates as if in neutral when I take my foot off the accelerator on downhill grades. I like to call it "bicycle mode".

I have found, my 2011 GX460 tranny will retard bicycle mode but if I tap the brake ever so slightly when it is doing it, it frees it up GO! Hope this helps those of you who love a free coaster! LOL
I think this bicycle mode retardation is what they call Downhill Assist.

Just curious, but isn't there a switch to turn that off? If there is, and I turn it off, will it continuously retard the momentum when I take my foot off the accelerator? I'll have to try that next...
We all like excellent gas mileage, so I'm perplexed why auto manufacturers would actually retard free wheeling bicycle mode momentum.

Disclaimer: Yes, I'm aware the same behavior is consistent with many Cruise Control systems. However, I like coasting in "neutral" on a downhill (without moving the shifter), and increasing speed without having to use expensive gas to do so.

Acrad

I think what you may be feeling is the lockup torque converter going out of lockup mode.

tecman

I typically don't mind this and occasionally don't want it so I bump the gas pedal to override it.