Hill Assist - HAC

[dcampen]

There has been some talk here about his but I couldn't find a pure thread on this subject so I will start this thread.

I have not payed much attention to HAC. The other day I was on a hill at the top of a driveway, the transmission was in D, I took my foot off the accelerator but didn't apply the brake and I started rolling backwards almost as if the transmission was in Neutral. What the heck, I thought there was supposed to be this HAC system to prevent that? What are other people's observations about the HAC system?

 

[Guest]

I thought it was only supposed to work so that you could let off the brake and then move to the gas without rolling back, not to keep you from rolling back after you let off the accelerator without actually hitting the brakes.

 

[dcampen]

I thought it was only supposed to work so that you could let off the brake and then move to the gas without rolling back, not to keep you from rolling back after you let off the accelerator without actually hitting the brakes.

Yeah, that might be it. I wonder though why it only works that one way. Afterall, when I have the transmission in "D", I am certainly not intending to go backwards. Why not have it hold my position when I let off the accelerator as well as when I let off the brake?

 

[boston_kevin]

Hill Start Assist

According to toyota documentation, it works while you are removing your foot from the brake to go to the accelerator. I believe there is a sensor (in combination with the gyro) that determines wheel rotation following a complete stop with the brake. If there is backwards wheel rotation after the brake is applied (and the tranny is in a forward gear), the brakes should momentarily be applied (not more than 3-5 seconds) to prevent the vehicle from rolling backwards.

I am assuming that they did this purposefully so you can rock yourself out of a rut if you get stuck. Sometimes, you have to rock the vehicle to get momentum to get out of a slick rut and you would not use the brake in this situation (brake would kill all momentum). Whereas stopping on a hill, you don't want to roll back, so the brake engages.

 

[dcampen]

I am assuming that they did this purposefully so you can rock yourself out of a rut if you get stuck. Sometimes, you have to rock the vehicle to get momentum to get out of a slick rut and you would not use the brake in this situation (brake would kill all momentum).

Ah Ha, I hadn't thought of that!

 

[lildevil23]

DAC problems.

I tried the DAC a few days ago...

I was on top of a hill..a good incline..asphalt/pavement surface...can reach 40MPH if going in neutral..so here is what I did..Put on 4LO..used maximum engine braking..but no physical braking or acceleration...but i still went down pretty fast..a lot of sensors came on..the DAC..vehicle stability..i didnt lock the central differential..is my DAC working??

 

[Good Times]

If you use DAC properly it will work.

First off, make sure you're in 4LO (which means you can't be on the friggin asphalt. MUST BE DIRT).

To get into 4lo... stop, neutral, shift to 4lo.

Second, make sure you're in 1st GEAR (the lowest of the lowest... i.e. "1" and not "D").

Once you're in 4LO and also in 1st gear, press the DAC button. Release the brakes and you should be able to go down the hill (5mph max) by the computer. Remember DAC only works on 4lo and 1st gear so if you're going 40mph something is WRONG.

Also if you're trying to go 40mph in 4lo... you've got problems in your head.... Keep it under 15-20 mph in 4lo... no need to tear anything up inside causing you to break something.

Also lock the center diff... if you need to be in 4lo and 1st gear, you'll want all the traction you can get. Therefore press every button possible (including the horns! hehe).

good luck!

 

[dcampen]

First off, make sure you're in 4LO (which means you can't be on the friggin asphalt. MUST BE DIRT).

I don't see any reason why I can't use 4LO on asphalt.

Also if you're trying to go 40mph in 4lo... you've got problems in your head.... Keep it under 15-20 mph in 4lo... no need to tear anything up inside causing you to break something.

He didn't say anything about trying to go 40 mph in 4LO. He said that the hill was steep enough that you would reach 40 mph if you went down it in neutral.

I think other people have commented that a DAC controlled descent was a little faster than they would have liked.

 

[Good Times]

Use 4lo on asphalt and you'll break alot of parts (btw, this also applies to locking the center. try locking the center and making a turn on a hard surface... you'll feel the binding real quick and figure out that you gotta unlock the center)... If you read any 4x4 manuals, it always state that 4lo should only be used on soft surfaces (which pretty much rules out asphalt). Ofcourse you are welcomed to try and see if it works (something I dare not try).

As for going 40mph... if you are in 4lo (and 1st gear) you'll only be able to go about 10-15mph (raw physical limitation on gearing). If you're in "D" then maybe 40-45? But at that point, there's no need to be in 4lo (way too much torque and you have a higher probability to break something). It's best to use 4lo in situations that require it. Outside of that, stay in 4hi and enjoy the ride.

And going 40mph down in neutral... well in neutral, the sky's the limit on how fast you can go.

If you use DAC properly, you'll be limited to about 3-5mph as the system is made to go that slow. Once DAC is enabled, you'll know as it'll be making a crap load of sounds. (beeps, screech, grinding, you name it, it'll sound like something is breaking...) definitely not a pleasant sound to hear as you use it. Personally speaking, I think the DAC doesn't work for all cases... Ofcourse it works on some cases but there are a few cases where you want to be going slower than 3mph... (i.e. 1mph).

Hope this clears some stuff up

 

[dcampen]

Use 4lo on asphalt and you'll break alot of parts

I don't believe this. There is no reason this should be. Does it say this in the Toyota manual?

(btw, this also applies to locking the center. try locking the center and making a turn on a hard surface... you'll feel the binding real quick and figure out that you gotta unlock the center)...

This is a completely different issue from being in 4LO. In 4LO the center diff is still unlocked unless you have explicitly locked it.

If you read any 4x4 manuals, it always state that 4lo should only be used on soft surfaces (which pretty much rules out asphalt).

Do you have any citations? I still don't believe this.

Ofcourse you are welcomed to try and see if it works (something I dare not try).

Sure, I will give it a try. Perhaps tonight. I drove my Camry to work today.

And going 40mph down in neutral... well in neutral, the sky's the limit on how fast you can go.

Well, no, your speed will be determined by the slope of teh hill given that there are still frictional losses even in neutral. And this was his point, to indicate the steepness of the hill.

 

[Good Times]

Here's a link on the basics of 4wd.

http://4wheeldrive.about.com/cs/offroadingtips/a/aa021203a_3.htm

"-Don't operate 4WD on hard dry surfaces."

"When to use 4L:
on wet, slippery surfaces
passing through sandy areas
on rough trails
through shallow water
rock-climbing
climbing steep hills
through mud
descending steep hills "

I brought up the part on the locking the center just incase people forget. I've forgotten it a few times and realized it once I was on dry land to feel the binding.

I'd experienced 35-40 degree drops (literally looking down) and have been able to go down on a controlled speed (1-2mph) using my brakes (and not using DAC as I wanted to go slower than what DAC offers at 3-4mph). The terrain was dirt but still solid enough that there was minimal slipping at the tires.

I've also experienced steep hills at Pismo (sand dunes) and believe me, there you can't use DAC and definitely don't want to be going 1-2mph either... I'd say more like 5-10 and then makin sure you have enough gas to bail out if you start to go horizontal.

If you're going 40mph downhill, that's pretty fast regardless of how steep it is. My guess is that he wasn't use the technology correctly. DAC only works on 4lo and 1st gear which even w/o using DAC, it'll be 15mph due to gearing limitation. Using DAC in conjunction w/ 4lo and 1st gear will limit you to 3-5mph as that is how is works.

Hope this sheds some light

I'll see if I can find my toy manual on 4lo and hard surfaces. I know my FJ40 has a diagram on the dash that says 4lo and 4hi must be on soft surfaces. But ofcourse our 4runners are able to drive on hard surfaces in 4hi...

 

[dcampen]

One caveat about my comments re 4LO being OK on asphalt. This applies to 4Gen 4Runners. Some other vehicles may also lock the center differential whenever the transfer case is put into low gear. I believe that some versions of the Nissan Xterra may lock the center diff whenever the vehicle is put in 4LO; in these cases you would not want to use 4LO on asphalt because teh center diff has been locked.

 

[dcampen]

Here's a link on the basics of 4wd.

http://4wheeldrive.about.com/cs/off...aa021203a_3.htm

"-Don't operate 4WD on hard dry surfaces."

And as you point out yourself, this info is very generic - saying not to operate 4WD on hard dry surfaces when we know that the 4Runner 4WD can be operated on hard dry surfaces. I have read a couple of books and more web sites on offroad driving and none of them were familiar with the 4Gen 4Runner 4WD system.

 

[Good Times]

I didn't know other vehicles lock the center diff when in 4lo. Although it's logical since whenever I use 4lo I've already locked the center diff.

As for being in 4lo and on asphalt, I seriously recommend you not attemping this feat. All this will do is damage your 4runner (which I don't want to see it happen at all). Plus if you think about it, you only need 4lo when you lose traction and need the extra umph to get you thru an obstacle. I know there aren't any obstacles on the road (except for the damn traffic on the fleeways!)

Keep in mind that our 4wd system is no different from older systems. Our system (v8 folks here) is a simple full time 4wd system (just like any ol full time 4wd system... we just have the extra wanna-be locker in the front and rear...). THis means that we drive in 4HI all the time. When needed we are able to drop down toe 4lo. but the rules are still the same in 4lo. Only use in soft surface in which you need to get over an obstacle.

But we are straying off topic somewhat though (but still cool as it's all good 411 for everyone).

 

[Iowa4Runner]

As for being in 4lo and on asphalt, I seriously recommend you not attemping this feat. All this will do is damage your 4runner (which I don't want to see it happen at all). Plus if you think about it, you only need 4lo when you lose traction and need the extra umph to get you thru an obstacle. I know there aren't any obstacles on the road (except for the damn traffic on the fleeways!)

To get technical, switching to 4LO when you're losing traction doesn't make much sense. Switching to 4LO gives you more torque, but if you're on a slippery surface you'll be unable apply that torque to the ground.

I've used 4LO a few times on dry pavement and the drivetrain made no indications of strain or damage. There are situations where 4LO could be useful on dry pavement, like pulling something heavy up a steep hill from a standstill. I see no technical reason for 4LO to cause damage if you're driving sensibly. If the Toyota owner's manual doesn't advise against using 4LO on dry pavement, I see no reason to avoid it.

 

[Good Times]

Ok lets clear some stuff up. I did my good ol homework and here's what I have learned

Btw I must thank dcampen for bringing this up as it's great discussing details like this as alot of the time it's overlooked.

I still feel that 4lo and asphalt shouldn't be one together. But there are scenarios (which I apparently didn't even think about) that you would use 4lo on solid surface. Scenarios such as towing a boat out of the lake (keep in mind that even though it's asphalt, it's wet therefore traction is greatly diminished) or even to start pulling something from a standstill.

Therefore, maybe I should rephrase my quote by sayin that 4lo and dry hard surface should not go together on prolonged drives. I'd say 30-50 yards shouldn't do any harm but driving in 4lo for an extended period and length could lead to problems on your tires (obviously) and drivetrain. (Iowa4Runner says it perfect "if you're driving sensibly").

But it all comes down to this... If you don't know what you're doing and have no knowledge... then you're better off no trying. I think the vast majority make mistakes only because they are not knowledgeable enough to make the right decisions. I for one am still learning (damn noob!) so I'm glad I looked this up. I don't own a boat nor jet ski so this wouldn't be an issue in the first place (too pooooor) but none the less something I have gained

 

[Kmvreter]

I agree but...

I agree for the most part with Good Times. I had a Jimmy before the T4R and it would bind up the drive train even in 4Hi on dry hard surfaces, especially evident when turning. Wouldn't even think to do it in 4Lo. But isn't the T4R different with a Torsen center diff. that allows slippage (as long as you haven't locked the diff?) That's how they accomplish the V8 to run as fulltime AWD or the V6 as 4WD. Or is this feature disabled in 4Lo?
Thanks.

 

[dcampen]

I had a Jimmy before the T4R and it would bind up the drive train even in 4Hi on dry hard surfaces, especially evident when turning. Wouldn't even think to do it in 4Lo.

But the 4Runner 4WD drive system is completely different from that in your Jimmy. This is why the 4Runner can be run in 4WD mode under all conditions.

But isn't the T4R different with a Torsen center diff. that allows slippage (as long as you haven't locked the diff?) That's how they accomplish the V8 to run as fulltime AWD or the V6 as 4WD.

Yes, the 4Runner 4WD system is different from other 4WD systems. The Torsen center diff is one difference as is the ATRAC system that takes the place of locking front and rear differentials. The capabilities and limitations of your Jimmy's 4WD system have no bearing on the capabilities and limitations of the 4Runner 4WD system.

Or is this feature disabled in 4Lo?

No, in the 4Runner, 4Lo is the same as 4Hi except that a different gear is being used in the transfer case. If you can drive on asphalt with the 4Runner in 4Hi then you should also be able to drive on asphalt with the 4Runner in 4Lo.

In fact, last night, I put my 4Runner in 4Lo-1st gear as I took it from my garage to the street. This involved 2 sharp 90-degree turns on asphalt and I experienced no binding, crow-hopping or other problems.

 

[dancojgl]

No, in the 4Runner, 4Lo is the same as 4Hi except that a different gear is being used in the transfer case.

I agree 99.9%, but...

The 4th Gen's have a center differential, not a transfer case. A transfer case operates like the locked center differential.

Bottom line: Locked center differential or vehicle w/ transfer case (e.g., 3rd Gen 4Runners and earlier), 4 wheel drive (hi or lo) on dry, hard pavement—BAD. Unlocked center differential (4th Gen) in 4WD on dry, hard pavement—OK.

As dcampen stated, the difference between 4WD HI and 4WD LO (4th Gen) is gear ratio.

—Dan

 

[Good Times]

No, in the 4Runner, 4Lo is the same as 4Hi except that a different gear is being used in the transfer case. If you can drive on asphalt with the 4Runner in 4Hi then you should also be able to drive on asphalt with the 4Runner in 4Lo.

Looking at it at that perspective you are indeed correct. But honestly, would you drive in 4lo for a prolong period on the asphalt? Probably not.

Thanks for correcting me though as looking at it that way, it definitely makes sense.

Btw, dancojgl... our 4wd system has a transfer case (w/o we wouldn't be able to switch to 4lo).