techwhiz4

Hi, does anyone know whether there is "truly" any difference between the Hill Start Assist Control (HAC) on 2006's IS250 AT and 2009's?

I know that the HAC is supposed to prevent the car from rolling backward for a moment when the driver takes their foot off the brake before applying the throttle on an incline.

One day I tested my 2009 IS250 RWD AT on a not-so-steep incline and realized that my car still rolls backward despite I can feel some resistance being applied but definitely not strong enough to hold the car.

I raised this issue to my dealer. I managed to test a used 2006 IS250 on an incline (steeper) at the dealership, and noticed that the car actually gets held still for a few moments and then starts rolling backward. On the same incline with my 2009 IS250, it just rolls backward with some "useless" resistance not holding the car at all.

The dealer then worked with Lexus Canada in investigating the HAC behavioral discrepancy on my car. They collected some computer data while my car was on the incline and the HAC was not performed as expected. Two weeks later, they replaced the Engine Control Unit (ECU) on my car since Lexus Canada wanted to ensure that it is not at fault.

However, the ECU replacement didn't resolve the problem; my car still rolls backward as before. The dealer re-collected some data and worked with Lexus Canada again. One week later, I got a statement from my dealer, claimed to be the information they got from Lexus Canada, saying that the HAC on 2009 and later IS250 AT has been designed to provide "weaker" resistance to prevent the car from rolling backward on an incline, and that the HAC behavior on my car is the same as other 2009 IS250 they had tested to confirm.

I'm not quite convinced of such a unbelievable answer from the dealer/Lexus Canada. As a result, please can any IS250 experts and/or owners shed some light on IS250 2009's HAC? Thanks a lot!

panda327

iTrader: (3)

i raised this issue here before too ...

my 2009 rolls back as well.. but on a fairly steep hill

Jetfire

iTrader: (3)

i'm gonna test this out tomorrow. my feet are usually fast enough that i don't notice.

panda327

iTrader: (3)

go try it on the hill on boundry street beside telus .. thats where i sometimes roll back sometimes i don't

ajfinoak

I live in the SF Bay Area- LOTS of hills. My '08 250 Auto rolls backward less than a foot and then yellow light flashes (traction control?) and car stops. Rather useless if someone is behind you. My '06 BMW 325 stopped immediately which made more sense.

GiantsFan

iTrader: (9)

^Mine does the same.. rolls back just a little bit before the resistance kicks in (on steep hills it's not strong enough to stop the car). On the other hand, my mom's 330ci doesn't even crawl back 1mm ...

The hill start assist is pointless.. .. I just have my left foot on the brake when I first press the gas....

Kvegas

I know this is an old thread but I'm sure people are still having this issue and spending a lot of money on this. My 2001 gs300 was doing the rolling back on steep hills and reversing uphill was the same but in reverse. It would also hesitate when accelerating from a stand still. After reading this forum, I almost changed my app sensor like everybody else but I decided to check some things. I noticed the throttle cable was kinda loose at the throttle body so I went to where the pedal is attached to the cable. There was a lot of play so I stretched the cable and attached a zip tie between the pedal and the cable to tighten the cable a little. It worked! Car leaves the line with no hesitation and stays in place when on a steep hill and when you take your foot off the brakes. No problem with reversing too. The connection between the throttle cable and the pedal has a spring and over time it must have worn out and was way too loose.

Bichon

Thanks for trying to help, Kvegas, but the 2nd generation IS has a computer controlled throttle-by-wire system. There is no throttle cable on our cars.