Dear Car Talk:
I have a 2023 Subaru Ascent, which is a great car. It has a feature called AVH, “Automatic Vehicle Hold,” which I unexpectedly love.
When stopped at a red light or in traffic, you depress the brake pedal and the AVH light comes on. You can then take your foot off the brake pedal, and the car will not move until you press the accelerator.
My question is, how does this work? Is the transmission switched to Park or is the brake locked in place? Or does it work another way? Just curious.
— Bob
“Brake hold” (it goes by different names by different manufacturers) is becoming a very common feature these days, and some people love it. You get to rest your foot while waiting for the light to change.
We’ve come a long way as a species from food, clothing and shelter, huh?
Personally, I hate brake hold because it doesn’t allow me to creep forward in traffic. And where I live, if you leave half a car length in front of you, someone’s going to cut in, which is totally unacceptable.
Anyway, it’s a clever feature, and it uses the car’s antilock braking system.
ABS was originally designed to prevent your wheels from locking up during emergency braking. To do that, it uses sensors at each wheel and a computer-controlled brake pump.
Well, over time, automotive engineers discovered that they could build upon those ABS components to create other safety systems. So, they used it to create traction control, which prevents wheels from slipping when you accelerate in snow or rain. And with some additional sensors, they used it to create stability control, which helps keep the car from skidding on turns and can even prevent some rollovers.
Brake hold is a relatively new use of the ABS equipment. Since there’s already a brake pump that’s operated by the car’s computer, brake hold — when turned on — simply employs that pump to keep the brakes engaged when you’re fully stopped. Then, once you touch the gas pedal, the computer knows that, too, via the throttle position sensor, and it tells the ABS pump to stand down and away you go.
So, it doesn’t put the car in Park. It’s simply an alternative method of engaging the brakes, just as you would do with your foot. Glad you enjoy it. Just remember to go the gym to make up for the exercise your right leg isn’t getting anymore, Bob.
