Does ABS work when sliding backwards?

I went wheeling yesterday with a large group in shallow snow and occasional ice conditions. [It was also 5-8F all day (brrr...), but that's not important to my question.] There was one particular small but steep hill (maybe 50 ft at 10-15%) that was giving all of us a challenge because of ice. I happened to have my rear locked on my first attempt. I lost traction right before the top, pushed the clutch in, hit the brakes, and proceeded to slide backwards down the entire hill. The trail is not straight and is lined with trees. Good times!

I believe ABS is disabled when the rear is locked on our trucks, but I watched several others in various rigs slide backwards down the same hill with all four wheels locked. It got me wondering if ABS works at all in those situations.

Anyone know?

I plan to do some experiments soon.

EDIT: I am talking about the ABS (anti-lock brake system), a system that prevents the wheels from locking up during emergency braking. I am not talking about the various other brake-based traction control systems.

EDIT: I mentioned ABS being disabled when the rear is locked to explain why my ABS didn't work, but it doesn't explain why it didn't work for the other rigs. I'm also wondering if mine would have worked with no locker engaged.

 

If your wheels are locked up and you are sliding, hang on.

 

I dunno, never thought of it to be honest, but interesting question for sure.
I would think without the traction features disabled, it would, given it has the hill roll back control feature (whatever the heck it's called).
I'll be curious if someone knows.

 

https://www.tacomaworld.com/threads/3rd-gen-traction-control-modes-explained.530695/

 

I'm not sure why you are linking that thread as it's about traction control. I'm talking about the ABS that prevents the wheels from locking while braking. I do realize that traction control piggybacks on ABS. Maybe I don't understand what you pointing out.

I mentioned ABS being disabled when the rear is locked to explain why my ABS didn't work, but it doesn't explain why it didn't work for the other rigs. I'm also wondering if mine would have worked with no locker engaged.

 

That was very common during Minnesota winter driving prior to the general availability of ABS. Sliding backwards down a twisting tree lined trail causes slightly more anxiety.

 

From the manual:


Also, I don't imagine ABS will kick in for your above scenario (for the others). Sounds like everyone was moving at slow speeds. I believe ABS will kick in based on wheel speed and a difference in speed between wheels. If all of the sensors are showing that the wheels are stopped, it has no idea the vehicle is sliding.

 

I think you are confusing ABS (anti-lock brake system) with traction control. Both use the same hardware, but the purpose is different. The purpose of ABS is to prevent the brakes from locking up the wheels during emergency braking. On the the other hand, traction control is for preventing wheel spin when accelerating.

Also, I mentioned ABS being disabled when the rear is locked to explain why my ABS didn't work, but it doesn't explain why it didn't work for the other rigs. I'm also wondering if mine would have worked with no locker engaged.

 

I'm not confusing the two. ABS is tied in to your wheel speed sensors. I had one go faulty on a previous vehicle and every time I applied my brakes, ABS engaged. Why? Because it saw 1 wheel at 0 MPH and all others moving.

 

https://youtu.be/c-2Qk43uJMA

I was in 3rd gen, 2nd gen in front of me and FJ ahead of him. All had no ABS in reverse, which why I luckily told the tacoma to let go of the brakes, but the FJ did not, and it hit the ditch.

This was years and years ago, right after I got rid of my XJ.

 

To put what @ktbell444 said in another way, if you are going slow enough (or it is slippery enough) that all 4 wheels lock at the same time, the ABS computer just assumes you are stopped. It doesn't know you are sliding so it doesn't engage.

Next time try staying off the brakes and feathering the clutch and gas to control your slide. I did that in an XJ once in a similar situation. Every time I touched the brakes it would slide and when I let them off (it was an auto) the wheels would spin just enough to hold me against the slope. Backed all the way down the hill like that. Good fun!

 

You said, "I believe ABS will kick in based on wheel speed and a difference in speed between wheels." That sounds like traction control, not ABS. In the cases I am referring to, no wheels were spinning. ABS is supposed to prevent that.

All of the traction control systems and ABS use the same hardware, so they are all related, but my question is specific to ABS.

 

4 channel ABS CANNOT work if a diff is locked....how would it independently control the rear wheel speeds while they're locked together?...not possible. Without a center diff, it's tough for them to make a front wheel spin a different speed than the rear also. Also, most vehicles disable the ABS under a certain vehicle speed since it does more harm than good at low speeds.

A-trac applies the brakes to manipulate the differentials, which is entirely different.

Keep in mind...if all 4 wheels are locked and you're sliding, the system thinks you're standing still.

 

Thanks! I suck at explaining things sometimes.

 

Based an my many years of driving in snow/ice conditions with ABS I don't think it works that way. It definitely activates at slow speeds when you slam the brakes on going forward. That happens to me 100 times a year maybe. When going backwards I'm not so sure of, especially when going backwards while the transmission is in forward.

 

I'm getting confused. Has noone slammed on the brakes while driving forwards on snow and had the ABS activate?

 

no. Abs cannot prevent all 4 wheels from locking up at low speed because it has no manner in which to determine that’s what’s happening. If you were going 2mph and a second later all 4 wheels are locked up, it thinks you’ve stopped.

computers cannot save us in every situation.

 

ABS and traction control do the same thing but for different events. Traction control works on acceleration, ABS for braking. Both monitor wheel speed variances. If all 4 wheels lock, ABS thinks you are stopped, as described earlier.

 

https://en.wikipedia.org/wiki/Anti-lock_braking_system

"Typically ABS includes a central electronic control unit (ECU), four wheel speed sensors, and at least two hydraulic valves within the brake hydraulics. The ECU constantly monitors the rotational speed of each wheel; if it detects the wheel rotating significantly slower than the speed of the vehicle, a condition indicative of impending wheel lock, it actuates the valves to reduce hydraulic pressure to the brake at the affected wheel, thus reducing the braking force on that wheel; the wheel then turns faster. "

Note the words "wheel rotating significantly slower than the speed of the vehicle".

ABS would be entirely useless if it could not prevent all 4 wheels from locking up during emergency braking. Have you guys lived in an area where it snows a lot?

 

This. If all 4 wheels are locked, the truck thinks it is stopped and will not engage ABS.