How does the RABS valve work?

[Shawn MacAnanny]

i know it's a rear antilock brake system but how does it work?

I don't believe there is a leveling sensor for rear loading like a proportioning valve that changes pressure based on bed height/load.

Does it run off of the ring gear sensor speed sensor for the obs trucks to detect lock up? I don't believe there are any front wheel speed sensors though? Or is there some sort of internal pressure sensor that releases if pressure out of it becomes lower than pressure in?

How does it know when your rear wheels lock up? Because if it's the same part from a srw and dually and you have heavier springs/air suspension and a dually and a 2500lb service body it would seem it would provide the same braking pressure to the rear as a srw and that may not be ideal. Especially if there is a leveling proportioning valve or sensor somewhere in the rear having heavier duty suspension.

 

[Shawn MacAnanny]

I found this article. Says the ring gear speed sensor. Not sure how true it is but I don't see how you can rely on average rear wheel speed to determine if the rear wheels are locking you without a front wheel speed reference.

http://www.aa1car.com/library/abs_kelseyhayes_rwal.htm

 

[79jasper]

That actually is the way it works. Lol
Not exactly "reliable."
And Iirc some F-SD trucks actually did have a leveling proportioning valve.
Towcat knows more on that.

Sent from my SAMSUNG-SM-G935A using Tapatalk

 

[Macrobb]

I'm guessing that when it sees a sudden jump from X speed(say, 30 miles per hour) dropping to 0(or close to) in a very short period of time, with the brakes engaged, it assumes that the wheels have locked.

Which might be true - if one wheel locks up, the speed seen at the differential will halve. If both lock up, it'll go to zero.
And it'll 'jump' vs smoothly dropping, especially at the time scales the computer is watching.

 

[no mufflers]

This got me thinking, I have been feeling a break pulsing coming from the rear of my truck. It feels like when the front rotors pulsate but just in the rear. It doesn't happen all the time iether.

Could it be the the RABS valve acting up? Or can the drums get out of round?

 

[KZF250]

I deleted mine in the shuttle bus. Was worried about locking up on panic stops but doesn't seem to be an issue and have had many cars/trucks vans without anti-lock anyway.

 

[Shawn MacAnanny]

This got me thinking, I have been feeling a break pulsing coming from the rear of my truck. It feels like when the front rotors pulsate but just in the rear. It doesn't happen all

I'm guessing that when it sees a sudden jump from X speed(say, 30 miles per hour) dropping to 0(or close to) in a very short period of time, with the brakes engaged, it assumes that the wheels have locked.

Which might be true - if one wheel locks up, the speed seen at the differential will halve. If both lock up, it'll go to zero.
And it'll 'jump' vs smoothly dropping, especially at the time scales the computer is watching.

So then does it immediately release and retry or does it stay released until incoming pressure is released? Like letting off the pedal entirely? Essentially if you rear wheels lock up you lose all rear braking as it dumps fluid pressure, it doesn't just release until it stops locking it seems

I had a pulsating braking and had an adjuster come loose off of the shoes in the rear. It still adjusted with tensions but one side was jammed between the shoe and backing plate. It felt like a very warped rotor

I also got a warped new drum from Napa. Both caused a warped feeling.

 

[no mufflers]

that's interesting, ill have to do some brake inspections soon I guess.

 

[Shawn MacAnanny]

I just completely rebuilt mine about 2-3k ago and had the rear brakes acting up the past few days. I suspected the rabs but went under to check the rear shoes and found I could see the adjuster cable off on the driver side. Sure enough I removed drum (16 lugs since I have wheel adapters on a dually) and I found the cable seems to be too short if you route it under the backing plate spring. If you route the adjuster over it holds the adjuster arm in the wright place. Must have come loose after I did the brakes and let that wheel completely back the shoes off. I routed it over the spring and readjusted everything.

 

[jwalterus]

how does it work?

There are 2 correct answers:
1) it doesn't
2) poorly




I just take out the bulb

 

[pelky350]

Yeah I haven't notice mine work on either of my trucks(bronco or 350) had break problems where wheel would lock up when first driving in morning no matter how gentle I was on the pedal and it never did anything, couple times I needed to stop in a hurry I controlled the wheel lock up better than the system does. my f350 has the light on saying antilock after I drive for a little does it mean anything?

 

[franklin2]

I deleted mine in the shuttle bus. Was worried about locking up on panic stops but doesn't seem to be an issue and have had many cars/trucks vans without anti-lock anyway.

I bypassed my rabs also, but realize you have even less than a earlier truck with a proportioning valve. In other words when you by pass the rabs, you now have a straight open line to the front brakes, and a straight open line to the rear brakes, no proportioning or any type of delay valves.

Mine works ok also, but I can lock up the rear tires in a panic unloaded stop, and I have 35 inch tires.

 

[Shawn MacAnanny]

My last truck had it bypassed and as a dually with a 2500lb bed it never once locked the rear wheels up. The fronts I had locked up a couple times.

 

[eacars]

I deleted mine in the shuttle bus. Was worried about locking up on panic stops but doesn't seem to be an issue and have had many cars/trucks vans without anti-lock anyway.

I had one lock up and do a 180 on a wet exit ramp about 30 years ago.. Truck may have been pre RABS system .I'm not sure . It was my sisters farm truck. Early 80's I'm guessing. When did the RABS system come out? I've gone through 2 of those ----ers since I bought my truck new in '94...Both my lights are on .. I think I have some air in there....Loose fitting or bad Master Seal..Still been a good Truck in the long run...

 

[eacars]

Now that both my ABS and brake lights are on, if I panic stop, it locks up.. I guess it's not working. I have to top off brake fluid more than normal... Leak. Right where the Master Cylinder #3, bolts to the vacumn booster. Still been a good truck in the long run...lol