Hello,

Have a 93 F250 and am wondering when the RABS is supposed to kick in. I remember when rear anti lock first came out on trucks, it was only supposed to work when the bed was loaded (can't remember if this was a ford/chevy or dodge function).

Reason I'm asking is that I'm able to get the rear brakes to lock up on my truck with no apparent abs function.

 

The RABS don't care for loaded vs. empty bed, it only looks at wheel speed - if tires lock it dumps the line pressure to reduce braking and get them rolling again. That said it takes the wheel speed from the differential, not the actual wheels, which can lead to what I experience today with my right wheels locking up and the left wheels continuing to roll - the RABS figured it out tho and did its job, but this tells me I need to tighten up my driver-side rear brake a tiny bit.

On your truck, do both rear wheels lock, or just one side? Do you get a nice and bright yellow "Rear Antilock" light on your gauge cluster? Also look at the HCU on the frame rail to see if it's not bypassed - some people think RABS is the root of all evil in the world and believing they can "pump" the pedal faster than the RABS cycles its valves, so they just bypass the entire thing with a piece of brake line. Some other folks are sneakier, and kill their RABS by performing internal surgery - they remove the large cap on the front of the HCU and remove the spring behind it then reinstall the nut, all looks like factory only after the first few times the HCU dumps the pressure its accumulator chamber fills up with brake fluid and cause no return spring stays full for all eternity, thus effectively disabling the RABS...

 

What you describe above is how ABS has worked for me on all my other cars that have had it .... sometimes it is scary when it pulses the brakes and feels like it is gonna rattle the entire car apart.

The scariest ABS I have ever seen is the RABS on my 90 F150. I thought the point was to NOT lockup the the wheels, mine ONLY locks up the wheels and has done so recently the past 2 days, yesterday when slowing quickly for some idiot on the highway, felt like the bed was being removed from the truck, and the smoke screen behind me proved the tire was locked up. I can't tell if it is both but I know the Driver side is locking. This event was the first time the light stayed on also, when I was off the highway it cleared when I killed and restarted.

I think I can do without RABS if it is not working correctly. What part is the likely candidate .... the hydraulics or the speed sensor? I am leaning to the hydraulic actuator as being the problem.

 

Non of my trucks ever seem to have a working RABS.

All 4 wheels lock up untill you let off the brake.

 

Both Ford trucks I've owned have had RABS, and both worked just fine--but you have to understand how to use it. Your front wheels will still lock up, so you still have to pump the brakes, just not as quickly as without RABS. Even once your front tires lock up, the action of the truck is different than without--the rear end stays behind you, it's just the front end that drifts when the front wheels lock up. Once you let off the brakes, the front will steer again, and you can resume braking. It's all in the owner's manual...

Jason

 

I can get the RABS on my 93 F350 crewcab to kick in if I try to lock the brakes but it doesn't generate enough of a difference between "on" and "off" to counteract how hard I can lock the brakes with my foot.

Since I know how to modulate the brakes when braking is impaired I don't need RABS or even ABS. That's because I know how to drive the "right" way.

Anyone can do this. Whenever you buy a new (or new to you) vehicle, you set up some cones in a parking lot and push the vehicle to it's very limits in acceleration, braking, cornering, etc. Get a feel for it. Practice it.

Then when your brain needs that information while under duress, you have experience to draw on.

 

I agree with knowing how to drive .... too bad Big Brother will not let us that do do so. I have never locked up the front wheels on my f150. The rears lock often, well at least one of them does.

Before I changed everything a year ago except the lines, carriers, backing plates, master cylinder, modulator and RABS actuator, one wheel would lock up easily, usually when brakes cold at slow speed. After replacing all springs, cables, hoses, wheel cylinders, calipers, discs and drum it does about the same thing.

Yesterday was the first time I could not let off the brake because doing so would have cuased me to buy a new bumper, hood and grill. It is also the first time the ABS light stayed on in the 5+ years I have owned this truck.

Is it possible I have the rear brakes misadjusted? This 1990 RABS with one actuator and 1 wheel speed sensor on an open differential would not even be called ABS by todays standards which are all 4 wheel ABS and sensors on each wheel so they can be individually controlled nowadays.

 

Sure they do - remove the ABS fuse and the indicator light

I have while demonstrating how ineffective RABS is to various friends over the years ;-)

My rears "drag early" if the truck sits for a couple of days, but after 3-4 brake applications the problem goes away.

The only time this doesn't happen is when I replace the rear drums/shoes together as a set. Then it's fine for a couple of months and then they "drag early" again when I first start out in the morning.

I've learned to tolerate this for 18 years.

The 81 F350 crewcab that this 93 F350 crewcab replaced did the very same thing and was solved the very same way - things were fine where everything was brand new, but after a few months, sticky sticky sticky when I leave in the morning.

I've tried a couple of times to take everything apart and "back in" the self-adjusters but once you back up and apply the brakes, they re-adjusted the problem comes back.

The last time I did the rears I was extremely generous with grease on everything but the mating shoe/drum surfaces and that did nothing.

I basically gave up.

ABS has come a long way and many vehicles these days have far better programming where modulation can be changed in speed and intensity more appropriately for a given situation. That's when it starts to become useful rather than an annoyance.

The reason for this is various road-course racing leagues started to allow ABS which then allowed engineers the opportunity to learn in extreme conditions and apply that to consumer vehicles years later.

I installed four wheel ABS in my old 75 Dodge D200 pickup using Suburban parts and a homemade controller, and I did that because I consider it irresponsible to overdo the horsepower and not overdo the brakes in an equal way. Stopping is as important as acceleration in my opinion.

Anyway, I eventually ripped it all off because even though I couldn't modulate the brakes with my foot anywhere near as fast as the ABS parts could, the thing is my brain could make faster/better decisions than I could code into my homemade controller.

Still, it was a fun exercise.

 

My RABS does function but I don't really like it or most other ABS systems for that matter I agree with Frederick that its important to go out and test the limits of your vehicles handling. I did this with all of my children soon after they got their license. We had a 2wd 86 Suburban and I made the kids lock up the brakes and do some skid recovery exercises. My daughter (now 30+) amazed her friends when she saved her Honda minivan after hitting black ice and recovering from a 90 deg skid on a local 4 lane undivided highway.
On my old dually ambulance I rigged the emergency brake so it wouldn't lock and I would use it to prevent the fronts from locking in slippery weather.
My current 95 ext cab handles different from the 94 old reg cab shorty I had before. Its much more predicable and I was surprised at how well it handles in emergency situations.
BTW I really like a manual tranny in bad weather.
regards
rikard

 

The longer the vehicle, the harder it is to get the back end to come around--which helps if you hit ice at highway speeds. There was a world of difference in handling between my reg-cab longbed and my x-cab longbed.

Personally, I like 4-wheel ABS. It's saved my bacon a few times where even though I was leaving lots of room in front of me, people got stupid ahead, and I could stop safely and confidently, with plenty of room to spare. You have to remember that the pulsing action of ABS reduces the braking power, so you have to really stand on the brakes if you're on pure ice, and you may have to take it out of gear once you're below about 10 MPH to get fully stopped.

Jason

 

This is also my experience with RABS on these trucks too, the system quietly goes about the job of keeping the rear end behind you when braking hard, it probably reduces overall braking power some but on must trucks it's probably not a significant amount. I have owned some earlier trucks that didn't have RABS and in those it was pretty much guaranteed you would not be pointing the same direction you were originally after a panic stop, so in that way RABS is a good thing.
But I'm not a fan or ABS overall, the system in my Subaru for example is far too sensitive and intrusive, to the point it drastically increases stopping distances on snow covered roads and that makes driving in those conditions much more unsafe than it needs to be.

 

Bleed your brakes, get some slotted rotors and hawk HPS pads. When you swap pads, make sure to lubricate the caliper pistons. Then take the truck out and stand on the brakes at 45 mph. If your front brakes still don't lock, then you have a component not functioning.

Your rears need adjustment. Both of them.

It certainly is possible to have the rears not adjusted properly. And if all of the components of your RABS with open differential were functioning as they should, the ABS function would work just fine.

My first truck was an 89 I6 with RABS and open 3.08 diff. When the rears are adjusted improperly, the ABS does not work right. And I never had a problem with that truck (or this truck) locking the front brakes.

Good Luck. Brake issues are scary if not rectified.

 

Frederic is exactly right about the brakes not locking when cold for the first few months after a full rear brack job, I only drive the truck about 4-6K a year ... so that may be why mine lasted about 14 months.

Guess I should adjust the rears. I am guessing the left is locking before the right, so the single sensored RABS on the open diff does not notice the left lock because the right is driving the sensor ring on the pumkin, and the ABS does not work, but the last two days I have driven the ABS light did come on and stayed on the last time, which leads me to believe it might have activated.

What is the proper adjustment for the rear drums, I have always tightened them all the way down and backed off about 1 turn as long as the brakes were not dragging on all my other drum brake cars.

I agree with all that the fronts should lock .... I just haven't ever done so. All components in the front are about 2 years old, all components of the rear are a year old. Fluid is DOT 3 and was fully bled both times and also about 6 months after I bought the truck in 06. I pumped a lot of red dirt out of the system ... guess the previous owners did not believe in bleeding the brakes every 2-4 years as I do.

The last 2 times this week the rears locked it was when I was driving on DRY pavement or asphalt.

When the roads ice in southern Louisiana, I take a snow day and do not drive.