Specifically the rear only systems on the 3/4 and 1 ton trucks.

Is it a stand alone system? Or is it controlled by the PCM?

The reason I ask is because I'd really like to retrofit my 73 F250 with rear ABS. One of my buddy's had an 87 F150 with rear only ABS and it looked pretty simple. I plan to swap a 10.25 or a dana 70 from a late model e350, so I'll have an abs sensor. The RABS valve looks pretty simple to install, but what controls it?

And before I really get into it, how well do these older systems work in the first place?

Sam

 

some will say they work great, but most will tell you don't bother with it because they are not worth the aggravation.
the RABS on my 88 F350 was disconnected and bypassed by ford before the truck had 500 miles on it because it would either have you kissing the windsheild, or running red lights because of no rear brakes. more often it was running reds.
they could not figure it out, so bypassed it. after that issue, all other 5 trucks had the RABS bypassed before i ever picked them up.
when the old RABS system was working, it was marginal at best. when it does not work it is a royal pain in the asterisk.
i only have one vehicle with working ABS, my 04 F-350 rack dump. and that one is soon to be bypassed because it is acting up also.
ABS is not the best thing since sliced bread like everyone makes it out to be.
if you know how to drive you do not need it. the big problem these days is most people do not know how to drive anymore with abs, collision avoidance, auto breaking, auto navigation, and all the other unnecessary crap put into cars and trucks.

 

I agree with tjc.
I would not waste the time to try to add abs to any vehicle, I have never been in a situation that I wish my abs worked. All it does is make stopping harder and more dangerous.

 

+1

The RABS on my F-150 seems overly sensitive. It activates way too easily and you end up not stopping when/where you want to.

On the other hand, it causes me to drive the speed limit and keep a larger buffer behind the vehicle in front of me...

The "first generation" RABS on the OBS trucks is pretty crappy. It's nothing like the latest generation on the newer models.

 

I wouldnt install it either, but to ansswer your question, it is a stand alone system with its own computer.
the computer monitors wheel speed, if it is slowing too fast, it activates a valve in the rabs valve to block fluid to rear brakes. If it still slows too fast it will open a valve to let some pressure bleed off into an accumulator in the rabs valve.

 

As designed the RABS systems on these trucks works well, in general it does it's job which is to keep the rear wheels behind the front wheels in a panic stop situation thus allowing the driver to maintain control. What it does not do and was never intended to do is reduce stopping distances, if that is what you are expecting from an ABS system then you completely misunderstand it's purpose. But if that is what you want to achieve then look instead at installing rear disc brakes, these will produce much more balanced stopping power which is most evident on wet or snow covered roads where stopping will be much less dramatic, and they are completely self adjusting so they require much much less maintenance than drum rear brakes which simply don't stay properly adjusted for very long.

 

I've had two trucks with the RABS: a '95 F-150 and my current '97 F-250. I can't say that I've ever noticed the RABS in the F-250. I don't know if that means it's working perfectly or if it just means that I haven't really tested it.

But the RABS on the F-150 worked great. Just like Conanski said, it made the rear end stay behind the front in a panic stop.

The best example of this was one time when I was screwing around in a parking lot. I got the truck into a slow spin without using the brakes. Then I slammed on the brakes while the truck was spinning. The skidding front tires immediately slid to the front while the rolling back tires acted like the fletching on an arrow. The spin stopped immediately and the truck stopped in a straight line.

In more real world usage, what I liked about it was that the brake proportioning was always right, whether the truck was loaded or empty. My '85 F-250 would lock the rear up too easily when empty, making me let off on the brake to maintain control at times, which made for longer stopping distances than would be required with RABS. And the rear brakes weren't as strong as they could have been when I had a big load on it, again giving longer stopping distances than it would have had with RABS.

So when the RABS is working well I see it as a really big plus. I never had any trouble with it in my F-150, and as I said, I don't know about the current F-250.

 

All mine ever did was drop the pedal out from under my foot during panic stops and make me crap my britches. I pull the fuse and warning light bulb on mine now and have been driving that way for 20 years.

 

Don't bother with it because they are not worth the aggravation.

Like what I did there? Lol.

Seriously, don't even bother. ABS does not reduce stopping distance, it just keeps the wheels from skidding out hence the name Antiskid (or antilock) Braking System. Nothing to do with stopping faster... it actually might even increase your stopping distance in some cases (i.e. in dirt, sand or gravel) because locking up your wheels in that stuff normally piles it up in front of the wheels which helps slow you down.

Mine never worked anyway, and when I plugged it in it messed up my PSOM so into the trash went the controller and warning light bulb. Repurposed the now-empty fuse slot for something else.

 

RABS on my 92 was worthless. Might have been a stuck valve or other problem, the parts were expensive enough that I never considered repairing it. As for adding it to an older truck, nope.. I'd do a manually adjustable proportioning valve instead of transplanting an obsolete, marginally functional at its very best, expensive system to a sweet old ride. You can do much better..

 

Good to know, I did some research and there are more modern ABS setups available to retrofit, I might look into those a bit more. Personally, abs has saved my butt a time or two, in a panic stop situation being able to just concentrate on steering and not trying to modulate the brakes is huge. A couple months ago I had to stop quick and VERY nearly slid sideways into the subaru in front of me! Somehow I had the presence of mind to back off the brakes and just steer into the median. I wound up next to the car eye to eye with the driver.

The main issue I have is when the back locks up, I have to back off on the pedal. For me an ABS system would definitely cut stopping distance, I could get a LOT more out of the front brakes. This truck weighs at least 6500 lbs, locking up the front brakes just isn't an issue.

Maybe an adjustable prop valve would be a better solution?

EDIT: OldRacerGuy, funny you mention the adjustable prop valve, you posted that idea just as I was typing this post.

Sam

 

I think the RABS in these trucks works fine.

Those that have theirs bypassed or the fuse pulled should install an adj prop valve to limit rear press. Full press to the rear brakes is an accident waiting to happen. Try a full panic stop at speed while slightly turning and see what happens.

 

"Full press to the rear brakes is an accident waiting to happen."
this is only true if you do not know how to drive.

i have never had ABS until i got the 88. and nothing i own or drive has abs except the 04. i average between 50 and 70 thousand miles per year, and not once ever had any problems stopping either at 4,200 lbs in the crown vic, or 240,000 lbs in the lowboy with a 375 cat excavator on the trailer..
besides no abs, i also learned how to drive in a truck that did not have seat belts, power steering, or power brakes.
no air conditioner, or radio either.

 

I bet you feel the same as I do about those newer vehicles that parallel park themselves.

 

A vehicle with no RABS has a proportioning valve on it to limit rear brake press and set a bios between front/rear. I wasn't referring to a vehicle without RABS.

I was talking about DISABLED RABS. When you disable the RABS you now have FULL press to the rear brakes -the same as the front brakes. Impossible to control on a panic stop even for a driver as experienced as yourself.