Gentlemen,

When I first got my aero the rabs system worked, I thought it was real abs. It is not. I had to make a panic stop from 65 mph and I was conscious of the "system" working, it simply turned off the rear brakes more and more until I felt like I only had front brakes. At that point I thought that the "pump" for repressurization stopped working. Then I found out there is no pump. As I read the numerout posts on this site, I discovered that that is how it is supposed to work. You guys are smart. It really is not a great system.

When my rabs failed, I had premature rear brake lockup which is dangerous. I fixed the problem by installing an adjustable proportioning valve in the rear brake circuit. $39.00 on ebay, new.

WOW. Does my Aero have brakes now. NO REAR LOCKUP even in wet weather I recommend this modification strongly. Brakes are better now than with the lame rabs system working.

Ken

 

Trucks (Aerostar classification) are designed to carry varying amounts of weight - from empty to fully loaded. The braking system ideally should compensate for the load range and distribute braking force proportionally. While this sounds easy in theory, it is expensive to implement. Other manufacturers do use some hybrid of load weight sensors and compensating brake valve arrangement. Ford, in the interest of simplicity (expense) chose to use the RABS arrangement to prevent rear wheel lockup. Unfortunately, as you just described, the RABS system cannot differentiate between loaded and unloaded conditions.

 

Gentlemen,

I really enjoy having all systems operational in my vehicles. It does bother me that my rabs is not working. The adjustable proportioning valve seems to be a reasonable fix both in cost and efficacy. I did mount my valve adjustment in a convenient place to adjust for load conditions. I can vary my rear brakes from 100% to 47% with a twist of a ****. I really was scared when the rabs was working on that high speed stop. The damned thing simply bled off my back brakes.

Ken

 

Two questions :

1. Where in the system did you install the proportioning valve?
2. How does this by-pass the RABS?

 

Dear maxwgn,

Simply install the device at the Master cylinder port serving rear brakes between cylinder and rabs valve. My rabs unit failed and I simply left it in the circuit, it does nothing. It requires a short piece of brake line from auto zone to do. It works VERY well. You could remove the defunct rabs valve but it was hard to do in my case.

Very, very good brakes now.

Ken

 

Well, i was curious, as the RABS unit could fail, preventing the rears from working at all...

If I do this, I will by-pass or remove the RABS unit completely, so it could not later effect performance at all...

 

That is a good idea, I really tried to do that on mine but the lines were frozen on. researched the valve design and it fails in the full open position, it requires electricity to close off. I think I will pull the fuse on the rabs unit and unplug the valve wire. thanks for making me think about that.

You will be happy with the results.

Ken

 

I am glad that I read this thread. I started reading around the forums to find out how to fix my RABS light problem. The yellow light comes on, blinks once, and stays on all the time everytime I start up my 91 Aerostar 4.0. The RABS is definatly not working concidering I locked up the rears the other day in the rain and went into a nice fishtail slide avoiding an accident. That really inspired me to see what I could do about fixing it. In truth the slide is really what helped. When I nailed my brakes, the rearend swung out to the left pointing me to the right. As I slid towards the car sideways, right at the magic moment I let off the brakes and swerved into the parking lane next to the curb. Thankfully there where no cars parked there but had there been, I would be parting out my 91 Aerostar. Now after reading this thread I wonder how it would have been different if the RABS was working. I am also amazed as to how well Big Red (my name for it) handled the extreme evasive maneuver. Anyways, thanks for the advice posted in this thread. I will definatly take ken1mod's adjustable proportioning valve modification into concideration based on what I have just learned about the lame RABS.

Thanks again,
P51man

 

Before getting too far into RABS diagnosis, you might check that the switch on the shuttle valve in the MC isn't gunked up with corrosion. It seems that the little chamber that the switch goes into gets wet with minutes amounts of fluid leaking in, and corrosion builds up. WHen it contacts the switch, it will be sensed as an error in the brake system, which can disable other systems, such as the RABS, the cruise control, the OD lock up, and, if you have it, the 4wd.