I'm just curious, can someone point me to information regarding the function and usefulness of the rear antilock system? I've worked with 4-wheel antilock systems and have a grasp of how they function. But the RABS only has the one tone wheel that I've noticed, so I'm curious as to how it operates (and how well it operates).

 

I don't know where to find a detailed write-up, but I understand the intention.

Most vehicles with high center of gravity, such as trucks and vans, will tend to unload the rear tires when braking due to "weight transfer". This can lead to rear tires locking up, which causes the rear end going out of control. A lot of these kind of cars have brake pressure valves that are adjusted by linkages connected to the chassis, so that as the rear end unloads, its suspension extends, which moves a lever that reduces pressure to the rear brakes.

Ford took a more active aproach to the Aerostar, installing a speed sensor in the rear axle and some kind of speed sensor for the front wheels and a small computer to tell when there is rear wheel lock-up. (I should have taken a closer look at my 2wd 91 model before selling it.) In the 4wd model, the front speed sensor is in the transfer case. But I have activated the RABS on the 2wd model in the rain once. But I've never tried the same hard stop with the RABS disabled to see what it feels like, so I can't say how much of a benefit it really is.

A 4 channel system would obviously be much better, as that can also prevent the front wheels from locking up, allowing you to continue to steer in slippery situations, and may also be deployed for assisting yaw control or anti-spin. But it would have been much more costly, especially during the time that the Aerostar was first introduced (86?).

 

Very basically, I think the Aero rabs system monitors vehicle speed & rear wheel rotation. If the rabs computer detects too rapid deceleration ( wheel lock-up) at the rear end , hydraulic pressure to the rear brakes is bled off into a small dump reservoir in the Combi valve block. The same block also adjusts pressure between the front & rear brakes.
Thats about all I know apart from the fact that a 20c 'O' ring in the combi valve block will cause problems with fluid loss & brake warning lights!

Aeroman.

 

Thanks guys. I understand what ABS does (I am an automotive engineer) and why. I know that Ford first started putting RABS on their trucks in the '80s to help prevent rear lock up when unloaded (thus trucks and not cars). What I was fishing for was more along the lines of how well does the system work. As in - how effective is it at doing what it is supposed to do? I was also curious as to what the Aerostar used as a front reference sensor.