If this only occurs with the steering turned to one side, there are a couple of possibilities. First of all, the two front wheels will be turning at different speeds and the ABS Computer will detect that. Based on the signals from the rear wheels, the computer is supposed to figure out the reason for one front wheel turning slower than the other one. It's either because one is making a smaller diameter circle and should be going slower, or one is about to lock up. If the computer thinks one wheel is about to lock up, it will block additional fluid flow to it, then bleed some pressure off. Once that wheel gets back up to speed, it will reapply more fluid pressure to it. With some systems, you will feel the brake pedal being pushed back at you.
It's important to note that the purpose of anti-lock brakes is not to help you stop on slippery roads, and it can't fix driver errors such as going around a corner too fast. It's purpose is strictly to prevent a loss of steering ability caused by skidding tires. Braking distances may increase a lot, especially on GM vehicles, but the goal is to maintain steering ability. That is fine and effective in preventing you from sliding down a hill on snow because while it allows you to slow down, it also allows you to steer on the way down. It doesn't help much when the other guy runs the red light and surprises you. Few people react fast enough to find a clear spot and to steer toward it in that panic situation.
Many cars DO have decreased stopping distances with ABS even though that wasn't the intent. Caprice Classic squad cars with ABS go and go and go, and they find them in the next county before they come to a stop, but at least they could steer. Those cars without ABS stop much faster. They use the Bendix-9 ABS system with nine valves in the hydraulic controller. The almost identical Bendix-10 used in the Dodge Dynasty has one extra valve to isolate the two rear wheels so they can be controlled separately. You will just about tear the seat belts off the hinges in a panic stop with that system. Stopping distances are very much shorter but it has to involve more than just that tenth valve because rear tires do such a small percentage of total stopping. Something else in the design of the car or the ABS system is responsible for the better stopping ability.
ABS systems also can't control a sideways skid regardless whether you're on or off the brakes. You have a different issue if the system is kicking in on dry pavement involving no skidding. The first thing to look at is to be sure all tires are the exact same size. When one tire is smaller or larger than the others, it will rotate at a different speed, and the computer will detect that. Most systems will see that while driving on a straight road, set a related fault code, turn on the yellow warning light, and turn the system off. Of even greater importance is if you have a 4wd model, it's not even good enough to have four tires the same size. They must be the same brand. Only the Astro Van and a few Jeep models use a solid all-time transfer case with all-time four-wheel-drive. Mismatched tire sizes puts a constant stress on the transfer case that WILL destroy it. People have even run into problems from buying two new tires one day and the other two identical tires a week later. That later pair could have come from a different plant, different assembly line, or could have an improved, (cheapened), tread compound that makes them rotate at a different speed. That can be enough to take out the transfer case. There have been a lot of lawsuits related to mismatched tires. The Ford Aerostar and Dodge Caravan all-wheel-drive, (which is different than "four-wheel-drive"), both use a viscous coupling in one drive shaft to allow the tires to turn at different speeds so mismatched tire sizes is not a problem as long as the vehicle doesn't have ABS.
The next thing to look at is the wheel speed sensors. The wires can break from the constant flexing from turning, but that would be detected by the computer and the yellow warning light would turn on. Metal chips can collect on some types of sensors and cause the signal to drop out, especially at lower speeds. The best way to find that is with a scanner that can access the ABS Computer and can display live data during a test drive. You might be able to catch one sensor going to "0" mph when no skid is occurring, but it's easier to use the record feature if it's on the scanner. You press the record button when the problem occurs, then view the sensor data slowly, later. Because the data passes through the scanner's memory, the recording actually starts a few seconds before the record button was pressed.
GM also has had a huge problem with wheel speed sensors that are built into the front wheel bearing assemblies. The slight, normal play that develops in the bearings is enough to let the speed signals drop out intermittently. That can happen in as little as 15,000 miles. From what I can remember, I don't think this applies to your van. I seem to recall the sensors are mounted externally on yours. I would check for rust or metal chips on the end of the front sensors. That can cause signal dropout, more at lower speeds.
One more uncommon thing to look at is the steering system. The Astro Van uses two idler arms that wear out WAY faster than on any other vehicle. If you tug on the center link, you'll see the right front wheel turn left and right a little. Besides causing accelerated tire wear and sloppy steering, it makes it impossible to maintain precise alignment settings. When the wheel turns an amount different than designed in, it is going to rotate at a different than expected speed. That might be enough for the ABS computer to think one wheel is slowing down too quickly. The problem would typically be more likely to show up the further you turn the steering, and it would typically cause a problem when turning in one direction more than the other direction.
Idler arms/ alignment
Thursday, December 29th, 2011 AT 1:09 AM
