Need lesson on RWAL

[keithert]

I think I infered it from reading your pages. Guess I was wrong. I'll have to read the owners manual. I did have a rental of a Cavalier back in 1990 that had rear wheel antilock. It did pulse when the brakes were locked. What does it feel like on one of our vehicles?

EDIT. Here is a good page that explains the RWAL system. The owners manual doesn't say that the light will be active but it does say that the system can pulse. From the following it sounds like it should pulse like other anti-lock systems.

How it works
The Kelsey-Hayes rear wheel antilock brake system obviously only affects the rear brakes. On trucks with 4WD, the antilock system only works in the two-wheel drive mode.

The conventional master brake cylinder and power booster supplies brake pressure to the EH valve, which contains two solenoid valves: a normally open isolation valve to block pressure from the master cylinder to the rear brakes during antilock braking, and a normally closed dump valve for relieving pressure in the rear brake circuits. The EH valve also contains a pressure accumulator for storing fluid pressure during the dump or release phase of operation, and a reset switch which allows the system to maintain proper brake pressure.

When the ABS control module detects a difference in the average speed of the rear wheels compared to the vehicle's overall speed, it initiates antilock braking. The ABS isolation solenoid is energized to prevent any further increase in brake pressure at the rear wheels, and then the ABS dump solenoid valve is opened to release pressure from the rear brake circuits so the wheels can regain speed and traction. Pressure is reapplied when both solenoids are de-energized and return to their normal positions. The cycle is repeated continuously for as long as ABS braking is needed or until the vehicle stops.

From the following does it sound like putting larger tires on a Trackick could cause issues with the system?

The DRAC module is calibrated to the final drive ratio and original equipment tire size of the vehicle. Replacing the original tires with ones of a different size or aspect ratio will change the speed sensor signal, which in turn can adversely affect the operation of the ABS system as well as torque converter lockup and the accuracy of the speedometer and odometer.

If non-stock tires are installed on a GM vehicle with a DRAC module, therefore, the DRAC module must either be replaced or recalibrated. On 1991 and earlier "C" and "K" pickups, the DRAC modules can be recalibrated for different tire sizes and axle ratios by changing the configuration of an 8 pin connector that plugs into the instrument panel circuit board connector. A DRAC recalibration kit from GM is necessary for this procedure. By referring to a speedometer calibration chart in the factory service manual, the correct pin positions can be determined for any tire size and gear ratio combination. The specified pins are then broken off the connector. This alters the DRAC circuits when it is replaced in the instrument cluster to recalibrate the vehicle speed signal.

[kick-fix]

ah , it must pulse, when the dump valve is activated.
my other car has full ABS and i learn never to let it pulse.
last time it did, was driving on a painted roof,
ahh, parking garage. roof.

dont think ours is recalibratable. only thing GM is the hood orniment.
wish i had a full Mitsubish/Suzuki i spec on it.would be interesting read.

[Bex]

I have never felt the brake pedal 'pulse' in any way, which led me to believe that maybe RWAL is dead. Also cannot find anywhere to do (sorry, slightly confusing) test of rear wheels on grass and front wheels on pavement. Have done this with all wheels on grass (neighboring field...neighbor not very happy with tires marks everywhere), and car stopped fine...as a matter of fact, stopped better than on gravelly, muddy road. But even with stopping well, no pulsing action in pedal. I have located dump valve, but have not gone under car to see if sensor on rear differential. And still cannot find the jumper by left foot, unless it is the square white jumper with black locking cap on it. Maybe will try to take a picture of it, if I can squish down enough under steering wheel with the camera. Will also have to study FSM more...seems there is no easy remedy here.

[fordem]

Originally Posted by keithert

From the following does it sound like putting larger tires on a Trackick could cause issues with the system?

The DRAC or Digital Ration Adapter Controller module is essentially a signal processor that takes the signal from the VSS (Vehicle Speed Sensor) and processes it before passing on to cruise control, ABS, etc.

Yes - theoretically - larger tires can affect ABS and presumably RWAL however these systems are designed to monitor "the rate of change" of the tire's rotational speed rather than the actual speed itself.

Question - have you ever looked at a car with a "space saver" spare tire ...

Does it use the same size wheel/tire as the road wheels? In my experience - No.
Does the space saver have the same rolling diameter as the road wheels? In my experience - No.
Does this difference in size affect the ABS? In my experience - No?

Bearing in mind that this is how the car comes from the factory, what does that say about ABS and different tire sizes?

[kick-fix]

it does warn you, it tells you to drive slow , real slow and to get the tire fixed
immediatly, EG: drive to tire store only with small tire. NOT TO WORK. or Vacation, or the the Summer home. 1000 miles away. it states a max speed and distance.
(read the tiny tire spec and the paper on it that came new with car. it's all there)
It is better than nothing, if you just read the application guide.

but we dont have a DRAC, this is NOT a GM design.
it IS a Suzuki in sheeps clothing, nothing more.
ABS is desided for a specific tire, (this is not ABS) so who knows.?

but as you well know the rooling resistance is diff. and the tire print is differ.
and if you UPSET this , then the modulation of the dump valve may be too heavy or too light , depending on which way you went.
in our case ,it would be too light. so stopping distance will increase. over normal RWAL action on stock tires.

but that is splitting hairs. ( unless you are driving in extreme conditions)

i run studs , soon.
so that is another matter,
and no, its not designed for studs , either. (which vary by ,wet and hard ice, radial)