RWAL brakes

[Lt.Del]

On my 91 sub, I want to finally get my RWAL brakes working correctly.  I have put it off for a while now--simply unhooked the wire at the combinatin valve. The brake light still flashes, which gets on my nerves.

My question is where do i find the ALDL so that I can check the voltage at the H and A pins.  I want to see if my VSS (speed sensor) needs replacing. (I know the VSS should read resistance of 900 to 2000 ohms) Basically, my symptoms are, when I rehook the line at the combo valve, is that even at low speeds, when applying the brakes, they activate the pulsing sensation and I can't stand it because it drastically increases my required stopping distances...in high traffic areas, I have come close to hitting cars in front of me because the brakes pulse so much that it increases required stopping distance.    That is why i Unhooked the wire at combo valve--to override the RWAL system.  

Another question:  where is the DRAC (ratio adapter) located? Behind glove box or somewhere?

Gonna conquer this once and for all--even if i have to replace the RWAL ECU, isolation / dump valve, combo valve, and VSS.

 

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

Edited by: SgtDel  at: 2/6/07 10:38 pm

[roundedline]

The ALDL is under the steering column, typically to the right of the e-brake rod.
The DRAC should be over near the ECM...

Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project

[Lt.Del]

Thanks Chris.  When it gets a little warmer out, I'm gonna try and diagnose the problem.  

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

[VileZambonie]

The DRAC is part of the instrument cluster.

DESCRIPTION
The Rear Wheel Antilock System (RWAL) is designed to reduce the occurence of rear wheel lockup during a severe brake application. The system functions by regulating the rear hydraulic brake line pressure. The pressure regulation is accomplished by a control valve which is located under the master cylinder.

The control valve is made up of two valves, a dump valve which releases pressure into an accumulator, and an isolation valve which holds rear brake pressure. The valve is controlled by a microcomputer which is part of the Electronic Control Unit (ECU). The ECU is mounted next to the master cylinder.

In a severe brake application as pressure is applied to the brake pedal the ECU is designed to permit the valve to do one of three functions, or a combination of all three. The ECU will allow the valve to:



Maintain the same amount of hydraulic pressure.
Release hydraulic pressure through the dump valve into the accumulator.
Increase the pressure by pulsing the isolation valve.
The ECU operates by receiving signals from the speed sensor which is located in the transmission, and the brake lamp switch. The speed sensor sends it's signal to the Digital Ratio Adapter Cluster (DRAC), which is part of the instrument cluster. If the axle ratio or tire size is changed it will be necessary to recalibrate the DRAC. Refer to Instrument Panel, Gauges and Warning Indicators / Speedometer Head/Service and Repair / Speedometer Calibration.

The Rear Wheel Antilock (RWAL) system is connected to the existing brake warning lamp located on the dash. An indication of the RWAL operation and a bulb check is performed each time the ignition is turned "on," the warning lamp will remain on for about two seconds. A RWAL system malfunction is indicated by the brake warning lamp. To determine the specific trouble refer to System Diagnosis / Procedures / Vehicle On Board Diagnostics / Reading Codes.



Reading the DTC's without the scan tool:


PROCEDURE
Diagnostic Trouble Codes (DTC) 's can be displayed by jumping terminals "A" and "H" of the Data Link Connector and observing the flashing of the "BRAKE" warning light. This should only be done when the brake lamp is "ON".
The terminals must be jumped for about 20 seconds before the code will begin to flash. Count the number of short flashes starting from the long flash. Include the long flash as a count. Sometimes the first count sequence will be short. However, subsequent flashes will be accurate. If there is more than one failure, only the first recognized code will be retained and flashed.

NOTES:


DTC's 6,9, and 10 will only turn the "BRAKE" lamp on while the fault exists. These are soft codes faults and can only be read with the Tech 1 scan tool.
Do not jump terminals "A" to "H" if the "BRAKE" lamp is not on. Jumping the terminals with the "BRAKE" lamp off will set a false DTC 9. This will turn the "BRAKE" lamp on and disable the antilock system.