RWAL stands for REAR WHEEL ANTILOCK. RWAL was standard equipment starting in 1988 on full size trucks.
1988 and newer trucks of that body style that do not have 4 wheel ABS have RWAL.
There are three brightness levels of the dash brake indicator light on RWAL equipped trucks.
Brightest – park brake
Medium – Combination valve (aka metering & proportioning valve)
Dim – RWAL System
Watch the light very closely and apply the parking brake. Did the light get brighter?
When the key is first turned on the RWAL module turns the dash brake indicator light on for 2 seconds, then it should go off.
You can unplug the wire on the Combination valve to see if it is the cause on the light coming on.
The RWAL system can store trouble codes, but it is only capable of storing one code at a time.
If the light is blinking the manual states you should still check the codes by turning the key on (engine not running) and put a jumper wire between terminal A and H of the ALDL. Count the brake light flashes.
You can retrieve RWAL codes without a scan tool.
If the instrument panel light is on, to retrieve RWAL codes connect a jumper wire between terminal H and terminal A of the ALDL connector with the key on (engine not running). Watch the dash brake lamp and count the blinks. Sometimes RWAL system send out a phantom blink before the code blinks start. It can take up to 20 seconds for the code to start coming out.
Checking the code with this method can set a false code 9. Checking for a code when no code is present sets a code 9, WHICH WILL DISABLE THE RWAL system.
Clearing codes
You can clear the code by removing the power to the RWAL module for 10 seconds. Multiple circuits send power to the RWAL module and the fuses may be labeled STOP/hazard and BRAKE. Removing a fuse labeled RWAL may not clear the code. GM information on which fuse/fuses to pull is not very reliable.
My understanding is, when the RWAL system has stored a trouble code, the antilock system will not function so the vehicle will perform similar to a non antilock brake vehicle.
Keep in mind the RWAL system does have components that are part of the hydraulic system of the vehicle and failures can occur that can cause loss of brakes.
Fault codes
Codes for Dodge RWAL, Ford RABS and GM RWAL applications are similar but have a few differences:
1. Should not occur (replace the controller if it does)
2. Open isolation valve circuit or defective ABS module
3. Open dump valve circuit or defective ABS module
4. Grounded or closed RABS or RWAL valve switch
5. Excessive actuations of dump valve during ABS braking
6. Erratic speed sensor signal
7. Shorted isolation valve or defective ABS module
8. Shorted dump valve or defective ABS module
9. High resistance or open speed sensor circuit (resistance should be 1000 to 2500 ohms)
10. Low speed sensor resistance (Dodge & Ford only). On Chevrolet trucks the code 10 is for the brake light switch circuit.
11. Open speed sensor circuit (GM only) 1993 manual states this is a invalid code.
12. Stop lamp switch circuit (Dodge & Ford)
13. ABS module speed processor (Dodge & Ford only)
14. ABS module program (Dodge & Ford only)
15. ABS module RAM memory failure (Dodge & Ford)
Beginning in August 1990 (this date does not match in all information on when the change took place), a change was made to the RWAL module on all General Motors vehicles. The change transforms codes 6, 9 and 10 into "soft" (non-latching) codes that only illuminate the warming light as long as the fault is present. If the fault is intermittent and no longer exists, the light will go out with the next ignition cycle.
A scan tool can be used on the newer GM applications to read any soft (non-latching) codes.
On the modules built after Aug 1990 grounding the "H" terminal on the ALDL connector erases soft codes.
