See Figures 1 through 8
Before reading trouble codes, perform the Diagnostic Circuit Check (1989-91 vehicles) or Function Test (8-93 vehicles) according to the appropriate charts for your truck. The charts are located earlier in this section. This test will aid in separating RWAL system problems from common problems in the hydraulic brake system. The diagnostic circuit check will direct the reading of trouble codes as necessary.
DO NOT ground the brake trouble code terminal of the ALDL/DLC if the BRAKE lamp is not on indicating a stored trouble code. Attempting to activate trouble code read out with the BRAKE lamp off may set a false Code 9. This will turn the BRAKE lamp ON and disable the anti-lock brake system.
See Figures 9 and 10
The computer control module (ECU/EBCM) will assign a code to the first fault found in the system. If there is more than 1 fault, only the first recognized code will the stored and transmitted.
Trouble codes may be read either though the use of Tech 1® or equivalent scan tool or by connecting a jumper wire from pin H on the ALDL/DLC to pin A. If the jumper method is used, the fault code will be displayed through the flashing of the BRAKE warning lamp on the dash. The terminals must be connected for about 20 seconds before the display begins. The display will begin with 1 long flash followed by shorter ones-count the long flash as part of the display.
Sometimes the first display sequence will be inaccurate or short; subsequent displays will be accurate.
If using a hand scanner, note if a soft code is stored, only the last recognized code will be retained and displayed on the scanner. Soft fault Codes 6, 9 and 10 (vehicles through 9) can only be read with a scan tool. On some models, Codes 1, 11 and 12 will not read on the scan tool and must be read using the jumper wire method.
Never ground terminal H of the ALDL to terminal A if the BRAKE warning lamp is not lit. Doing so will usually set a false code 9 and illuminate the BRAKE warning lamp. With the brake lamp on, the RWAL system will be disabled.
Stored trouble codes must be cleared with the ignition switch OFF . NEVER attempt to clear codes while the ignition switch is in the ON position or the computer module will likely be destroyed. Remove the ECMB fuse for at least 5 seconds, then reinstall the fuse.