Four Wheel Antilock Brake System (4WAL)
This article has been updated with TSB 91-228-5ACCESSING AND READING ABS DIAGNOSTIC TROUBLE CODES
ALDL Connector Terminal Identification.:
Trouble codes are read by jumping terminals "A" and "H" of the Assembly Line Diagnostic Link (ALDL) and observing the flashes of the "Antilock" warning lamp. The terminals must be jumped for a few seconds before the code will begin to flash. The "Antilock" warning lamp will flash in a manner similar to the "Service Engine Soon" lamp.
IMPORTANT: The manual flashout method is NOT recommended. This method, (which is covered in the applicable service manual) does not give the detailed diagnostic information that the Tech 1 tool is capable of, such as reset switch status and real time wheel speed display. Additionally, when using manual flashout to read newly introduced RWAL soft codes technicians may cause the codes to be cleared and a false code 9 to set (reference Dealer Service Bulletin No. 91-101-5). Erratic grounding of the ALDL terminals can also cause false codes to be set.
GM, therefore, recommends the use of the TECH 1 tool over manual flashout in all diagnostic procedures involving both RWAL and 4WAL.
IMPORTANT: When using the Tech 1 remember to use a scan tool with a serial number above xx002000 along with tool number TK2650 (4WAL cartridge, special RWAL 4WAL adapter, and harness).