RWAL & 4WAL SERVICE PROCEDURES READING TROUBLE CODES

SUBJECT: READING TROUBLE CODES

MODELS: 1988-91 TRUCKS EQUIPPED WITH ANTI-LOCK BRAKE SYSTEMS

Trucks equipped with either Rear Wheel Anti-lock (RWAL) or 4 Wheel Anti-Lock (4WAL) will store a trouble code when a fault is detected with the system. On vehicles with RWAL the red BRAKE light will be illuminated when a fault is detected. On vehicles with 4WAL the amber Anti-Lock light will be illuminated. There are two methods that can be used to read the stored trouble codes, manual flashout and Tech 1.

The manual flashout method is NOT recommend- ed. This method, (which is covered in STG manual 15005.05-2) 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 Truck Service Bulletin 91-5E-31, Corporate Number 065021). Erratic grounding of the ALDL terminals can also cause false codes to be set.

GMC Truck 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).

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.