4WAL BRAKING SYSTEM OPERATION AND CHARACTERISTICS. GMC Truck S15 Pickup - 4WD 1991

For 1990-2009 cars

# 4WAL BRAKING SYSTEM OPERATION AND CHARACTERISTICS

#### SUBJECT: 4WAL BRAKING SYSTEM OPERATIONAL CHARACTERISTICS

VEHICLES AFFECTED: 1992 C/K SUBURBANS, 1992 K UTILITIES, 1991-92 S/T UTILITIES AND 1990-92 M/L VANS

Some owners of 1992 C/K Suburbans, K Utilities, 1991-92 S/T Utilities, and 1990-1992 M/L Vans may notice an audible operation occur at 8 mph during acceleration following each ignition cycle provided the brake is not applied. The audible operation is the 4WAL system performing a pump and solenoid functional check which assures proper operation of the 4WAL system. The check is a normal function of the system.

In addition, during a hard braking situation when the 4WAL system may become active, the owner might notice the following occurrences:

* Pedal feedback at the driver's foot as the 4WAL system rapidly makes pressure changes.

* An audible operation consisting of a pump motor sound or solenoid clicking.

These conditions are considered normal and no attempt should be made to repair them.

If your diagnosis of the vehicle confirms a properly operating vehicle, please explain the operation of the braking system to the customer, and provide the customer with a copy of the "FOUR WHEEL ANTI-LOCK (4WAL) BRAKING SYSTEM OPERATION" letter attached to this bulletin. Further action should not be necessary.

If your diagnosis shows that the vehicle is not operating properly, consult the proper service manual for further definition.

#### FOUR WHEEL ANTI-LOCK (4WAL) BRAKING SYSTEM OPERATION

Your vehicle is equipped with the four wheel anti-lock (4WAL) braking system.

You may notice a 1/2 second long "moaning" sound coming from the front end at 8 mph during acceleration. This will happen each time the vehicle is turned on provided the brakes are not applied. The "moaning" sound is the 4WAL system performing a pump and solenoid functional check which assures proper operation of the 4WAL system. The check is a normal function of the system.

In addition, during a braking situation when 4WAL may become active, you might notice the following occurrences:

* The brake pedal pulsating as the 4WAL system rapidly makes pressure changes.

* A "moaning" type sound consisting of the pump motor sound or solenoid clicking.

These functions are considered normal and indicate that the system is working correctly. Replacement or modification of the braking system components will not change or reduce the noise. Attempts to reduce the noise will only give you, the customer, a false sense of vehicle unreliability and the inconvenience of having your vehicle out of service.

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.