RWAL FUNCTIONAL TEST INFO (RWAL/ENGINE CONTROL UNIT). GMC Truck S15 Pickup - 4WD 1988

For 1990-2009 cars

# RWAL FUNCTIONAL TEST INFO (RWAL/ENGINE CONTROL UNIT)

#### SUBJECT: REAR WHEEL ANTI-LOCK BRAKE/ENGINE CONTROL UNIT AND TECH-1 RWAL FUNCTIONAL TEST INFORMATION

#### VEHICLES AFFECTED: 1988-92 "C/K, R/V, S/T G, M" TRUCKS

THE TECH 1 DOES NOT TEST RWAL COMPONENTS OR MAKE A DETERMINATION IF A COMPONENT IS GOOD OR BAD ON 1988-92 C/K, R/V, S/T, G AND M TRUCKS. THE TECH 1 USES A "FUNCTION TEST" TO TEST THE RWAL SYSTEM, AND THROUGH THIS TEST IT CAN BE DETERMINED IF THE ECU IS GOOD OR BAD. TO UNDERSTAND THE "FUNCTION TEST" A BASIC UNDERSTANDING OF THE ECU IS NEEDED.

THE ECU RECEIVES INPUT, INTERPRETS THE INFORMATION, AND DECIDES IF THE RWAL SYSTEM SHOULD BE ACTIVATED -- OUTPUT. IF THE ECU DOESN'T RECEIVE INPUT, OR RECEIVES INCORRECT INPUT IT CAN'T MAKE A DECISION TO - OUTPUT. CONVERSELY, THE ECU COULD RECEIVE GOOD INPUT, BUT CAN'T OUTPUT BECAUSE OF BAD CONNECTIONS OR WIRES. IN BOTH CONDITIONS, YOU COULD LOSE RWAL AND SET A CODE. BELOW ARE THE MOST FREQUENTLY SET CODES. THEY ARE ALL EITHER AN INPUT TO ECU OR RECEIVE OUTPUT FROM THE ECU -- NOT THE ECU. BY USING THESE CODES THE ECU IS SAYING; "I HAVE DETECTED A PROBLEM" AND IDENTIFIES THE AREA OF MALFUNCTION.

CODE 4 GROUNDED ANTILOCK VALVE SWITCH CODE 6 ERRATIC SPEED SIGNAL CODE 9 OPEN/GROUNDED CIRCUIT TO THE SPEED SIGNAL CODE 10 BRAKE LAMP SWITCH CIRCUIT

OBVIOUSLY, REPLACING THE ECU WILL NOT SOLVE THESE PROBLEMS.

BY USING THE TECH 1 WE CAN DETERMINE, IF THE ECU IS FUNCTIONING CORRECTLY. THE TECH 1 PERFORMS A "FUNCTION TEST" ON THE RWAL SYSTEM, BY FURNISHING THE ECU WITH THE CORRECT INPUTS. THE ECU THEN ACTIVATES THE RWAL SYSTEM. WHEN THE RWAL SYSTEM IS ACTIVATED THE ISOLATION/DUMP VALVE CAN BE HEARD AND FELT.

THE RWAL "FUNCTION TEST" SHOULD BE DONE AT LEAST THREE TIMES:

1. IF THE RWAL SYSTEM PASSES AT LEAST TWO OF THE THREE TIMES, THE ECU IS GOOD. (THIS MEANS NO CODES NO RWAL LIGHT.)

2. IF THE RWAL SYSTEM FAILS (A CODE IS SET OR THE RWAL LIGHT IS ON), THIS DOESN'T MEAN THE ECU IS BAD.

3. IF A CODE IS SET AFTER THE TEST, DIAGNOSIS FOR THAT CODE SHOULD BE FOLLOWED.

4. IF THERE IS NO CODE, BUT THE RWAL LIGHT IS ON, ALL RWAL ELECTRICAL CONNECTIONS SHOULD BE CHECKED. 95 PERCENT OF THE TIME ELECTRICAL CONNECTIONS ARE THE CAUSE.

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.