 | ||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||
|
ABS Light On, Codes Atored Signal Trouble In 1990-93 4WAL-Equipped GM TrucksPosted 8/7/2000By Ralph Dahlen
Vehicles System Problem Operation Code 61 relates to the right front, Code 62 relates to the left front and Code 63 relates to the rear wheels. The reset switch is a normally closed switch that opens with a calibrated difference in pressure against spring tension. The switch will open during an ABS stop in the pressure decrease step of the cycle to signal the computer that a pressure decrease did occur. Also, the switch or switches will open in a non-ABS stop if there is a hydraulic failure in the foundation brakes, such as a leaking wheel cylinder/caliper or air in the system. Diagnosis It is common to see these codes on a 1990-93 GM truck equipped with 4WAL. The ABS and BRAKE warning lamps will be illuminated if the fault is present as a hard failure with one or more of these codes stored. The ABS and BRAKE warning lamps will stay on until the problem is fixed and the codes cleared. If a truck comes in with ABS Code(s) 61-62-63, the first thing to do is to check for any problems with the foundation brakes. Keeping in mind that a pressure differential problem in any channel can cause one of these codes, check for fluid leaks at all four wheels and follow the lines up to the EHCU and then the master cylinder. Repair any leaks and also check for new brake parts that may have been previously installed. Air in the system can set these codes so be sure to go through bleed procedures after a repair has been made that opens the hydraulic system. There is a specific bleed procedure that must be followed when air is introduced before the EHCU or when the air is at the EHCU itself. This bleed procedure is not necessary if air is kept at the wheels, such as when replacing a brake hose or wheel cylinder. With this type of repair, just bleed the brake at the wheel, paying attention not to let the channel lose too much fluid. If the channel does go dry, air will enter the EHCU and then the more involved bleed procedure must be done, first at the EHCU, then at the wheel. If no leaks are found, nor any hydraulic brake components recently installed, the problem is likely a faulty reset switch(es). These switches are internal to the EHCU and are not accessible for testing or replacing. If a switch fails, the entire EHCU must be replaced. If replacing the EHCU, bleed the unit and road test. Previous codes are not an issue since the ABS computer is part of the EHCU assembly and has a clear memory. If not replacing the EHCU, the codes can be cleared with a scan tool or manually with an A to H jumper routine at the DLC. The manual method of clearing codes consists of an A to H jumper at the DLC two times with a two-second duration each time the jumper is made, followed by a two-second wait in between. Road test the truck, stopping numerous times at varying speeds to determine if the fault will return. If the fault does return, replace the EHCU and bleed the system. During production year 1993, GM eliminated the reset switches from the 4WAL EHCU. Vehicles affected are all 4WAL equipped trucks, model years 1990-92, plus many, but not all, 1993s.
|
|||||||||||||||||||||||||||||||||
Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
 | ||||||||||||||||||||||||||||||||||
This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time.
