The Anti-lock Braking System (ABS) was first introduced on J-body cars in 1992. ABS provides the driver with 3 important benefits over standard braking systems: increased vehicle stability, improved vehicle steerability, and potentially reduced stopping distances during braking. It should be noted that although the ABS-VI system offers definite advantages, the system cannot increase brake pressure above master cylinder pressure applied by the driver and cannot apply the brakes itself.
The ABS-VI Anti-lock Braking System consists of a conventional braking system with vacuum power booster, compact master cylinder, front disc brakes, rear drum brakes and interconnecting hydraulic brake lines augmented with the ABS components. The ABS-VI system includes a hydraulic modulator assembly, Electronic Control Unit (ECU), Electronic Brake Control Module (EBCM) or Electronic Brake and Traction Control Module (EBTCM) (these are all different "Computer Control Modules" which differ depending upon vehicle year and application), a system relay, 4 wheel speed sensors, interconnecting wiring and an amber ABS warning light.
The ECU/EBCM/EBTCM monitors inputs from the individual wheel speed sensors and determines when a wheel or wheels is/are about to lock up. The ECU/EBCM/EBTCM controls the motors on the hydraulic modulator assembly to reduce brake pressure to the wheel about to lock up. When the wheel regains traction, the brake pressure is increased until the wheel again approaches lock-up. The cycle repeats until either the vehicle comes to a stop, the brake pedal is released, or no wheels are about to lock up. The ECU/EBCM/EBTCM also has the ability to monitor itself and can store diagnostic codes in a non-volatile (will not be erased if the battery is disconnected) memory. The ECU/EBCM/EBTCM is serviced as an assembly.
The ABS-VI braking system employs 2 modes: base (conventional) braking and anti-lock braking. Under normal braking, the conventional part of the system stops the vehicle. When in the ABS mode, the Electromagnetic Brakes (EMB) action of the ABS system controls the two front wheels individually and the rear wheels together. If the one rear wheel is about to lock up, the hydraulic pressure to both wheels is reduced, controlling both wheels together.
BASIC KNOWLEDGE REQUIRED
Before using this section, it is important that you have a basic knowledge of the following items. Without this basic knowledge, it will be difficult to use the diagnostic procedures contained in this section.Basic Electrical Circuits
- You should understand the basic theory of electricity and know the meaning of voltage, current (amps) and resistance (ohms). You should understand what happens in a circuit with an open or shorted wire. You should be able to read and understand a wiring diagram.Use Of Circuit Testing Tools
- You should know how to use a test light and how to use jumper wires to bypass components to test circuits. You should be familiar with the High Impedance Multimeter (DVM) such as J 34029-A. You should be able to measure voltage, resistance and current and be familiar with the meter controls and how to use them correctly.
The ABS-VI contains sophisticated onboard diagnostics that, when accessed with a bidirectional scan tool, are designed to identify the source of any system fault as specifically as possible, including whether or not the fault is intermittent. There are 58 diagnostic fault codes to assist the service technician with diagnosis.
The last diagnostic fault code to occur is identified, specific ABS data is stored at the time of this fault, and the first five codes set are stored. Additionally, using a bidirectional scan tool, each input and output can be monitored, thus enabling fault confirmation and repair verification. Manual control of components and automated functional tests are also available when using a GM approved "Scan" tool. Details of many of these functions are contained in the following sections.
Enhanced Diagnostic Information, found in the CODE HISTORY function of the bidirectional scan tool, is designed to provide the service technician with specific fault occurrence information. For each of the first five (5) and the very last diagnostic fault codes stored, data is stored to identify the specific fault code number, the number of failure occurrences, and the number of drive cycles since the failure first and last occurred (a drive cycle occurs when the ignition is turned ON and the vehicle is driven faster than 10 mph). However, if a fault is present, the drive cycle counter will increment by turning the ignition ON and OFF . These first five (5) diagnostic fault codes are also stored in the order of occurrence. The order in which the first 5 faults occurred can be useful in determining if a previous fault is linked to the most recent faults, such as an intermittent wheel speed sensor which later becomes completely open.
During difficult diagnosis situations, this information can be used to identify fault occurrence trends. Does the fault occur more frequently now than it did during the last time when it only failed 1 out of 35 drive cycles- Did the fault only occur once over a large number of drive cycles, indication an unusual condition present when the fault occurred- Does the fault occur infrequently over a large number of drive cycles, indication special diagnosis techniques may be required to identify the source of the fault-
If a fault occurred 1 out of 20 drive cycles, the fault is intermittent and has not reoccurred for 19 drive cycles. This fault may be difficult or impossible to duplicate and may have been caused by a severe vehicle impact (large pot hole, speed bump at high speed, etc.) that momentarily opened an electrical connector or caused unusual vehicle suspension movement. Problem resolution is unlikely, and the problem may never reoccur (check diagnostic aids proved for that code). If the fault occurred 3 out of 15 drive cycles, the odds of finding the cause are still not good, but you know how often it occurs and you can determine whether or not the fault is becoming more frequent based on an additional or past occurrences visit if the source of the problem can not or could not be found. If the fault occurred 10 out of 20 drive cycles, the odds of finding the cause are very good, as the fault may be easily reproduced.
By using the additional fault data, you can also determine if a failure is randomly intermittent or if it has not reoccurred for long periods of time due to weather changes or a repair prior to this visit. Say a diagnostic fault code occurred 10 of 20 drive cycles but has not reoccurred for 10 drive cycles. This means the failure occurred 10 of 10 drive cycles but has not reoccurred since. A significant environmental change or a repair occurred 10 drive cycles ago. A repair may not be necessary if a recent repair can be confirmed. If no repair was made, the service can focus on diagnosis techniques used to locate difficult to recreate problems.