See Figure 1
The Delco Moraine Anti-Lock Brake System (DM ABS-III) was used on 1988-91 vehicles. This system operates on all 4 wheels. The system is designed to reduce the tendency of 1 or more wheels to lock while braking. In most cases, the conventional power brake system of the vehicle performs the braking function; anti-lock braking occurs only when a combination of wheel speed sensors and a microprocessor determines a wheel (or wheels) is about to lose traction during braking. The DM ABS-III then adjusts the brake pressure to both front wheels independently and/or both rear wheels to reduce the tendency of the wheel(s) to lock-up. This system helps the driver maintain control during hard braking on a wide range of road surfaces and driving conditions. The driver can minimize stopping distance and bring the vehicle to a controlled stop. The DM ABS-III cannot increase the brake pressure above the master cylinder pressure and can never apply the brakes by itself.
The DM-ABS III system provides improved braking by regulating the amount of force at any wheel to a value which will prevent locking and by keeping all 4 wheels at or near the same speed during braking. These 2 functions combine to provide the driver with the shortest possible stop while maintaining control of the vehicle.
When the ignition switch is turned ON , the system enters its initial cycle. At this time, a clicking noise may be heard; this is normal and results from the controller cycling the relays and solenoids to check circuitry. While the vehicle is operating, the controller is constantly monitoring the system as well as reacting to brake inputs. If any faults are found, the controller assigns a fault code and stores the code for future retrieval. The amber warning light on the dash will be illuminated when a fault is detected.
The amber ABS dash light will inform the driver of system status. If the light is flashing, the controller has detected a fault but is still allowing the ABS system to operate. If the light is on constantly (no flash) the controller has detected a fault and disabled part or all of the ABS system. The front and rear wheel portions may be individually disabled by the controller. The amber warning light refers only to the ABS system; with ABS disabled, the vehicle will still have conventional braking capabilities if that system is not damaged.
The red brake warning lamp can indicate conditions damaging to the ABS system, such as low fluid. If the red lamp is on with the amber lamp, attend to the conventional brake system before diagnosing ABS. The brake system must be operating properly before ABS diagnosis.
When the brake pedal is depressed, the controller is alerted that the brakes have been applied and monitors the speed of each wheel. If impending lock-up or uneven wheel speeds are detected, the controller cycles the solenoids on and OFF rapidly. This can occur independently on each front wheel or on the rear wheel which begins to lock first. When the solenoids are turned on or OFF, brake hydraulic pressure is applied or released at the each wheel in an attempt to equalize the deceleration rate.
During ABS operation, the system is varying the line pressures very quickly. The driver should not try to assist the system by pumping the brake pedal; hard, firm and continuous application is recommended for best ABS response. During ABS operation, the driver will feel a pulsing in the brake pedal as the line pressures change rapidly. Additionally, the clicking of the solenoids may be heard as well as momentary tire noise (screech or chirp). Both noises are normal for the system. Rapid speed change at a wheel may result in the wheel appearing to lock momentarily, resulting in road noise. Any wheel which locks and stays locked for more than 1 second is not normal; the system should be inspected immediately.
The DM ABS-III is designed only as a skid prevention system. It cannot operate properly if the base power brake system is defective. Dragging brakes, defective wheel bearings, etc. will not allow proper ABS operation.
Anti-lock Brake Controller
Located below the right front seat, the brake controller monitors the speed of each wheel to determine impending lock-up and, when necessary, activates the appropriate solenoid(s) to adjust brake pressures. The controller also monitors the complete system for malfunctions, provides diagnostic information and shuts the system down if a serious fault is detected. The control unit is not serviceable and must be replaced as a complete assembly.Powermaster III Unit
The Powermaster III is an integral power booster and modulator designed to provide both normal power assist and anti-lock braking. Brake fluid pressure for both normal and ABS braking is created and maintained by a combination of an electric motor/pump and an accumulator. The accumulator is pre-charged to approximately 1200 psi (8274 kPa) with nitrogen gas.
The electric pump maintains system pressure between 2200 psi (15,169 kPa) and 2700 psi (18,616.5 kPa). During an ABS stop, fluid to the wheel units is modulated by 3 solenoid assemblies. One solenoid is assigned to each front brake; both rear brakes are controlled by 1 solenoid.Control Solenoids
Each solenoid assembly can apply, hold or release fluid pressure in the line it controls. The left and right front brakes can be controlled individually since each is controlled by its own solenoid. However, since there is a single solenoid for both rear brakes, they are modulated together when either rear wheel approaches lock-up. (Since the rear brake modulation is based on the lowest wheel speed, this type of system is known as a Select Low 3-channel System.) The solenoids are located on the Powermaster III and are controlled by electrical signals from the Brake Controller.Wheel Speed Sensors
The wheel speed sensors generate an AC voltage as a magnetic toothed ring passes a stationary coil. The frequency of this voltage, which increases with wheel speed, is used by the Brake Controller to determine wheel speed. By comparing wheel speeds during braking, the Anti-lock Brake Controller determines impending wheel lock.
At the front, the speed sensor rings are located on the outer CV-joints, directly below the wheel speed sensor. No repair or air gap adjustment to the speed sensor units is possible or permissible.
The rear wheel speed sensors and rings are integral parts of the rear wheel hub and bearing assemblies. Each sensor and ring is self-contained and sealed from the environment. If a sensor or ring requires is damaged or malfunctioning, the entire rear hub and bearing assembly for that side must be replaced. No repair or air gap adjustment is possible or permissible.Proportioner Valve
Proportional control of brake line pressure to the rear brakes takes place only after a preset input pressure has been reached. Above this preset pressure, the valve limits outlet pressure to the rear wheel brakes at a set percentage of the total system output. If malfunctioning, the proportioner valve must be replaced; the separate components of the valve cannot be serviced.Interconnecting Wiring
The interconnecting wiring is made up of 3 specific harnesses:
Two separate jumper harnesses extend from each rear wheel speed sensor to the rear harness through underbody connectors. Additionally, 2 separate jumper harnesses extend from each front wheel speed sensor to the main ABS wiring harness.
The wiring in these jumper harnesses cannot be repaired. If the wiring is damaged, the entire jumper harness must be replaced.
The Powermaster wiring harness can be serviced separately. The main ABS wiring harness also contains and ABS power center. This power center includes the front and rear solenoid enable relays, a 10 amp fuse for the brake controller, a 15 amp fuse for the power feed to the rear wheel control solenoid, a 30 amp fusible link for the front solenoid power feed and a 30 amp fusible link for the Powermaster III pump motor.
The Powermaster III is equipped with connectors using Connector Position Assurance (CPA) locking pins. These pins assure correct alignment and retention when snapped securely. The pin must be removed before separating the connector and always reinstalled after re-connection. Make certain the rubber connector seals are in place on each connector before assembly.Front and Rear Enable Relays
See Figure 2
Located on the ABS power center, these relays are grounded by the brake controller when the system contains no detectable faults. Once the enable relays are grounded, voltage is applied to the solenoids. If the brake controller detects a fault, loses power or loses ground, the controller de-energizes the ground to the front enable relay. The amber light on the dash will come on to indicate loss of ABS function. The controller may detect a fault in the rear circuits and disable only the rear; if this occurs the amber light will not come on.Brake Switch
The brake pedal switch signals the brake controller that the brakes are in use. The switch must be functioning and properly adjusted for the ABS to perform correctly.Brake Warning Indicator
The red brake warning lamp will light after the ignition switch is in the RUN position to indicate that any or all of the following conditions exist:
Each of these conditions can damage the brake and ABS systems or reduce there efficiency. If the red light remains on after starting, the vehicle should be checked as soon as possible.
The amber Anti-lock warning lamp will light after the ignition switch is in the RUN position to indicate that either of the following conditions exist:
Both the red and amber lights can be tested for bulb function; they should be lit when the ignition switch is turned to START . Additionally, the amber anti-lock warning lamp should light for approximately 3 seconds after the ignition switch is turned to RUN while the system undergoes initialization.