For 1990-2009 cars

(1) | Reservoir
(2) | Master Cylinder
(3) | High Pressure Accumulator
(4) | Normally Open Valve
(5) | Normally Closed Valve
(6) | Left Front
(7) | Right Rear
(8) | Pump
(9) | Low Pressure Accumulator
(10) | Left Rear
(11) | Right Front

The purpose of the MGH-25 antilock brake system (ABS) is to minimize wheel slip during heavy braking. MGH-25 performs this function by monitoring the speed of each wheel and controlling the brake fluid pressure to each wheel independently during a braking event. This allows the driver to retain directional stability and better steering capability.

### 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 potential, voltage, current, amperes, and resistance, ohms. You should understand what happens in a circuit with open or shorted wire. You should be able to read and understand a wiring diagram.

• | Use of circuit test tools--You should be familiar with a digital multimeter (DMM), and be familiar with the meter controls and how to use them correctly. You should be able to measure voltage, resistance, and current. You should know how to use jumper wire to bypass components for testing circuits.

### Antilock Brake System (ABS) System Components

Caution: Refer to ABS Component Handling Caution in the Preface section.

The MGH-25 ABS consists of a conventional hydraulic brake system plus antilock components. The conventional brake system includes a vacuum booster, master cylinder, front disc brakes, rear drum brakes, interconnecting hydraulic brake pipes and hoses, brake fluid level sensor, and the brake warning lamp indicator.

The antilock brake system (ABS) components include a hydraulic unit, an electronic brake control module (EBCM), system fuse, four wheel speed sensors, one at each wheel, interconnecting wiring, ABS indicator, the de-coupled dynamic rear proportioning (DDRP) indicator which is connected to the parking lamp, and the rear drum brakes. For the general layout of this system, refer to Antilock Brake System Component Views .

The hydraulic unit with the attached EBCM is located between the surge tank and the fire wall on the left side of the vehicle.

The basic hydraulic unit configuration consists of hydraulic check valves, two solenoid valves for each wheel, a hydraulic pump, two accumulators, and two dampers. The hydraulic unit controls hydraulic pressure to the front calipers and the rear wheel cylinders by modulating hydraulic pressure to prevent wheel lockup.

### Base Braking Mode

(1) | Reservoir
(2) | Master Cylinder
(3) | High Pressure Accumulator
(4) | Normally Open Valve
(5) | Normally Closed Valve
(6) | Left Front
(7) | Right Rear
(8) | Pump
(9) | Low Pressure Accumulator
(10) | Left Rear
(11) | Right Front

The baseline braking mode of the MGH-25 antilock brake system (ABS) used in this vehicle is a diagonal split system. In this system, one master cylinder circuit supplies pressure to the right front and the left rear brakes. The other circuit supplies pressure to the left front and the right rear brakes. All valves in the hydraulic modulator are in their normal, non-energized positions as shown in the drawings found the hydraulic diagram.

### Antilock Braking Mode - Apply

(1) | Reservoir
(2) | Master Cylinder
(3) | High Pressure Accumulator
(4) | Normally Open Valve
(5) | Normally Closed Valve
(6) | Left Front
(7) | Right Rear
(8) | Pump
(9) | Low Pressure Accumulator
(10) | Left Rear
(11) | Right Front

If during the pressure hold or pressure decrease mode the electronic brake control module (EBCM) senses that wheel slip has reduced, the EBCM will increase the pressure to the affected wheels by applying master cylinder pressure. The inlet valve is opened and the outlet valve is closed, now base brake master cylinder pressure can be applied to the wheel.

### Antilock Braking Mode - Hold

(1) | Reservoir
(2) | Master Cylinder
(3) | High Pressure Accumulator
(4) | Normally Open Valve
(5) | Normally Closed Valve
(6) | Left Front
(7) | Right Rear
(8) | Pump
(9) | Low Pressure Accumulator
(10) | Left Rear
(11) | Right Front

When the electronic brake control module (EBCM) senses the wheel slip, the EBCM closes the inlet valve and keeps the outlet valve closed in the brake pressure modulator valve in order to isolate the system. This holds the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.

### Antilock Braking Mode - Release

(1) | Reservoir
(2) | Master Cylinder
(3) | High Pressure Accumulator
(4) | Normally Open Valve
(5) | Normally Closed Valve
(6) | Left Front
(7) | Right Rear
(8) | Pump
(9) | Low Pressure Accumulator
(10) | Left Rear
(11) | Right Front

If during the pressure hold mode the electronic brake control module (EBCM) still senses wheel slip, the EBCM will decrease the pressure to the affected wheels. The inlet valve is left closed and the outlet valve is opened. The excess fluid/pressure is temporarily stored into an accumulator within the brake pressure modulator valve (BPMV), until the pump can return the fluid to the master cylinder reservoir.

### De-Coupled Dynamic Rear Proportioning (DDRP)

De-coupled dynamic rear proportioning (DDRP) is a proportioning system to maintain vehicle stability during braking. In normal braking condition, equal wheel speed should be maintained for efficient and balanced braking. In hard braking conditions, a vehicle requires relatively less brake force at rear wheel due to vehicle's weight transfer to the front. DDRP maintains desired brake pressure to the rear wheel by using the antilock brake system (ABS) rear inlet and outlet valve in order to provide efficient braking and vehicle stability. In the DDRP system, the power to the rear hold valve solenoid is provided from the ignition. If the following fault conditions are existing, the red brake warning lamp will be illuminated.

• | Two wheel speed sensors inoperative on same axle

• | Rear inlet solenoid inoperative

• | Battery 2, motor input, short to ground

• | Battery 1, electronic brake control module (EBCM) input, open or short to the ground

• | Motor ground open or short to battery

• | EBCM ground open or short to battery

• | Ignition open or short to ground

#### De-Coupled Dynamic Rear Proportioning (DDRP) Failure Modes

Part | Hypothesized Failure | De-Coupled DRP
--- | --- | ---
ABS Warning Lamp | Brake Warning Lamp | D-DRP Status
Sensor RF | Short or Open | ON | -- | Degraded
Sensor LF | Short or Open | ON | -- | Degraded
Sensor RR | Short or Open | ON | -- | Degraded
Sensor LR | Short or Open | ON | -- | Degraded
2 Sensors, Same Axle | Short or Open | ON | ON | Disabled
1 Front and 1 Rear Sensor | Short or Open | ON | -- | Degraded
Motor | Short to Ground - LOW Side | ON | -- | Degraded
Short to Ground - HI Side | ON | -- | Degraded
Short to Battery - LOW Side | ON | -- | Degraded
Short to Battery - HI Side | -- | -- | None
Motor Circuit Open | ON | -- | Degraded
Motor Stalled | ON | -- | Degraded
Front Apply Solenoids | Short or Open | ON | -- | Degraded
Front Release Solenoids | Short or Open | ON | -- | Degraded
Rear Apply Solenoids | Short or Open | ON | -- | Degraded
Rear Release Solenoids | Short or Open | ON | ON | Degraded
System Relay | Open, Unable to Turn ON | ON | -- | Degraded
Shorted On, Unable to Turn OFF | -- | -- | None
Battery 2, Motor | Short to GND | ON | ON | Degraded
Open | -- | -- | Low Voltage
Ground, Motor | Open or Short to Batt | ON | -- | Enabled
Battery 1, EBCM, Solenoids | Open or Short to GND | ON | ON | Disabled
Ground, EBCM, Solenoids | Open or Short to Batt | ON | ON | Disabled
Ignition | Open or Short to GND | ON | ON | Disabled
Brake Switch | Not Applicable | -- | -- | --
Serial Communication | Open or Short | ON | ON | Enabled

### Electronic Brake Control Module (EBCM)

The electronic brake control module (EBCM) performs the following primary functions in order to provide efficient braking and vehicle stability. In the de-coupled dynamic rear proportioning (DDRP) system, power to the rear hold valve solenoid is provided from the ignition. If the following fault conditions are existing, the red brake warning lamp will be illuminated.

• | Monitor wheel speed sensor inputs

• | Detect wheel slip tendencies

• | Control the brake system while in the antilock control mode.

• | Monitor the system for proper electrical operation.

The EBCM continuously checks the speed of each wheel to determine if any wheel is beginning to slip. If any wheel slip tendency is detected, the EBCM commands appropriate valve positions to modulate brake fluid pressure in some or all of the hydraulic circuits to prevent wheel slip and provide optimum braking. The EBCM continues to control pressure in individual hydraulic circuits until a slipping tendency is no longer present. Also the EBCM continuously monitors the antilock brake system (ABS) for proper operation. If the EBCM detects an error, it can disable the ABS function and turn ON the ABS warning lamp in the instrument cluster. The EBCM also controls the display of the ABS DTCs while in diagnostic mode.

### Solenoid Relay

The solenoid relay provides power to the pump motor and solenoids. The switch in the relay is normally open, but is commanded to close during initialization. The relay switch will remain closed for the remainder of the drive cycle as long as no diagnostic trouble codes (DTCs) set which require the switch to open. If a DTC sets which requires the relay to be commanded OFF, battery voltage will be removed from the pump motor and solenoids for the remainder of the current drive cycle and antilock brake system (ABS) cannot function. The relay is an integral part of the electronic brake control module (EBCM) and cannot be serviced separately.

### Wheel Speed Sensors and Rings

A wheel speed sensor is present at each wheel. The sensors transmit wheel speed information to the electronic brake control module (EBCM) by means of a small alternating current (AC) voltage. The signal is transmitted to the EBCM through interface that can cause false or noisy wheel speed sensor input to the EBCM.

### Antilock Brake System (ABS) Warning Lamp

The antilock brake system (ABS) warning lamp is located in the instrument cluster and will illuminate if a malfunction in the ABS is detected by the electronic brake control module (EBCM). The ABS warning lamp informs the driver that a condition exists which results in turning OFF the antilock brake function. If only the ABS warning lamp is ON, normal braking with full power assist is available.

Conditions for the ABS warning lamp to turn ON are as follows:

• | ABS malfunction detected--As previously described, the ABS warning lamp turns ON when a problem has been found in the ABS.

• | Instrument panel cluster bulb check--When the ignition is turned to ON, the ABS warning lamp will turn ON for approximately 3 seconds and then turn OFF.

### Brake Warning Lamp

The red brake warning lamp is in the instrument cluster and will illuminate to warn the driver of abnormal conditions in the brake system which may result in reduced braking ability. The lamp will illuminate when the parking brake is applied or not fully released, or if the brake fluid level switch is closed. Closed is when the brake fluid is low in the master cylinder reservoir. When the brake fluid level switch is closed, low condition, a brake warning lamp will stay illuminated until the condition has been repaired. Also some failure modes in MGH-25 system will illuminate the lamp to let the driver know de-coupled dynamic rear proportioning (DDRP) is disabled.