Controller Antilock Brake (CAB)

The Controller - Antilock Brake (CAB) is a small computer that monitors the brake system and controls the system while it functions in antilock mode. The CAB is located under the battery tray and uses a 60-way system connector. The power source for the CAB is through the ignition switch in the RUN or ON position.

NOTE: The Controller - Antilock Brake is NOT on the CCD Bus.

The primary functions of the CAB are:
- to detect wheel locking tendencies
- to control fluid modulation to the brakes while in antilock mode
- to monitor the system for proper operation
- to provide communication to the DRBII while in diagnostic mode

The CAB continuously monitors the speed of each wheel, through the signals generated at the wheel speed sensors, to determine if any wheel is beginning to lock. When a wheel locking tendency is detected, the CAB isolates the master cylinder from the wheel brakes. This is done by activating the isolation valves. The CAB then commands the appropriate build or decay valves to modulate brake fluid pressure in some or all of the hydraulic circuits. The fluid used for modulation comes from the booster servo circuit. The CAB continues to control pressure in individual hydraulic circuits until a locking tendency is no longer present.

The CAB has several useful diagnostic capabilities.
- First, fault codes are retained in non-volatile memory until either erased by the Technician using the DRBII, or erased automatically after 50 ignition cycles (key cycles).
- Second, more than one fault code can be stored at a time. The number of faults stored is displayed by the DRBII when commanded by the Technician. The number of key cycles since the most recent fault was stored is also displayed.
- Third, using the DRBII, most functions of the CAB and the ABS system can be accessed by the Technician for testing and diagnostic purposes.

Start-Up Cycle
The start-up cycle takes place immediately after the ignition switch is turned on. It is an electrical check of some basic electrical functions such as the system relay and the antilock warning light relay. During this check the ANTI LOCK lamp is illuminated and is extinguished at the completion of the test. The total test duration is approximately 1-2 seconds.

Drive-Off Cycle
The drive-off cycle takes place when the vehicle reaches about 3 miles per hour the first time after an ignition reset. During this test, the modulator solenoid valves are activated briefly to test their function.

Latching versus Non-Latching Faults
Some faults detected by the CAB are latching; that is, the fault is set and the ABS is disabled until the ignition switch is reset. Thus, the ABS is disabled even if the original fault has disappeared.

Other faults are non-latching; that is, any warning lights that are illuminated are only illuminated as long as the fault condition exists. As soon as the condition goes away, the light(s) are extinguished, although a fault code will be set in most cases.