Both FWD and AWD systems monitor and compare wheel speed based on the inputs from the wheel speed sensors. The brake pressure is controlled according to the impending lock-up computations of the ABS control unit.
On the FWD vehicles using a 3-channel system, if either front wheel approaches lock-up, the controller actuates the individual solenoid for that wheel, reducing pressure in the line. Impending lock-up at either rear wheel will engage the rear control solenoid; hydraulic pressure is reduced equally to both rear wheels, reducing the tendency of the rear to skid sideways under braking.
The AWD vehicles incorporate a G-sensor into the system to give the ABS control unit an acceleration signal; this is used in conjunction with engine rpm and wheel speed signals to determine high or low friction road conditions. If the system detects impending wheel lock at, for example, the left front wheel, the solenoid controlling the left front/right rear circuit is activated to reduce line pressures. The select-low valve (common to both channels) reacts to the pressure change and reduces the pressure to the other rear wheel. This system overcomes some of the inherent problems of applying ABS to all wheel drive vehicles.
The system incorporates an idle-up control which raises engine speed to 1800 rpm during braking. This eliminates engine braking during ABS-engaged stops and allows the control system to apply maximum stopping effort.
Also found on the AWD is a delay valve which prevents simultaneous front and rear wheel slip by momentarily delaying rear wheel cylinder pressure when ever front wheel line pressure increases sharply. This insures more even application of braking force to the ground and better vehicle control for the operator.
Three separate relays aid the operation of the ABS system. The ABS motor relay (for the pump motor) and the ABS valve relay are located together immediately adjacent to the hydraulic unit under the hood.
The ABS power relay is mounted on a separate bracket next to the anti-lock control unit in the right rear quarter panel. Each of the relays may be replaced in the usual fashion, although care must be taken to release wiring connector clips before removing the relay.
FLAT BATTERY REMEDY
When booster cables are used to start the engine when the battery is completely flat and the vehicle is immediately driven without waiting for the battery to recharge itself, the engine may misfire, and driving may not be possible. This is due to the fact that the ABS consumes a great amount of current for it self-check functions; the remedy is to either allow the battery to recharge sufficiently, or to disconnect the electrical harness connector for the ABS circuit, thus disabling the ABS system. The ABS warning light will illuminate when the connector is disconnected.
After the battery has been sufficiently charged, engage the ABS electrical connector and restart the engine. Check to be sure the ABS warning lamp is not illuminated.
DEPRESSURIZING THE SYSTEM
The ABS system requires no special system depressurization prior to the opening of hydraulic lines or bleeding of the system.