Brakes (ABS ECU): Description and operation

ABS ECU

The ABS ECU controls the operation of the ABS modulator to provide the ABS, ETC and HDC functions. It also operates warning lamps in the instrument pack to provide the driver with status information on each function. The ABS ECU is attached to a bracket below the RH front seat, beneath a protective cover. Incorporated into the ABS ECU are integrated circuits and software for system control and diagnostics. Three electrical connectors interface the unit with the vehicle wiring.

The warning lamps consist of:

• An amber ABS graphic.
• An amber TC graphic.
• Two inclined vehicle graphics for HDC, one amber (fault), which includes an exclamation mark, and one green (information).

The warning lamp bulbs are serviceable items that can be renewed from the rear of the instrument pack.

When the ignition is switched on, the ABS ECU performs a bulb check of the warning lamps as part of the power up procedure. The ABS warning lamp is extinguished briefly, after 1.3 to 2 seconds, then remains illuminated until vehicle speed exceeds 4.4 mph (7 kph). The ETC and HDC warning lamps are extinguished after 4 to 5 seconds. If a fault warning lamp remains illuminated after the lamp check, a fault has been detected and repair action is required.

The ABS ECU continually calculates vehicle speed using the inputs from all four ABS sensors. Vehicle speed is used as a reference against which individual wheel speeds are monitored for unacceptable acceleration or deceleration. Vehicle speed is also output to the instrument pack for the speedometer.

ABS operation

The purpose of ABS is to prevent vehicle wheels locking during brake application, thus maintaining vehicle steerability and stability. This allows the vehicle to be steered while the brakes are applied, even under emergency conditions, and to avoid obstacles where there is sufficient space to redirect the vehicle.

WARNING: ABS is an aid to retaining steering control and stability while braking. ABS cannot defy the natural laws of physics acting on the vehicle. ABS will not prevent accidents resulting from excessive cornering speeds, following another vehicle too closely or aquaplaning, i.e. where a layer of water prevents adequate contact between tyre and road surface. The additional control provided by ABS must never be exploited in a dangerous or reckless manner which could jeopardise the safety of driver or other road users. The fitting of ABS does not imply that the vehicle will always stop in a shorter stopping distance.

NOTE: During normal braking the feel of the brake pedal on vehicles equipped with ABS will be the same as that on non ABS vehicles. During anti-lock braking operation the driver will experience feedback in the form of a pulsating brake pedal and solenoid/pump motor noise from the ABS modulator.

The anti-lock braking function is automatically enabled whenever the ABS modulator is in the normal braking mode.

While the anti-lock braking function is enabled, if the ABS ECU detects a wheel decelerating faster than the average, indicating it is about to lock, it operates the ABS modulator in the ABS braking mode for the affected wheel.