Control Functions
ABS, EBD and TRACSControl functions
ABS warning light
This is located on the instrument panel (dashboard) and is activated if the ABS system control module detects a fault in the ABS system. When the ignition is switched on or the engine starts, the control module switches the warning light on. The light then goes out after approx. 2 seconds if no fault is detected. It the warning light is on, the ABS system is not working but normal braking power is still available~ The warning light flashes when the ABS system DTM is activated.
Warning! ABS and EBD are not available while Volvo Diagnostics is activated Take great care when test driving as there is a serious risk of the wheels locking when braking
When the control module DTM is disengaged, the warning light comes on for approx. 2 seconds and then goes out if no fault is detected.
The warning light is controlled by a relay on the instrument panel (dashboard). When the ABS control module grounds the signal to the relay, the relay breaks the light's ground and the light goes out. When the ABS control module increases the voltage over approx. 7V, the relay opens and the light comes on.
TRACS warning light
This is located on the instrument panel (dashboard) and comes on if TRACS is disengaged via the switch, if there is a fault in the TRACS system (when the brake discs on the front wheels are in danger of overheating) or if there is a fault in the ABS system. When the ignition is switched on or the engine starts, the control module switches the warning light on. The light then goes out after approx. 2 seconds if no fault is detected.
When the control module DTM is disengaged, the warning light comes on for approx. 2 seconds and then goes out.
If the rear wheels lock while TRACS is engaged (e.g. if the parking brake is applied), the control module will disengage TRACS and switch to ABS. The TRACS warning light may come on and then go out when ABS is disengaged.
Hydraulic unit
The hydraulic unit (1) is integrated with the control module and is located in the engine compartment. The hydraulic unit consists of a hydraulic pump (2) of the eccentric piston type and an electric motor (3) in a valve block with inlet valves (4) and outlet valves (5) and a low-pressure accumulator (6). The electric coils for the inlet and outlet valves are located in the control module.
The hydraulic unit must not be taken apart for repairs, but must always be replaced as a complete unit.
The hydraulic pump, which is operated by a 12V DC motor, supplies brake fluid to the brake circuits when ABS/TRACS is in use.
The pressure in the brake circuits when using ABS and EBD is governed by the pressure in the master cylinder, and is directly proportional to the force the driver applies to the brake pedal. The hydraulic unit is oversized so that it can always deliver the necessary flow and pressure.
The control module controls, via the inlet and outlet valves and the hydraulic pump, the pressure to the different brake calipers when braking under ABS, EBD or TRACS.
The valve block has three internal circuits, one for the LH front wheel, one for the RH front wheel and a common circuit for the rear wheels.
The front wheels are controlled individually, while the rear wheels are under common control, depending on which wheel threatens to lock first.
The valve block has six valves for controlling the brake pressure, three inlet valves and three outlet valves. So, each front wheel has an inlet valve and an outlet valve, while the rear wheels have a common inlet valve and outlet valve between them. In stand-by mode, the inlet valves are open and the outlet valves closed. The outlet valves act as non-return valves between the brake calipers and the hydraulic pump.
The valve block has two low-pressure accumulators, one for the primary circuit (1) and one for the secondary circuit (2).
The accumulators are located between the outlet valves and the hydraulic pump and quickly absorb the brake fluid returning from the brake calipers when using ABS, EBD or TRACS and which the hydraulic pump cannot suck back. This ensures that the brake calipers can release quickly and that the wheels can speed up quickly.
Cars fitted with TRACS have a different hydraulic unit. The difference is that:
The brake pipe (1) between the master cylinder primary circuit and the hydraulic unit is larger in diameter. This gives the brake fluid pipe more capacity and ensures that the hydraulic pump has sufficient brake fluid when using TRACS.
The valve block has an extra inlet valve (2) (TRACS inlet valve) which closes the connection between the hydraulic pump and master cylinder primary circuit when TRACS is activated. The valve is electrically operated and is normally open.
The valve block has an extra hydraulic valve (3) (TRACS valve) which closes the connection between the master cylinder primary circuit and the hydraulic pump inlet side when the brake pedal is depressed and pressure builds up in the primary circuit. The valve is hydraulically operated and normally open.
The valve block has an overflow valve (4). If the hydraulic pump supplies more brake fluid than is normally required for control purposes, the excess brake fluid is fed back to the hydraulic pump inlet side via the overflow valve. The valve block has an overflow valve (4). If the hydraulic pump supplies more brake fluid than is normally required for control purposes, the excess brake fluid is fed back to the hydraulic pump inlet side via the overflow valve. The overflow valve also controls the maximum pressure in the system under TRACS control.
ABS
When braking, the control module (4) receives a signal from the brake light switch (5) that braking has started. The control module then switches to stand-by mode for ABS control. The signal from the brake light switch is not necessary for ABS control, but makes for gentler adjustment when ABS first cuts in.
A wheel sensor (1) by each wheel sends signals to the ABS control module. The control module uses these signals to calculate the car's reference speed. If there is a risk of the wheels locking, the control module triggers the ABS system hydraulic unit (4) and the hydraulic pressure to the wheel concerned is adjusted to avoid the wheel locking.
The pressure in the circuit is controlled so that the maximum possible braking effect is transmitted to the rod, which occurs when the wheel is rotating at approx. 20% slippage relative to the road surface.
The ABS system does not engage until the car's road speed exceeds approx. 7 km/h (4.4 mph) (i.e. there is nothing to stop the wheels locking under approx. 7 km/h (4.4 mph)).
ABS can be divided into three different stages:
1. Maintaining brake pressure. (Constant pressure)
The wheel is threatening to lock. The control module closes the inlet valve to the circuit to prevent the brake pressure increasing any further. If the wheel still continues to slow down, the brake pressure must be increased.
2. Reducing the brake pressure. (Pressure dumping)
The control module opens the outlet valve to reduce the brake pressure and allow the wheel to speed up. If the wheel starts turning too quickly, the brake pressure must be increased.
3. Increasing brake pressure. (Pressure build-up)
The control module closes the outlet valve and opens the inlet valve, increasing the pressure and slowing the wheel down.
Steps 1, 2 and 3 are then repeated until braking ends or ABS cuts out.