BMW 318/325/M3/525/535/M5 1989-1993 Repair Information

Description & Operation


One of the greatest contributions to automotive safety was the advent of anti-lock braking systems. Anti-lock braking systems (ABS) allows maintaining directional control of the vehicle during braking. While benefits from ABS can be derived on dry pavement driving, the most substantial benefits are witnessed under adverse traction conditions.

Braking systems operate on the principle that motion energy is removed from the vehicle in the form of heat and dissipated. The brake calipers squeeze the brake pads against the rotors and slow the rotors. This does not stop the vehicle; the friction of the road surface against the tires is what actually slows the vehicle. The brakes merely provide the retarding force for the tires. If the tires can not maintain a level of traction with the road surface, the best braking system can not slow the vehicle.

If during braking, 1 or more tires hit a section of low traction, the braking force applied by the calipers will overwhelm the available traction at the tire contact patch. As a result the tire will slide instead of roll. If we look at the contact patch of the tire as the car rolls down the road, we would see that the tire has a relative speed of zero compared to the ground. Under braking the relative may increase so there is a slight percentage of slip between the tire and the road surface. A small percentage of slip is acceptable and friction force will rise, slowing the car. If the percentage rises too high and the tire is no longer rolling, the friction force drops tremendously and the tire can not provide lateral or longitudinal traction.

Driving in the wet or snow, loose gravel or sand, or any other kind of low traction surface can cause the tires to lock and loose directional stability. ABS monitors the rotation of the tires and compares the speed of each. If the speed of 1 or more tires drop drastically below that of the others during braking, the ABS controller will cut hydraulic pressure to that wheel until it is rotating at the same speed as the others. This will provide the best chance of maintaining directional control of the vehicle.

In the case of the 325iX, four wheel drive vehicle, a deceleration sensor is needed to sense the relative motion of the car. Since the possibility exists that 1 or more wheels can be spinning due to acceleration, not braking, a sensor independent of the wheel speed sensors is needed.

ABS can not perform miracles. If the laws of physics are exceeded, the car can leave the roadway. ABS can only help to maintain control. Go too fast into a turn and mash the brakes, ABS or not, the tires can only do so much and control may be lost. Driving too fast in the rain or snow is a recipe for trouble. ABS is a tool to make driving safer, not a cure-all for bad driving habits.

ABS can be useful in dry ground driving in the same way it is in low traction situations. ABS, once eschewed by the racing world, has now been gladly accepted and used on vehicles ranging from rally cars, World Touring Car Championship M3's and Formula 1 cars.


Speed Sensors

The speed sensors are located at each wheel and provide the speed reference to the control unit. The speed sensors are permanently magnetized inductive sensors that read pulses from a tooth wheel on each hub. A voltage signal is generated as each tooth passes through the magnetic field. The sensors are replaceable. The tooth wheels are integral with the wheel hubs and are replaced with the complete hub.

Control Unit

The control unit contains all the signal conditioning circuitry and the output circuits. The output circuits control the hydraulic unit to adjust the line pressure to each caliper. The unit is located under the dash panel on the left side in the E30 3 Series, above the glove compartment in the E36 3 Series and in the forward position of the electronics box in the engine compartment of the E34 5 Series. If a problem is sensed, the control unit will light the instrument panel warning lamp.

Hydraulic Unit

See Figures 1 and 2

Click image to see an enlarged view

Fig. Fig. 1: Hydraulic unit of the anti-lock brake system

The hydraulic unit, located in the engine compartment contains valves and a pump. The valves have 3 positions; pressure build-up, pressure hold and pressure drop. As the tires locks, the control unit informs the valve to hold the pressure. If the tire remains locked, the control unit will allow the valve to drop the pressure until the tire starts to turn. The control unit will allow the valve to start building pressure to start the cycle over again.

The pump returns the brake fluid taken from the wheel cylinder while the pressure is lowered. The pump is designed to maintain separation of the 2 braking circuits.

During operation of the anti-lock system, a pulsing may be felt at the brake pedal and a clicking heard from the hydraulic unit. This is normal and informs the operator that the ABS is in the functioning mode.


While parts of the of the ABS system can be removed and replaced by the owner of the vehicle, special tool, technique and procedure are needed to check the operation and performance of the system once repairs are completed.

To bleed the hydraulic system, a necessary step after the hydraulic system has been repaired, the BMW Diagnostic Tester or equivalent must be utilized to electronically open the internal valve. Only this way can the system be completely bled.

To check the operation of the ABS system after any of the electronic portions have been replaced or disconnected, the BMW Diagnostic Tester or equivalent must be used.

It is recommended to allow a licensed and trained professional to complete repairs on the ABS system. Most repairs are straightforward, but the diagnosis and testing of the system can enter a different realm. The safety and integrity of the braking system is at stake.