Anti-Lock/Traction Control: Description

NOTE: Repair information for CL500, E300TD or ML320, is very limited or not available from manufacturer.

CAUTION: See ANTI-LOCK BRAKE SAFETY PRECAUTIONS article in GENERAL INFORMATION.

NOTE: Anti-Lock Brake System (ABS) may be integrated with possible combinations of Acceleration Slip Regulation V (ASR V), Brake Assist System (BAS), Electronic Stability Program (ESP) or Electronic Traction Support (ETS) on E320 AWD models and may be referred to as Traction Systems. Due to combined ASR, BAS, ESR and ETS warning light, check combined systems for DTCs.

The Bosch-designed Anti-Lock Brake System (ABS) uses a hydraulic unit with 3 fast-switching solenoid valves, 3 or 4 speed sensors, an electronic control unit with a wire harness with relays. Electronic control module monitors wheel speeds for all systems. Yellow ABS warning light comes on as ignition is turned on and goes out when engine is running. The ABS system is programmed to activate after 7.5 MPH. See Fig 1-8.

When vehicle speed is more than 3 MPH, Built-In Test Equipment (BITE) checks ABS. If any fault is found, ABS warning light comes on again. When ABS warning light goes on, ABS is switched off and conventional brake system takes over.

Acceleration Slip Regulation V (ASR V) and ABS hydraulic components are combined in hydraulic unit. ASR V will operate at all speeds. If one or both wheels lose traction, hydraulic pressure will be applied to appropriate brake calipers and engine torque is limited until vehicle has regained control. A Yellow driver warning light in speedometer will flash during loss of traction. Most ASR V functions can be switched OFF using ASR V control switch mounted on center console. Traction control will then engage only between 24 and 50 mph.

Brake Assist System (BAS), if equipped, is located in BAS brake booster, is designed to compliment ABS system by maximizing vehicles braking capacity during emergency braking. BAS solenoid valve is actuated by BAS or ESP control module and opens a poppet valve allowing atmospheric pressure to back side of the rolling diaphragm of brake booster increasing brake pressure. Maximum boost is applied to brakes quicker to help reduce braking distances. If ABS and BAS/ETS warning lights illuminate simultaneously, a fault may exist in either system. BAS functions will remain, despite warning light.

Electronic Stability Program (ESP), if equipped, enhances directional control and reduces driving wheel spin of vehicle under any driving condition. Over or under steering of vehicle is counteracted by engine torque reduction as a result of a signal over CAN data bus to ME control module and applying brakes to appropriate wheel to counteract vehicle movement. ESP/BAS warning light will flash indication activation of ESP functions. If BAS/ESP warning light illuminates and remains on while engine is running, a malfunction has been detected in either system. With BAS/ESP warning light illuminated, partial engine output will be available. Most ESP functions can be disabled using ESP control switch mounted on center console switch.

Electronic Traction System (ETS) is used on E320 AWD vehicles. ETS control module receives input from 4 speed sensors and brakelight switch. ETS control module processes input signals into output signals to solenoid valves, self-priming high pressure/return pump, ETS warning light, ABS warning light and ETS or BAS/ETS warning light. Based on input signals, ETS control module uses one of 3 operating modes, normal mode, ABS control mode or ETS control mode.

In normal mode, no wheel speed differences exist which require a control mode. All solenoid valves are de-energized (normal) position. This operating mode is used when ETS and ABS systems are deactivated due to a fault.

In ABS control mode, ETS control module determines if ABS control is needed. Appropriate solenoid valves are triggered by the ABS circuit in ETS control module, releasing hydraulic pressure to appropriate wheel until wheel speed difference does not exist.

In ETS control mode, if one or more wheels loses traction when starting off or accelerating, ETS control module actuates the self-priming high pressure/return pump, switchover solenoid valve, appropriate solenoid valves for rear axle and inlet solenoid valve (Bosch) which then applies hydraulic pressure to corresponding brake caliper until calculated drive slip is reached. Left and right solenoid valves are triggered separately. Operation of the brake pedal interrupts ETS control mode. At 50 mph, ETS switches off at front wheels and at 80 mph, ETS switches off at rear wheels. ETS warning light, located in speedometer, will flash as soon as traction is lost. If BAS/ETS warning light illuminates while ETS warning light flashes (with engine running), ETS is being switched off temporarily on one or more wheels to prevent overheating of brakes. If BAS/ETS warning light illuminates and does not go out, a malfunction may exist in either BAS or ETS system. Check for DTCs in both systems.

If battery voltage is less than 11 volts, when ignition is turned on and test speed exceeded, ABS remains off until alternator increases voltage to more than 11 volts. Warning light will then go out. After repairing ABS system, perform a light test to check system. Ensure light goes out after 7 MPH.

ETS systems prevents driven wheels from spinning during initial drive off and while accelerating within prescribed speed ranges. ETS warning light (Yellow), located at center of speedometer, illuminates if a wheel speed does not correspond to vehicle speed and will remain on for one second after vehicle stability is regained.

NOTE: The entire ABS system must be checked when repair includes any ABS component or if units are replaced following an accident.

NOTE: For more information on brake system, see BRAKE SYSTEM article under BRAKES.

Fig 1: Identifying & Locating ABS/ASR V Components With ETS (C230 & C280 Up To 5-97)