From BMW To Jeep, The Teves Mark 20 Is One Of The Most Common ABS Systems On The Road.
Introduced in Europe in 1995, the Teves Mark 20 system was first used on domestic applications in 1997 on Jeep Wrangler, Cherokee & Grand Cherokee, Chrysler Town & Country minivans, Lincoln Mark VIII, Honda Civic and BMW Z3 models. In 1998, it replaced other ABS systems on the Dodge & Plymouth Neon, Chrysler Cirrus, Plymouth Breeze and Dodge Stratus. In 1999, it replaced the Mark 4 system on the Chrysler LHS and Concorde, and was added to the Chrysler 300M and Sebring.
Made by Continental/Teves (Teves was formerly owned by ITT Automotive), the Teves Mark 20 ABS system is the successor to the Teves Mark 4 ABS system, and features a more compact, integrated design that combines the electric pump, solenoid valves and controller into one assembly. The Mark 20 is 40 percent lighter than the Mark 4 system, has traction control capability and is less expensive to manufacture. It is a nonintegral ABS system that can be configured for either three- or four-channel operation on front- or rear-wheel drive vehicles.
TEVES Mark 20 ABS SYSTEM COMPONENTS
The Mark 20 unit consists of a light weight aluminum valve body, that holds the ABS valves and pump, two low pressure accumulators, an electric motor and a control module. The block that houses the valve body is called the "Integrated Control Unit" or ICU.
On Jeep and Chrysler LH applications, the ICU is located on or under the left front fender. On Neons, it is located under the master cylinder. On Chrysler Cirrus and Sebring, Plymouth Breeze and Dodge Stratus models, the ICU is on the right side of the suspension crossmember. On the minivan applications, it is tucked behind the steering rack on the left side of the suspension crossmember.
The solenoids that operate the ABS valves are part of the control module rather than the valves, so there are no external wires from the controller to the ABS valves. The part that houses the control module and solenoids is called the "Controller for Anti-lock Brakes" or CAB.
On applications that have a three-channel ABS setup and split the brake hydraulics front and rear, six ABS valves are used (two for each front wheel circuit, and two for the combined rear brake circuits). On applications that use a four-channel ABS setup and split the hydraulics diagonally (RR & LF, LR & RF), eight ABS valves are used (two for each brake circuit). Two isolation valves are added for applications that have traction control. The Mark 20 system also has a pedal-mounted brake switch. The ABS system will function without input from the switch, but the switch is there to help the ABS system engage as soon as the pedal is depressed.
On applications with traction control, there is a switch to deactivate the system, which is normally enabled any time the ignition is turned on. TRAC ON and TRAC OFF lamps inform the driver of system status. The controller may deactivate the traction control system temporarily after it has been engaged for a certain period of time to prevent heat buildup in the brake linings and possible brake fade. ABS braking is not affected by the status of the traction control function.
TEVES MARK 20 ABS SYSTEM OPERATION
Wheel speed in the Mark 20 applications is monitored by conventional magnetic wheel speed sensors (one at each wheel). The sensors generate an A/C signal, whose amplitude and frequency is proportional to the rotational speed of the wheel. Readings can be adversely affected by metallic debris accumulating on the ends of the sensors, or by an excessive air gap between the sensor and tone ring.
The controller looks for a difference in wheel speeds when braking, and initiates ABS braking if one or more wheels are decelerating at a faster pace than their companions, indicating the wheel is losing traction and is on the verge of locking up.
The ABS inlet and outlet valves in the valve block are pushed open by the solenoids in the controller when ABS braking is needed. When the controller detects that a wheel is about to lock up, it energizes the inlet valve solenoid causing the normally open valve to close. The normally closed outlet valve remains closed, so fluid pressure in the brake line is held steady. This is the "isolation" phase of the ABS braking cycle.
If the wheel is still about to lock up, the controller begins the "decay" phase of the ABS cycle. The inlet valve is still energized to prevent any further pressure from reaching the brake, and the outlet valve is energized to open it so pressure can be released back to the accumulator in the valve body. The pump motor is also energized during the ABS cycle to pump fluid from the accumulator back to the high-pressure side. The decay cycle releases the brake and allows the wheel to regain speed and traction.
The final part of the ABS cycle is the "build" phase. Pressure is reapplied to the brake by de-energizing both the inlet and outlet solenoids so they will return to their normal positions (inlet open, outlet closed). The pump keeps running to route fluid from the accumulator back to the high side of the system. This keeps the brake pedal from dropping. The system continues the isolate-decay-build cycle until ABS braking is no longer needed or vehicle speed drops below three mph.
NOTE: During an ABS stop, the driver must maintain firm pressure on the brake pedal for everything to work properly. If he pumps the pedal, he will deactivate the ABS system and lose the traction the ABS system was trying to restore.
On applications that have traction control, the pump is also used to generate hydraulic pressure as needed to one or both drive wheels to limit wheel spin.
After the completion of an ABS stop, there may be some residual pressure in the brake lines. This residual fluid is returned to the master cylinder in one of three ways depending on the application. On front/rear split applications (Jeep), the pump continues to run for a few seconds. On the diagonally split systems (FWD cars), the ABS outlet valves are opened for a short interval when the vehicle starts to move again so the accumulators can purge. On vehicles with traction control, the isolation valves are cycled to release any residual pressure.
On 1999 Jeep Grand Cherokee models and 2000 model year Neons, a variant of the Mark 20 system called the Mark 20e is used. The system is functionally identical to the Mark 20 except that it uses a different type of wheel speed sensor. This is necessary because the Mark 20e system handles more than ABS and traction control. It also controls front-to-rear brake proportioning during normal braking. This eliminates the need for a separate proportioning valve in the brake system, and is called an "Electronically Variable Brake Proportioning System" or EVBPS.
The advantage of EVBPS is that it can fine-tune rear brake balance to changing load, road and braking conditions. A conventional brake proportioning valve is calibrated to reduce brake pressure to the rear wheels by a fixed amount. When the brakes are first applied, pressure to the front and rear brakes is the same. But as the driver presses harder on the pedal, pressure to the rear brakes is typically reduced by up to 50 percent. This is necessary to prevent the lightly loaded rear wheels from locking up and skidding. But a fixed calibration is always a compromise. It works well enough for normal driving and braking on dry pavement, but may not work so well if a vehicle is heavily loaded or is driven on a wet or slick road.
Some vehicles have rear brake proportioning valves with a mechanical linkage attached to the rear suspension. These are typically used in pickup trucks and minivans because the weight on the rear wheels can change significantly depending if the vehicle is empty or heavily loaded. The linkage reacts to changes in ride height that occur when weight is added to the back of the vehicle. Increasing the load lowers the ride height, moves the linkage and changes the position of a metering valve inside the proportioning valve to increase the amount of pressure routed to the rear wheels when the brakes are applied.
With EVBPS in the Mark 20e system, there is no proportioning valve so brake pressure to both rear wheels is regulated electronically. The controller constantly monitors rear wheel speed and adjusts the amount of pressure to the rear wheels by opening the normally closed rear outlet ABS valves. By changing the frequency and duration at which both rear outlet valves are pulsed open, the controller can regulate rear brake pressure as needed.
MARK 20E WHEEL SPEED SENSORS
To accomplish the EVBPS function, the Mark 20e system requires a special type of wheel speed sensor. The wheel speed sensors used in all other ABS systems produce an alternating current (AC) voltage signal that changes in frequency and amplitude with the speed of the wheel.
The "magneto-resistive" wheel-speed sensors used with the Mark 20e system work like a Hall effect crankshaft position sensor and generate a direct current (DC) digital voltage signal. A 5 volt reference voltage is sent to each sensor by the controller. The resistance of the sensor changes with wheel speed, producing a digital DC speed signal that is much more precise. This allows the controller to detect subtle changes in wheel speed that can be used for the EVBPS brake proportioning function as well as to detect underinflated tires. The air gaps on the Mark 20e magneto-resistive sensors are not adjustable. The factory air gap specification is 0.28 to 1.42 mm.
On the Jeep applications with the Mark 20e system, there is also a special "acceleration switch" (also called a "G-switch") to modify the operation of the ABS system according to traction conditions (important when driving offroad or on slick pavement). The acceleration switch contains three separate mercury switches (G1, G2 & G3), each oriented in a different direction to monitor the vehicle�s motions. The top of the switch housing has an arrow indicator that must face up and point forward. If the housing is misaligned, it can have an adverse effect on the operation of the ABS system.
Under normal driving conditions, all three G switches are normally closed. Switch G1 and G2 monitor forward deceleration, while G3 monitors rearward deceleration. If a certain G force is exceeded, it will open one or more switches, causing the ABS controller to select the appropriate control program for best braking.
TEVES MARK 20 ABS DIAGNOSTICS
When the ignition is turned on, the ABS controller receives a "wake-up" call and illuminates the warning lamps for a few seconds to check the bulbs. It also checks continuity with all four wheel-speed sensors, and checks its own internal software. If any problems are detected, a fault code is stored in memory and the ABS system is disabled.
If a hard fault is detected while the vehicle is being driven, a fault code will be stored in memory, the ABS warning lamp will come on and the ABS system will be disabled.
To diagnose the system, a scan tool is required (DRB III or equivalent). Fault codes are displayed as messages on the tool. Each message will indicate a particular circuit or component. You must then refer to the appropriate diagnostic chart in a service manual to isolate the faulty component in the circuit or confirm the fault.
Normal bleeding procedures are the same with the Mark 20 and 20e systems � unless air has entered the master cylinder or ABS unit. This requires a special bleeding procedure using a DRB III scan tool or equivalent.
The procedure is as follows:
1. Bleed foundation brakes (bleed each line using the sequence recommended for the vehicle).
2. Run the "Bleed Brakes" program on the scan tool. This will cycle the ABS valves in the ICU allowing any air that is trapped inside to be purged through the brake lines.
3. Bleed the foundation brakes again.