See Figure 1
The Teves Mark IV ABS system is used on 1991-93 vehicles covered by this information. The system allows individual modulation of braking force at each wheel as well as providing traction control at the drive wheels if the vehicle is so equipped. The ABS system employs a conventional diagonally-split braking system and additional ABS components. The system allows the operator to maintain directional control during heavy braking situations by preventing locked-wheel skidding.
On vehicles equipped with the Traction Control System (TCS), wheel spin at the drive wheels is reduced by selective application of the wheel brakes if vehicle speed is below 25 mph (40 km/h). The TCS system is designed to improve traction on loose or wet surfaces and aid in getting the vehicle moving from rest.
Braking is performed by the conventional brake system until the Electronic Brake Control Module (EBCM) detects a wheel beginning to lock, or the brake pedal has traveled beyond a specified limit. The EBCM will then command the system into anti-lock mode.
Electric signals are received by the EBCM from each wheel speed sensor. By comparing these signals to each other and to pre-programmed values, the EBCM can determine that one or more wheels is approaching lock-up. The EBCM activates the Pressure Modulator Valve (PMV) to control fluid pressure to the wheels. The PMV assembly has 2 valves per hydraulic circuit; the EBCM operates these valves electrically to control brake pressure. The valves may be placed into 3 modes: pressure increase, pressure reduce or pressure hold. By switching rapidly between these 3 modes, the EBCM can reduce pressure at the locking wheel, slow the most rapid wheel to the average or maintain the wheels just below the point of lock-up.
During the pressure increase mode, the inlet valve within the PMV is open, allowed to enter the particular hydraulic circuit. As this mode cycles, pressure and fluid is reduced within the master cylinder, causing the brake pedal to drop nearer the floor. When the pedal travel exceeds a pre-determined level, the EBCM will activate the PMV pump motor. Pump pressure is applied to the wheel circuit(s) in need and/or the master cylinder hydraulic circuits. With pressure applied to the master cylinder hydraulic circuits, the pedal will rise until the brake travel switch is de-activated, switching the pump off.
During vehicle operation, the EBCM performs a series of self-tests, checking both electronic and hydraulic systems for vaults. If detectable problems exist, the EBCM will set a fault code and/or illuminate the ANTI-LOCK dash warning lamp. The ANTI-LOCK warning lamp is in addition to the normal BRAKE warning lamp which warns of problems in the conventional brake system.Traction Control System (TCS)
The traction control system is additional to the ABS system and is not found on all vehicles with ABS. With traction control, brake pressure is increased to reduce wheel spin; with ABS, line pressure is reduced to allow greater wheel spin. The EBCM monitors the speed of the drive wheels through the wheel speed sensors; if wheel slip is detected below 25 mph with the brakes not applied, the EBCM enters the traction control mode.
Once in traction control mode, the EBCM commands two isolation valves in the PMV into operation. The isolation valves close to separate the non-driven wheels and the master cylinder circuits from the drive wheels. Once isolated, brake fluid pressure may be applied to the drive wheels without affecting the other circuits. The EBCM turns on the PMV pump motor and begins cycling the inlet and outlet valves to slow the slipping wheel(s).
Since the pump motor supplies more fluid than required, two pressure relief valves allow excess fluid to return to the master cylinder reservoir. If the brakes are manually applied by the operator during traction control operation, the PMV pressure switch and the brake pedal switch signal the EBCM to disable traction control and allow manual braking. Additionally, manual braking can override the traction control system at any time; the isolation valves act as one-way check valves, allowing manually-applied brake fluid pressure to enter the system.
Vehicles equipped with TCS are also be equipped with a manual switch to disable the traction system. When the switch is engaged, canceling system operation, a TRACTION OFF dashboard warning lamp will be illuminated. Some vehicles also have indicators showing that the TCS is engaged, either by lighting a dashboard lamp or displaying a TRACTION ENGAGED message.
Electronic Brake Control Module (EBCM)
Located on the inner firewall, the EBCM is a solid-state computer which monitors the speed of each wheel as well as the electrical status of the PMV assembly. Once wheel lock or wheel slip (with TCS) is detected, the EBCM operates the inlet and outlet valves in the PMV to control wheel speed. The valves can be cycled very rapidly, allowing control to be maintained in all operating situations.
The EBCM will disable either the TCS or ABS if a fault is detected. In most cases, the system will remain disabled for the remainder of the driving cycle, even if the fault self-corrects before the next key-on cycle. If either low system voltage is detected at the EBCM or low brake fluid level is detected at the PMV, the systems(s) will be disabled until the condition is corrected. Once the sensors indicate normal voltage or fluid level, the system(s) are enabled, even if the vehicle has not been shut off and restarted.
If the control unit detects an electrical fault, it will disable the TCS and/or ABS system immediately and illuminate the appropriate dash warning lamp to advise the operator. A fault code will usually be set in memory. Although it is possible for just the TCS system to be disabled, any fault in the ABS system will also cause the TCS system to be disabled.
The fault codes may be retrieved when the EBCM is placed into the diagnostic mode. A bi-directional hand scanner is required to retrieve or clear stored codes; no warning lamp flash outputs are used.Wheel Speed Sensors
A wheel speed sensor at each wheel transmits speed information to the EBCM by generating a small AC voltage relative to the wheel speed. The voltage is generated by magnetic induction caused by passing a toothed sensor ring past a stationary sensor. the signals are transmitted through a pair of wires which are shielded against interference. The EBCM then calculates wheel speed for each wheel based on the frequency of the AC signal received from the sensor.Warning Lamps
See Figure 2
Certain models equipped with traction control may cause a pilot lamp to light or a message to be displayed when the system is engaged. These signals are a convenience to the operator but are not system warning lamps.BRAKE
The red BRAKE warning lamp in the instrument panel illuminates to alert the operator of conditions in the conventional braking system which may affect braking or vehicle operation. The lamp will illuminate when the parking brake is applied, the brake fluid in the master cylinder reservoir is low or when the ignition is switched to START , allowing the function of the bulb to be confirmed. Illumination of this lamp during vehicle operation signals a potentially hazardous condition within the brake system.ANTI-LOCK
The amber ANTI-LOCK dashboard warning lamp is lit when the EBCM detects a fault within the ABS system. When this lamp is lit, the ABS system is disabled. If only the ANTI-LOCK lamp is lit, full power-assisted braking is available without the added benefit of the anti-lock system.
This warning lamp applies specifically to the traction control system. It will light if the EBCM detects a problem specific to the TCS or if the operator has switched the system off manually.
On vehicles equipped with traction control, the EBCM is programmed to prevent brake overheating. During heavy braking conditions, if traction control mode is entered and the controller determines that the brakes are too hot to function properly, the TCS is disabled and the warning lamp is lit. After a period of time, usually 15-20 minutes, the system is enabled and the warning lamp extinguished. This protective behavior is normal operation and should not be mistaken for an intermittent system failure.
Additionally, the EBCM monitors the signal from the transaxle temperature switch. If transaxle temperature exceeds approximately 320°F (11°C), the controller will disable the traction system, allowing both the brakes and the transaxle to cool. When the temperature switch closes again, at approximately 300°F (140°C), the EBCM will keep the system disabled for an additional 3-5 minutes to allow further cooling. The dash warning lamp will be illuminated during the period of disablement.TCC/ABS Brake Switch
This normally closed switch is integral to the normal electrical/vacuum-release brake switch. It monitors position of the brake pedal, opening when the pedal exceeds 40 percent of total travel. The EBCM then turns on the pump in the pressure modulator valve assembly, circulating fluid and forcing the pedal upward until the switch closes.Fluid Level Sensors
The brake fluid reservoirs on both the master cylinder and pressure modulator valve contain fluid level sensors. If the master cylinder reservoir becomes low on fluid, the red BRAKE warning lamp will be lit, signaling a potential system hazard. If the fluid level in the PMV reservoir becomes too low, the amber ANTI-LOCK warning lamp is lit and the ABS/TCS system disabled until the level returns to minimum.Pressure Modulator Valve (PMV) Assembly
The assembly consists of the pump and motor, the valve body and the fluid reservoir. The reservoir provides fluid to the pump while the pump circulates the fluid to the master cylinder and maintains pedal height. The valve block contains the inlet and outlet valves for each wheel circuit.
When equipped with traction control, the PMV contains the isolation valves used to block pressure in the master cylinder and rear wheel circuits. A pressure switch within the PMV detects operator application of the brakes during TCS operation; when this signal is received, the EBCM immediately disables TCS function and allows manual braking.
The PMV is not serviceable except for replacement of the reservoir.Proportioner Valves
Each rear wheel circuit contains a proportioning valve to limit brake pressure to the non-driven wheels. These valves improve front-to-rear brake balance during normal braking.