- Joined
- January 26, 2004
- Messages
- 6,113
- Reaction score
- 22
- City, State
- Mechanicsville, Virginia
- Year, Model & Trim Level
- 2004 Acura TL
The ABS System
Antilock Brake Systems (ABS) and traction and stability control systems are rapidly growing. Most vehicles today have ABS as standard equipment. These systems add yet another group of electronically controlled systems to the increasingly complex modern vehicle.
Modern ABS can be discribed as electronic/hydraulic pumping of the brakes for straight-line stoppping under panic conditions. Good drivers are use to pumping the pedal to avoid lockup and control loss, but now ABS takes over and more effecient. Remember that a tire on the verge of slipping produces more friction with respect to the road than one that is locked and skidding. Once the tire(s) lose grip, friction is decreased and the vehicle takes longer to come to a stop.
Pressure Modulation is what makes ABS what it is. This releases and applies the brake pedal to keep the wheels from becoming locked. When the brake pedal is pumped or pulsed, pressure is quickly applied and released at the wheels. ABS can modulate the pressure to the brakes as often as 15 times per second. The steering ability of a vehicle is reduced if the front wheels are locked, and the stabilty of the vehicle is reduced if the rear wheels are locked; ABS ensures that maximum grip froce and stabilty of the vehicle. An ABS Control Module calculates the slip rate of the wheels based on the Vehicle Speed Sensor (VSS) and the speed of the wheels - which then uses the information to control the brake fluid pressure.
An ABS Module:
The ABS Module is the control computer for the Antilock Brake System which is normally located inside the trunk of the wheel housing (older vehicles) or mounted to the master cylinder or part of the hydralic control unit. As stated above, it monitors system operation and controls Antilock function when needed. It relies on input from the VSS's and feedback from the hydralic unit to determine if the antilock brake system is operating correctly and to determine when the antilock mode is required. The Module has it's own self-diagnostic function including numerous numeric trouble codes. The module can also be known as an ECU (Electronic Control Unit), EBCM (Electronic Brake Control Module), the ABS Controller, or the ECM (Electronic Control Module).
Indicator lights - most ABS systems have two indicator lamps. One is tied into the ABS system while one is united with the base braking system. The ABS Lamp (pictured above) is orange/amber in color and when the light is on the ABS system is disabled, but normal braking will still be available. All vehicles have a red warning light (the other indicator light). This lamp is lit when there is a problem with the brake system or when the parking brake is still engaged. On start up, the ABS light will illuminate and then time out. If the light does not time out or blinks rapidly - these are codes - and states that there is a problem with the system and notifys the owner that there is a problem and the ABS system is disabled while the light is on. Most common problems with ABS lights are wheel speed sensors. Speed Sensors are used for a variety of things; including transmission operation, speedometers, Tire Pressure Monitoring Systems, and yes - ABS systems.
Brake Warning Light:
Speed Sensors are used for many applications (as stated above), but are used greatly during ABS operation. Speed sensors are mounted sturdy to the hub/spindle/knuckle and are spaced out a certain air gap between the reluctor wheel (ABS Ring - like a crank sensor) that produces an AC signal and sends it to a buffer which changes it to a DC signal and sends it to the ECU of the ABS. NOTE: To test an VSS is simple: Connect a Digital Multimeter to the two leads, turn the meter to AC Volts, and spin the wheel and see if you get a reading. Check to see if it's in the correct specs. ANOTHER NOTE: Modern Cars (like the Pontiac G6) are now coming with different VSS. If you do the old method of testing a VSS you will not get a correct reading and will be replacing a good part. The new sensors have three wires which have a signal wire, power, and ground. These signal wires need to be jumped to the connector when testing or the ABS system will shut down as it will see it as one of the ABS sensors being disconnect and a bad reading will occur. Read testing information before testing.
A AC Current Generating VSS (Speed Sensor):
Types Of ABS Systems: There are 4 systems (1,2,3, or 4 channel) that are used, but mostlyeither one, three, or four. The set up is pretty basic. For the one channel system, there is only one wheel speed sensor which is in the rear (usually on top of the rear axle) which monitors the rear wheels together. Ford use to use this a lot on their trucks. The 3-channel system is what we have on the Explorers. There are two sensors up front, one at each wheel with the ABS Rings on the CV Axles, and then one rear sensor on the differential/rear end with an ABS Ring on the Carrier. This is common to find on SUVs and Trucks. The 4-channel system is used on most independent rear FWD vehicles (Toyotas, etc.). These use sensors at each four wheels and is the most complex design.
Bleeding Brakes A common mispractice of car repair is (when pushing back caliper pistons for new pads) to push the old dirty fluid back up to the master cylinder. This fluid will go through the ABS module as well. Doing this may cause trash/dirty fluid to get into the module and cause problems. When pushing the piston back, open the bleeder and allow the old fluid to go out that way. This ensures no trash going into the Hydralic Unit.
Brake Fluid is needed for any brake system. The wrong fluid can cause problems. DOT 3 is usually recommend for most passenger vehicles (cars, trucks, and SUVs), but putting the wrong fluid in may result in different braking characteristics. The main difference is that DOT3 and DOT4 absorb water, while DOT5 doesn't. Most cars use DOT 3 fluid from the factory. Brake fluid can eat paint, so get it off as soon as possible.
Brake Fluids
ABS System Flaws As everyone thinks that the ABS systems are the greatest things alive when it comes to safety; but when studies are done, they come up just the same. In studies done by the USA Insurance Institute for Highway Safety they state: "Cars with anti-lock brakes are more likely to be involved in fatal crashes than cars without them/" Why is this? People can't drive - thats the main reason. Other reasons comply with conditions : While ABS can usually shorten braking distances on wet and icy roads, there are some limitations in other conditions. It may actually take longer to brake on loose stones or fresh snow because they cannot build up in front of the wheels as they would when the wheels are locked.
For more information go here (with pictures):
How It Works
-Drew
