ok this is how it works...just going off memory because I dont have SI on this computer.
its a delphi ebc-325c abs module in our trucks (over 8600gvw)
it has a charge pump motor, EBCM, EHCU, 2 front wheel speed sensors, 1 rear wheel speed sensor.
It is a 3 channel ABS system, NOT 4 like most cars. The rear wheel circuit is common and both wheels are controlled individually.
When you first start the truck and wheel speed over 15mph is detected, it goes thru a test and compares all sensors to make sure they are within range and all agreeing. Then it goes into a standby mode. When the ABS is not active, it doesnt do anything, the fluid pressure from the master cylinder bypasses right thru it. Thats why if the ABS fails you still have normal braking. There are two lines coming form the master cylinder obviously because its a dual circuit system like evry car since the 1960's. One for the rear brakes one for the front. When the EBCM senses one wheel slowing down faster in relation to the others, or a rate of decel that is over a certain threshold, it goes to work. It looks at and controls each front wheel individually and the rear wheels together.
The EHCU (electrohyhdraulic control unit) is what does the actual work, the EBCM is the brain and sensory perception. OK so the EBCM/EHCU has three modes. Pressure hold, pressure increase, and pressure decrease. There are a series of dump valves and isolation valves, and of course the pump motor. So when, say the left front wheel loses traction and starts to lock up or slow down way fatser than the others. The EBCM first activates the isolation solenoid which diverts braking pressure from the "bypass" to the actual EHCU, so now all of your brake pedal pressure has to go thru the EHCU and it can take over control if it wants to, so theoretically you have just lost control of braking and the EBCM is calling all the shots until the wheels regain traction or you let off the pedal. So once the EBCM has control of the one or more wheels that require ABS intervention, then it goes into "pressure hold" mode. The isolation solenoid holds the braking pressure constant to the wheel so even if you let up a little or press down on the pedal a bit more, its not going to make any difference. The EBCM then looks at the skidding wheel sensors speed, and if the simple pressure hold act allows the wheel to spin again then all it well. But if there is still too much pressure to allow teh wheel to regain traction, it goes into pressure decrease mode. It activates teh dump solenoid on that wheel circuit to bleed off brake pressure until the wheel starts spinning again. As soon as its bled off enough pressure to allow the wheel to start spinning again, it goes into pressure increase mode. The dump valve closes and it then uses the charge pump to build up braking pressure on that wheel until the wheel locks up again. Then the wheel starts to lock up again it goes into pressure decrease mode, and the cycle repeats until the truck stops skidding or you come to a complete stop. Thats the pulsing that you feel in the pedal. If the ABS event is long enough to completely bleed off all the availble pressure that you are putting ont eh pedal, thats why you feel the pedal go towards the floor and then push back up at you, its the pump building up pressure again.
The EBCM also controls the front/rear proportioning on our trucks; there is no standard proportioning valve. GM calls it "dynamic rear proportioning". If the front wheels lock up before the rear wheels, the EBCM cranks proportioning full rear and then does its thing with the valves and solenoids.
The ABS in our trucks works pretty well IMO on dry and wet pavement. You have to give it some credit because controlling an 8000 pound truck with a hugely different front/rear weight bias is not easy. But its not perfect. The Delphi EBCM's are much better than the old Kelsey-Hayes systems in the GMT-400 trucks (88-98), but definetly not as good or advanced as the Bosch EBCM's in the GMT-900 light duty trucks. The LMM's still have the delphi EBCM. Bosch wrote the book on ABS and stability control and if you drive a GMT-900 tahoe, suburban, 1500 silverado, you will know the difference the first time you use it.
That being said, there are some road conditions (ice, snow covered ice, etc) and offroad dirt/mud, where ABS is a huge PITA. Thats why I put mine on a swtich. For those few times when I dont want to have ABS get in the way.
You have to remember though...the point/theory of ABS is NOT (neccessarilly) to stop in a shorter distance, its to allow you to maintain control in a hard braking situation so you can swerve to avoid an accident without skidding sideways or something. Most of the time if you just do straight line braking and are a really good driver who has experience with non-ABS vehicles, you can almost always stop in less straight line distance than ABS can.
ok wow I dont ask me how I just rememebr this stupid stuff
but so onto the line lock. It would require taking the EBCM apart and wiring something in manually or using a tech 2. If someone has an extra EBCM Id like to give it a shot, but Im not about to go hack up my own.
ben
