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ABS System Basics

Discussion in 'Under the Hood' started by ExplorerDMB, February 11, 2006.

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  1. ExplorerDMB

    ExplorerDMB Moderator/Technician Moderator Emeritus

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    [​IMG]
    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:
    [​IMG]


    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:
    [​IMG]

    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):
    [​IMG]


    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
    [​IMG]

    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
     
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  3. gijoecam

    gijoecam Village Idiot Elite Explorer

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    Decent write-up, but on our trucks, there are exactly two things that can make the BRAKE light come on: One is the parking brake, the other is a low fluid level in the brake fluid reservoir. That light is NOT tied into the ABS.

    But otherwise a pretty good write-up. :)

    -joe
     
  4. ExplorerDMB

    ExplorerDMB Moderator/Technician Moderator Emeritus

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    I never said it was tied into the ABS. I said one was (the ABS light) but the other was united with the braking system. I forgot about the low fluid; but yes you're right, that will cause the light to come on.

    -Drew
     
  5. gijoecam

    gijoecam Village Idiot Elite Explorer

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    I wasn't clear on what you were saying.... my bad. My point is that the BRAKE light won't be lit for any ol' problem with the brakes, only the parking brake switch and the low fluid level sensor will cause it to illuminate.

    -Joe
     
  6. ExplorerDMB

    ExplorerDMB Moderator/Technician Moderator Emeritus

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    Right, on our Explorers. Some vehicles have sensors in the pads instead of wear tabs that make noise.

    -Drew
     
  7. rdb1764

    rdb1764 New Member

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    ABS control module

    My wife's 01 Sport apparently has a bad ABS control module. ABS kicks in when the vehicle nearly comes to a stop, allowing it to creep forward a foot or two more, but the problem was getting worse. A few months ago, she took it to a local shop here in Aiken because of the extensive hours I was putting in at work, and they said it was the left front caliper. She had them replace it. Well, the vehicle is from Michigan, so the caliper does look nicer now (Just to have a matching set, I replaced the other about a month later). So the problem was starting to get a little worse as time (and the truck) rolled on. Not having good diagnostic tools, and having had a few experiences with the Ford dealership here that left me feeling less then confident in their abilities, I knew we had either a sensor or control module issue. Knowing the front sensors were integral to the hubs and knowing that replacement of any components may involve reprogramming, I was hesitant to do what my local Ford Dealer is favorable of, which is swapping out components until the problem does or does not go away (Yes, there is a story here). So, I started asking around and found the most favorable shop to be less than a mile down the road. I took the vehicle there and they determined the problem to be the ABS control module. did I mention the ABS light was NOT coming on except during ignition when it is supposed to. Anyway, I had them remove the fuse for the ABS. Vehicle brakes just fine now, but with a sale likely to be in its near future, I am considering getting the ABS back on line. I believe the CM is likely the issue, as I do have confidence in the shop I took it to. Anyway, from a historical standpoint, I may have caused the issue in the first place, as (did I mention this is a Michigan vehicle) I could not get the left front caliper bleed valve loose when I change the pads prior to the aforementioned caliper replacement. I reluctantly pushed the fluid back up into the system to retract the piston. :( Anyway, I am having trouble finding good info on the procedure for replacing the ABS CM. I know the module is only about $130. What I do not know is the reprogramming requirements for it if it is replaced and what equipment I would need to do it and where I might get said equipment from (borrowed). I do know a guy that knows a guy... Actually, i do know the guy...
     
  8. ExplorerDMB

    ExplorerDMB Moderator/Technician Moderator Emeritus

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    The situation your describing is very close to a bad wheel speed sensor. Did you ever have anyone scan it and give you codes?
     
  9. GRNMACHINE

    GRNMACHINE Well-Known Member

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    Nice writeup and thanks!! rdb, there is a TSB addressing this problem and as DMB said, it points to a front sensor. I will look for the TSB no., I am sure it is on the site somewhere though. It basically refers to a false slow speed activation of the ABS. If coming to a stop and the ABS kicks in and pulls to the right-replace the left sensor. Pulls to the left-replace the right sensor (obviously). Testing the sensors as DMB is the right thing to do. I plan on tackling this tomorrow, have a new OEM FORD hub with sensor on hand if needed. The sensor was over $200 from Ford, I was able to get the entire Hub with sensor for less than half.
     
  10. rdb1764

    rdb1764 New Member

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    Codes

    I do not know if someone has scanned it. I assumed the shop I took it to yesterday did, but the second response about the wheel speed sensor failure verifies what I had been thinking all along, that it is the left wheel sensor. one of the first things I noticed when this started happening is that the vehicle pulled ever slightly to the right, so I deduced that it was an issue with the left wheel sensor. I will call the shop Monday, as my wife, not myself, picked up the vehicle and I have not spoken to Steve personally yet. I will ask exactly what they did do to determine where the problem was. Would fault codes still present if the ABS trouble light was not indicating? Anyway, changing out the hub should be fun. Anything special about it electronically/programming-wise? Also, where is a good place to find TSBs? I also need to change out the driver's side door actuator. it is getting pretty weak. I have been in that door before. It seems everything in this explorer is locked up tighter than a... well, very hard to get to! This is no exception. The good thing is, my wife is much happier now that I replaced the dead blower fan motor for the HVAC! Poor vehicle. Everything is going wrong at once! Thanks for all your help guys!
     
  11. rdb1764

    rdb1764 New Member

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    Tsb

    Found a TSB for this. 98-14-8, already posted on this site. Interesting. also, good experience in navigating the site to find pertinent information.
     
  12. GRNMACHINE

    GRNMACHINE Well-Known Member

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    Yep that is the TSB. I think last time I was looking for a TSB (non Ford Explorer) I just did a yahoo/google search. I would assume the ABS codes would still be logged but I am no expert on that. Re: HUB, it was pretty straight forward. No programming needed. Just swapped it out while doing balljoints. It is attatched to the spindle, needed an air gun to remove the bolts. There is a great write up on here with pictures. Pretty much anthing I have looked for is here! Heh Heh
     
  13. geo1o1

    geo1o1 New Member

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    rdb1764, I have the same issue with my ford explorer sport, wat did you determine the problem was, my car goes a foot or 2, when i am braking between 10-5 mph, and pedal goes all the way down onces the car is stop, also the car tends to turn to the right, when i am braking, wat was you problem on the explorer

    thanks
     

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