Smooth stop, or trouble
looking for a place to happen?So there you are. You've once again
managed to turn another mangled hunk of metal and plastic into something
that resembles the vehicle that came from the factory. This was one of the
good ones-no hidden damage to argue with the insurance company about, no
six-month wait for parts and the paint is a dead-on match. To top it all
off, the customer couldn't be happier with the work you've done. He didn't
find a single thing wrong. You happily wave good-bye to each other, and
another job comes to an ideal conclusion.Or so you think. Bright and early the next
morning, the car comes back. As you approach the vehicle, you just can't
convince yourself that he has returned to once again compliment your work.
The problem? An anti-lock brake (ABS) light is on. What did you do wrong?
Maybe nothing, maybe everything. What you could have done-but didn't -may
make the difference in profit or loss on this job.Let's take a look at what should and
should not be done concerning ABS during collision repair.What is ABS?First of all, contrary to the name
"anti-lock braking system," it isn't a braking system at all. It
is a steering control system. The primary function of ABS is to prevent
the wheels from locking up, under panic stopping conditions, to allow the
driver a chance to avoid the oncoming collision that is quickly filling up
the windshield. If the wheels are not turning, the driver has no more
control than a passenger in the rear seat does. ABS also makes some
assumptions about the capabilities of the driver to actually avoid the
accident, but that is a topic for another time.OK, so it's a steering control system. But
can it stop the vehicle in a shorter distance, like the manufacturers
claim? Yes it can, and in the majority of cases, it does. But there are
some situations, such as loose road conditions, where certain types of ABS
units can actually increase the stopping distance. We'll get into how and
why that can happen a little later.What Does ABS Do? How Does it
Work?ABS actually "pumps" the brakes
during a hard stop to prevent the tires from locking up. It can pump one,
two, three or all four wheels, depending on the system and how many wheels
are actually approaching lock-up. While the system doesn't actually pump
the brake pedal, it has valves inside its hydraulic unit that open and
close to reduce the brake pressure. This allows the brake pads or shoes to
"loosen" their grip and permits the wheels to start turning
again, or it prevents the wheels from skidding in the first place. Other
valves then operate to reapply the brakes, which makes up the other half
of the pumping action.Anti-lock units are available from several
different manufacturers, including Kelsey-Hayes, Delphi, Bosch, Teves and
Delco/Bosch. All of these units have differences in the way they operate,
but for the most part, they all use the same basic components to perform
their functions. Each of them is also further divided into one of two
broad categories known as three-channel or four-channel systems.A three-channel system operates the front
wheels independently and the rear wheels together, whereas a four-channel
system operates all four wheels independently. There have also been some
single-channel systems in years past that operated only the rear brakes.
Those were referred to as RABS or RWAL and were used primarily on trucks.On a typical ABS system, you will find the
following components:Wheel Speed Sensors (WSS): A
vehicle will have two of these for a three-channel system and four on
a four-channel system.Vehicle Speed Sensor (VSS): This
is used only on three-channel systems for rear wheel speed reference.
The VSS is located in the transmission or on the rear of the transfer
case for 4WD vehicles.Rear Wheel Speed Sensor (RWSS):
This serves the same function as the VSS, but it is located in the
differential housing. A vehicle with an RWSS may still have a VSS, but
it won't be used for ABS.Brake Switch: Located behind the
brake pedal, this switch tells the computer that the driver has
pressed the brake. This switch is typically different from the switch
that activates the brake lights.ABS Computer: Commonly referred
to as the electronic brake and control module, (EBCM), this unit
receives signals from the speed sensors and brake switch. It also
controls the operation of the ABS valves and pump motor (if equipped),
performs diagnostics and stores codes.Modulator Assembly: The
modulator assembly is the primary hydraulic unit in an ABS system. It
contains all of the valves, accumulators and pumps necessary to
control the pressures in each of the brake lines.VSS Calibrator: Some systems use
this electronic interface to convert the VSS signal into a signal the
EBCM can use. It is also used to give the EBCM information on tire
size for some units.ABS Lamp: This amber indicator
on the instrument panel is designed to alert the driver of an ABS
problem.Traction Control Light: Used on
some vehicles equipped with traction control systems (TCS), this light
informs the driver of a potential problem with the TCS. Because
traction control uses the wheel speed sensors, and possibly some other
ABS components, TCS errors may also affect ABS operation.Hydraulic Pump Motor: This motor
is used to operate the hydraulic pump, which is usually located in the
modulator assembly. Hydraulic motors/pumps are used to produce
pressurized fluid to reapply the brakes during an ABS stop or to
remove the fluid that has been discarded during the
"release" portion of the cycle. Some systems, such as the
Delphi VI, have three motors.Regardless of the specifics of any given
system, they all have two things in common: what makes them engage and
what makes them disengage. In order for ABS to activate, it must have a
brake switch input from the driver and it must have information on the
rate of deceleration of the wheels, as determined by the wheel speed
sensors or VSS.For the system to deactivate, it must
either lose the brake switch signal (brake pedal is released), or the WSSs
must drop to 0 (less than 3 mph). In other words, the ABS will not
disengage until the vehicle comes to a complete stop or the brake is
released. With that in mind, consider the following test.Take a vehicle equipped with ABS to a
place where there is adequate space and where traffic is not a concern.
Bring the vehicle up to speed and apply the brakes sufficiently hard to
cause the ABS to activate. While holding the brake pedal, press the
accelerator to the floor and observe the action of the vehicle. You will
find that the vehicle continues to move forward even though the brake
pedal is fully depressed. This condition occurs because the ABS pumps, or
pulses, the brakes, and the anti-lock operation will continue until the
WSSs drop essentially to 0.In this case, the full-throttle condition
prevents the WSSs from slowing enough to deactivate the ABS. It is
basically the same as if the brake pedal were pressed intermittently while
driving at a steady speed. The vehicle continues to move but at a slower
speed. This is a condition I refer to as ABS run-on, and you should be
aware of how and why it can happen. You should also know that it is not an
abnormal condition.Now, let's go back to your customer with
the ABS light on...What Should Have Been Done?Several steps could have been followed
that may have caught this problem before the vehicle left the shop. The
steps fall into one of these broad categories, which we will discuss in
detail:inspection/road testing;diagnostics:ABS light on/off;flash codes;scan tool information;flowcharts;system flushing;repair/replacement procedures;computer reprogramming/tirecalibration.Road Test/InspectionBecause of their crumpled state when they
arrive at your shop, most vehicles will not lend themselves to being road
tested until after they are repaired, but a thorough inspection is a
relatively easy thing to do, as well as a good idea.Some of the things to look for include cut
or broken wires (especially in the immediate area of the impact), fluids
leaked and spilled on components or computers, and broken wheel speed
sensor reluctor rings. Also, look for signs that the ABS computer or
modulator may have been under water, mud in and around the sensors or
electronics, and severed brake lines. Any of these may have allowed
contaminants to enter the system. This inspection will not only help
prevent a comeback, but it may also need to be brought to the attention of
the insurer.Once the vehicle repair is complete, a
road test needs to be performed to check for proper operation of the
system. It isn't enough to turn on the key and have the ABS light go out.
Some codes and/or problems will not occur until the vehicle reaches a
given speed or until the ABS unit actually attempts an anti-lock stop.The best check for proper operation is to
make the vehicle perform an anti-lock stop from a speed in excess of 35
mph. The reason for that speed is that some ABS units will not perform
certain systems checks until the vehicle has traveled at least 35 miles
per hour. Obviously, this test needs to be performed in a safe area where
traffic isn't a problem and an ABS failure won't cause a concern. Keep in
mind that ABS failures do not adversely affect the base brake system, and
if an ABS failure occurs, the hydraulic system still operates normally.If the ABS passes the basic anti-lock stop
test, you can conduct one more performance test. Drive the vehicle at
highway speeds to ensure the ABS light doesn't come on. I've had this one
bite me before, and this is why: Wheel speed sensors are made of permanent
magnets with a wire wound around a metal core. When WSSs are tested with
an ohmmeter, only the wire is actually checked. If the magnet has been
weakened by the impact (fracture) or for some other reason, the ohmmeter
test won't show it. In fact, the AC voltage test won't necessarily show a
failure here, either. As the magnet weakens, its ability to operate at
higher speeds deteriorates, although it may still operate properly at
lower speeds. If the system operates as it should at legal highway speeds
without turning on the ABS light, the system should be considered fully
operational. One note here: It is not necessary to perform an anti-lock
stop from highway speeds. The previous stop from the lower speed should be
sufficient.DiagnosticsHere are some aids for diagnosing ABS
problems:ABS Light-The most basic form of
anti-lock system diagnostics is the ABS light. When the key is first
turned on, the ABS light should illuminate for a few seconds and then go
out if all systems are functional. Remember that this is only a static
test and does not mean there will be no other faults when the system is
operated. If the light stays on, it means that the ABS system is
non-functional. It does not necessarily imply there is a problem with the
ABS itself. There are conditions, not related to the ABS unit, that can
cause the system to be inoperative, such as low system voltage, a third
brake light bulb out (some systems), blown fuses, etc.Flash Codes-Some older ABS units
have the ability to provide codes by flashing the light on the instrument
panel. These were gradually phased out as scan tool diagnostics and data
became available. From the early to mid '90s many vehicles had both
capabilities, but flash codes were mostly eliminated after 1995 in favor
of scan tool diagnostics only. Refer to the appropriate service manual for
the procedure to activate flash codes if the vehicle you are repairing has
that capability.Scan Tool-For the better part of 10
years, ABS codes and data have been available on a variety of scan tools.
As a general rule, the older the vehicle, the less information there is
available from the ABS computer. On newer vehicles, the information
available includes active and intermittent codes, code histories, wheel
speeds for each wheel, brake switch status, applied voltages, current
software programming and others. For the collision repair technician,
pulling codes and checking wheel speeds for proper operation will most
likely be of the greatest importance.For instance, a vehicle traveling in a
straight line should obviously have the same speed reading for all four
wheels. An intermittent increase or decrease in the speed of any wheel
will show up here before it will actually set a code. That information can
then assist the technician in solving a potential problem before it gets
out the door.If an active or history code is found, a
look at service manual flowcharts will most often lead to the problem and
the required fix. Bear in mind that some codes will not activate the ABS
light. These codes indicate a problem that is not significant enough to
shut down the system, so the lamp is not illuminated. Because of this, it
is a good idea for all vehicles to have their ABS scanned before the
vehicle is released to the customer, even if the light went out after the
key-on sequence.Another capability scan tools offer is a
manual check of the internal modulator valves for proper operation. Many
vehicles are only able to perform diagnostics on electrical components and
circuits, and as such, stuck valves or other mechanical problems are not
picked up by the ABS diagnostic system. If a road test shows the ABS to be
operating incorrectly but no code is set, these manual tests may help to
locate the source of the problem. Familiarizing yourself with the tests
available on different systems will likely prove to be helpful with those
no-codes-but-doesn't-work-right situations.One final word about codes and scan tools:
Always make sure to clear all ABS codes before releasing any vehicle to a
customer. Some units will change the way they operate simply because a
code is present, whether there is a current problem or not. It is simple
to do and may eliminate a no-problem-found comeback headache.Flowcharts-Diagnostic flowcharts
are widely available for almost all ABS applications on the market today.
They are found in the OEM service manuals and in aftermarket repair
materials in both paper and electronic form. Now, please don't anybody
tell me about all of the errors you've found in flowcharts throughout the
years. I've heard it, and I've experienced it. No, they are not
perfect-nothing is. But in an industry such as collision repair, where ABS
diagnostics is not a daily function for most, the flowcharts may likely
turn out to be your best friend. There are typically charts for each code
available, as well as charts for operational problems (symptoms) where
there is no code. While some may think using flowcharts is as being as bad
as "asking for directions," remember that there is no labor
operation for "confused myself and wasted time." Use the
flowcharts, especially if ABS repair is something you don't do frequently.See Sample
FlowchartSystem FlushingYou wouldn't believe the looks I get when
I mention brake system flushing as a repair procedure. Let me explain why,
and you can decide for yourself.If you've ever done any brake work, you no
doubt found a grayish-black gooey substance in the disc calipers and wheel
cylinders. That substance is water, and it is there for two reasons.
First, it's because brake fluid is a desiccant, which means it absorbs
water. And second, it's because water is heavier than brake fluid, which
causes it to sink to the lowest point. Therefore, it ends up in the
calipers and wheel cylinders. But with the introduction of ABS modulator
units and their sometimes-complex internal plumbing, some of that water
now finds a home in the anti-lock brake unit.Caliper pistons and wheel cylinder pistons
operate relatively slowly even in panic stop conditions. They have
internal rubber seals to prevent leakage, and those seals also permit them
to have a slightly larger bore-to-cylinder clearance. But an ABS modulator
valve must operate up to 15 times per second and, as such, cannot use
rubber seals. Because there are no seals, the tolerance between the valves
and their bores must be much closer. If some of that gooey, watery
substance has collected in the modulator, it may well cause the valves to
seize and become inoperative. This problem may be worse in vehicles that
have been sitting for an extended period of time waiting for parts,
insurance approval or scheduling. If the ABS unit isn't operated on a
regular basis, even on the start-up test, the water can cause damage,
which may not show up until later, just as has happened in this current
comeback scenario.The solution is simple: Flush the entire
brake hydraulic system. I've seen "failed" ABS units fixed with
a simple flush, and I've seen units replaced at a considerable expense
that may have only needed to be flushed. Incidentally, some manufacturers
recommend that all ABS equipped vehicles have the brake system flushed and
refilled every two years, and some of those will even cover the first one
under warranty.The procedure is simple: Using the two-man
system, a pressure bleeder or a vacuum bleeder, remove all of the fluid
until it runs clean from all four wheels. Then use a scan tool to activate
the function test several times. This test will run the pump motor and
cycle the valves, which allows any old fluid trapped in the unit to be
forced out. Then repeat the bleed and function test procedure to remove
any remaining contaminated fluid. More than once, an expensive but
unnecessary repair has been avoided by this simple procedure.One last thing: You may find that some
manufacturers do not recommend the use of a vacuum bleeder for this
process. I've used one for years without a problem. You decide for
yourself.Repair/ReplacementFor the most part, ABS units do not have a
high parts count, and as such, most of the repairs are done by simple
replacement. But there are a few exceptions and traps you need to look
for, including wheel speed sensors and toothed sensor rings.As a general rule, WSSs with broken wires
should be replaced rather than repaired because the wires must remain
flexible to allow for suspension travel. Splicing or soldering will
eliminate that flexibility and possibly cause premature failure, although
other ABS system wires should have no problems with properly installed
splices. Additionally, if a repair is done on wires that were twisted,
make sure that you re-twist the wires when finished. ABS units operate on
very low voltage signals, and twisting the wires helps protect them from
outside electrical noise that may interfere with the operation of the ABS.When installing new sensors, make sure
they are re-routed exactly as they came from the factory. This is another
step the designers have taken to protect the sensor signals from outside
electrical noise. Also make sure the sensor you have is for the correct
side of the vehicle. Sometimes, the only difference between the left side
and the right side is the length of the sensor wire. I have seen an
installation where a longer wire sensor was installed on the opposite
side, and the excess cable was wound in a loop to keep it out of the way.
As it turned out, that loop acted like an antenna, and it pulled in all
sorts of electrical noise that affected the operation of the system. That
was an unnecessary headache that nobody needed.As far as the toothed sensor rings are
concerned, there are three major things to look for. The first is cracked
or chipped teeth. Even small chips missing from the teeth can be the
source of problems. If any parts of teeth are missing, replace the ring.The second concern is a cracked or
fractured ring. This is more likely to happen in a vehicle that has been
wrecked. Sometimes the fracture is hard to find, but it may allow the ring
to slip when the wheel is turning. A slipping ring will give incorrect WSS
readings even though it may look fine and seem to be adequately secure. A
scan tool will show the changes in the wheel speed sensor output for a
vehicle with this problem.The third area to watch for is missing
rings on new parts. All new brake hubs do not come with the reluctor rings
already installed. This is especially true for trucks. If the ring isn't
installed before the parts are replaced, I can guarantee an ABS light, a
code and the opportunity to start over with that part of the repair. Been
there, done that, don't repeat my mistakes.Computer Reprogramming/Tire CalibrationThroughout the years, ABS computers have
become increasingly sophisticated, and their applications have been
expanded. Today, rather than building separate computers for each vehicle
a manufacturer produces, one or two programmable computers may be applied
to many models. This makes matters less complicated in terms of parts, but
it adds a new dimension to the repair process.If an ABS computer requires replacement, a
reprogramming-capable scan tool is required, along with access to the
manufacturers' ABS program database. Programming is pretty much a
step-through procedure that will be easy to follow on the scan tool. The
area where you must apply caution is in entering the exact information
about the vehicle.For instance, the procedure may ask if the
vehicle is equipped with TCS. Some versions of a certain model will have
TCS and some won't, and the only difference could be which transmission it
has. If the computer is programmed for TCS and the vehicle doesn't have
it, the computer will throw codes. If it isn't programmed for TCS and the
vehicle is equipped for it, the system will never work. Other specifics
the program may require include tire size, axle ratio, transmission type,
cruise control and vehicle identification numbers (VINs). Apply some
common sense caution, and after a time or two, you'll be an old pro at the
programming game.One final item concerning programming:
Some vehicles, mostly trucks, have the ability to alter the operation of
the ABS based on tire size. As such, if a set of different sized tires is
installed on a vehicle, the ABS needs to be updated. On older models, this
may require replacement of the VSS calibrator to one that is
pre-programmed for that tire size and axle ratio. But newer vehicles can
be reprogrammed through the scan tool. Simply bring up the tire
calibration section, select the new tires from the available list and the
scan tool will do the rest. Keep in mind that vehicles without this option
should not have the tires replaced with a different size. The computer
will operate as if it still has the original size, and ABS operation will
be adversely affected.So there we have it. Was there anything we
could have done to prevent the return of this customer? Did we do all of
the checks we should have done? Did we miss something? Maybe, maybe not.
But hopefully you've picked up something from all this and are now better
prepared to prevent unnecessary comebacks in the future.
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