2001 DODGE TOWN AND COUNTRY air condition

CONDITION POSSIBLE CAUSES CORRECTION
SCRAPING (METAL-TO-
METAL).1. Foreign object interference with
brakes.1. Inspect brakes and remove foreign
object.
2. Brake shoes worn out. 2. Replace brake shoes. Inspect rotors and
drums. Reface or replace as necessary.
OTHER BRAKE CONDITIONS
CONDITION POSSIBLE CAUSES CORRECTION
BRAKES CHATTER 1. Rear brake drum out of round or
disc brake rotor has excessive
thickness variation.1. Isolate condition as rear or front.
Reface or replace brake drums or
rotors as necessary.
BRAKES DRAG (FRONT
OR ALL)1. Contaminated brake fluid. 1. Check for swollen seals. Replace all
system components containing rubber.
2. Binding caliper pins or bushings. 2. Replace pins and bushings.
3. Binding master cylinder. 3. Replace master cylinder.
4. Binding brake pedal. 4. Replace brake pedal.
5. Brake lamp switch not adjusted
properly and mounting bracket is bent.5. Straighten mounting bracket and
replace brake lamp switch.
BRAKES DRAG (REAR
ONLY)1. Parking brake cables binding or
froze up.1. Check cable routing. Replace cables
as necessary.
2. Parking brake cable return spring
not returning shoes.2. Replace cables as necessary.
3. Service brakes not adjusted
properly (rear drum brakes only).3. Follow the procedure listed in the
adjustment section.
4. Rear disc brake parking brake not
properly adjusted.Adjust parking brake shoes.
BRAKES GRAB 1. Contaminated brake shoe linings. 1. Inspect and clean, or replace shoes.
Repair source of contamination.
2. Improper power brake booster
assist.2. Refer to power brake booster
diagnosis and testing.
EXCESSIVE PEDAL
EFFORT1. Obstruction of brake pedal. 1. Inspect, remove or move obstruction.
2. Low power brake booster assist
(vacuum leak).2. Refer to power brake booster
diagnosis and testing.
3. Glazed brake linings. 3. Reface or replace brake rotors as
necessary. Replace brake shoes.
4. Brake shoe lining transfer to brake
rotor.4. Reface or replace brake rotors as
necessary. Replace brake shoes.
EXCESSIVE PEDAL
TRAVEL (VEHICLE
STOPS OK)1. Air in brake lines. 1. Bleed brakes.
2. Rear drum brake auto-adjuster
malfunctioning.2. Inspect and replace drum brake
components as necessary. Adjust rear
brakes.
RSBRAKES - BASE5-5
BRAKES - BASE (Continued)
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CONDITION POSSIBLE CAUSES CORRECTION
EXCESSIVE PEDAL
TRAVEL (ONE FRONT
WHEEL LOCKS UP
DURING HARD
BRAKING)1. One of the two hydraulic circuits to
the front brakes is malfunctioning.1. Inspect system for leaks. Check
master cylinder for internal malfunction.
PEDAL PULSATES/
SURGES DURING
BRAKING1. Rear brake drum out of round or
disc brake rotor has excessive
thickness variation.1. Isolate condition as rear or front.
Reface or replace brake drums or
rotors as necessary.
PEDAL IS SPONGY 1. Air in brake lines. 1. Bleed brakes.
2. Power brake booster runout
(vacuum assist).2. Check booster vacuum hose and
engine tune for adequate vacuum
supply. Refer to power brake booster
diagnosis and testing.
PREMATURE REAR
WHEEL LOCKUP1. Contaminated brake shoe linings. 1. Inspect and clean, or replace shoes.
Repair source of contamination.
2. Inoperative proportioning valve
(non-ABS vehicles).2. Refer to proportioning valve
diagnosis and testing. Replace valve as
necessary.
3. Improper power brake booster
assist.3. Refer to power brake booster in the
diagnosis and testing section.
STOP/BRAKE LAMPS
S TAY O N1. Brake lamp switch out of
adjustment.1. Replace brake lamp switch.
2. Brake pedal binding. 2. Inspect and replace as necessary.
3. Obstruction in pedal linkage. 3. Remove obstruction.
4. Power Brake Booster not allowing
pedal to return completely.4. Replace power brake booster.
VEHICLE PULLS TO
RIGHT OR LEFT ON
BRAKING1. Frozen brake caliper piston. 1. Replace frozen piston or caliper.
Bleed brakes.
2. Contaminated brake shoe lining. 2. Inspect and clean, or replace shoes.
Repair source of contamination.
3. Pinched brake lines. 3. Replace pinched line.
4. Leaking piston seal. 4. Replace piston seal or brake caliper.
5. Suspension problem. 5. Refer to the Suspension group.
PARKING BRAKE -
EXCESSIVE HANDLE
TRAVEL1. Rear drum brakes or rear disc
brake parking brake shoes out of
adjustment.1. Adjust rear drum brake shoes, or
rear parking brake shoes on vehicles
with rear disc brakes.
STANDARD PROCEDURE - BASE BRAKE
BLEEDING
NOTE: This bleeding procedure is only for the vehi-
cle's base brakes hydraulic system. For bleeding
the antilock brakes hydraulic system, (Refer to 5 -
BRAKES - ABS - STANDARD PROCEDURE)CAUTION: Before removing the master cylinder
cover, thoroughly clean the cover and master cylin-
der fluid reservoir to prevent dirt and other foreign
matter from dropping into the master cylinder fluid
reservoir.
5 - 6 BRAKES - BASERS
BRAKES - BASE (Continued)
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REMOVAL - FRONT DISC BRAKE SHOES
(DISC/DRUM BRAKES)
(1) Raise the vehicle. Refer to HOISTING in
LUBRICATION AND MAINTENANCE.
NOTE: Perform Step 2 through Step 5 on each side
of the vehicle.
(2) Remove the front tire and wheel assembly.
(3) Remove the two brake caliper guide pin bolts
(Fig. 14).
(4) Remove the disc brake caliper from the disc
brake adapter and hang out of the way using wire or
a bungee cord. Use care not to overextend the brake
hose when doing this.
(5) Remove the brake shoes from the disc brake
caliper adapter.
CLEANING - DISC BRAKE SHOES
WARNING: DUST AND DIRT ACCUMULATING ON
BRAKE PARTS DURING NORMAL USE MAY CON-
TAIN ASBESTOS FIBERS FROM PRODUCTION OR
AFTERMARKET BRAKE LININGS. BREATHING
EXCESSIVE CONCENTRATIONS OF ASBESTOS
FIBERS CAN CAUSE SERIOUS BODILY HARM.
EXERCISE CARE WHEN SERVICING BRAKE
PARTS. DO NOT SAND OR GRIND BRAKE LINING
UNLESS EQUIPMENT USED IS DESIGNED TO CON-
TAIN THE DUST RESIDUE. DO NOT CLEAN BRAKE
PARTS WITH COMPRESSED AIR OR BY DRY
BRUSHING. CLEANING SHOULD BE DONE BY
DAMPENING THE BRAKE COMPONENTS WITH AFINE MIST OF WATER, THEN WIPING THE BRAKE
COMPONENTS CLEAN WITH A DAMPENED CLOTH.
DISPOSE OF CLOTH AND ALL RESIDUE CONTAIN-
ING ASBESTOS FIBERS IN AN IMPERMEABLE
CONTAINER WITH THE APPROPRIATE LABEL. FOL-
LOW PRACTICES PRESCRIBED BY THE OCCUPA-
TIONAL SAFETY AND HEALTH ADMINISTRATION
(OSHA) AND THE ENVIRONMENTAL PROTECTION
AGENCY (EPA) FOR THE HANDLING, PROCESSING,
AND DISPOSING OF DUST OR DEBRIS THAT MAY
CONTAIN ASBESTOS FIBERS.
INSPECTION - DISC BRAKE SHOES
Visually inspect brake shoes (pads) for uneven lin-
ing wear. Also inspect for excessive lining deteriora-
tion. Check the clearance between the tips of the
wear indicators on the shoes (if equipped) and the
brake rotors.
If a visual inspection does not adequately deter-
mine the condition of the lining, a physical check will
be necessary. To check the amount of lining wear,
remove the disc brake shoes from the calipers.
Measure each brake shoe. The combined brake
shoe and its lining material thickness should be mea-
sured at its thinnest point.
²For front disc brake shoes, when a set of brake
shoes are worn to a thickness of approximately 7.95
mm (5/16 inch), they should be replaced.
²For rear disc brake shoes, when a set of brake
shoes are worn to a thickness of approximately 7.0
mm (9/32 inch), they should be replaced.
Replacebothdisc brake shoes (inboard and out-
board) on each caliper. It is necessary to replace the
Fig. 13 Removing Inboard Shoe
1 - INBOARD BRAKE SHOE
2 - HANGER WIRE
3 - CALIPER ASSEMBLY
4 - RETAINING CLIP
5 - PISTON
Fig. 14 Brake Caliper Mounting (Typical)
1 - BRAKE HOSE
2 - ADAPTER MOUNTING BOLTS
3 - BANJO BOLT
4 - CALIPER GUIDE PIN BOLTS
5 - 14 BRAKES - BASERS
BRAKE PADS/SHOES - FRONT (Continued)
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CLEANING - DISC BRAKE SHOES
WARNING: DUST AND DIRT ACCUMULATING ON
BRAKE PARTS DURING NORMAL USE MAY CON-
TAIN ASBESTOS FIBERS FROM PRODUCTION OR
AFTERMARKET BRAKE LININGS. BREATHING
EXCESSIVE CONCENTRATIONS OF ASBESTOS
FIBERS CAN CAUSE SERIOUS BODILY HARM.
EXERCISE CARE WHEN SERVICING BRAKE
PARTS. DO NOT SAND OR GRIND BRAKE LINING
UNLESS EQUIPMENT USED IS DESIGNED TO CON-
TAIN THE DUST RESIDUE. DO NOT CLEAN BRAKE
PARTS WITH COMPRESSED AIR OR BY DRY
BRUSHING. CLEANING SHOULD BE DONE BY
DAMPENING THE BRAKE COMPONENTS WITH A
FINE MIST OF WATER, THEN WIPING THE BRAKE
COMPONENTS CLEAN WITH A DAMPENED CLOTH.
DISPOSE OF CLOTH AND ALL RESIDUE CONTAIN-
ING ASBESTOS FIBERS IN AN IMPERMEABLE
CONTAINER WITH THE APPROPRIATE LABEL. FOL-
LOW PRACTICES PRESCRIBED BY THE OCCUPA-
TIONAL SAFETY AND HEALTH ADMINISTRATION
(OSHA) AND THE ENVIRONMENTAL PROTECTION
AGENCY (EPA) FOR THE HANDLING, PROCESSING,
AND DISPOSING OF DUST OR DEBRIS THAT MAY
CONTAIN ASBESTOS FIBERS.
INSPECTION - DISC BRAKE SHOES
Visually inspect brake shoes (pads) for uneven lin-
ing wear. Also inspect for excessive lining deteriora-
tion. Check the clearance between the tips of the
wear indicators on the shoes (if equipped) and the
brake rotors.
If a visual inspection does not adequately deter-
mine the condition of the lining, a physical check will
be necessary. To check the amount of lining wear,
remove the disc brake shoes from the calipers.
Measure each brake shoe. The combined brake
shoe and its lining material thickness should be mea-
sured at its thinnest point.
²For front disc brake shoes, when a set of brake
shoes are worn to a thickness of approximately 7.95
mm (5/16 inch), they should be replaced.
²For rear disc brake shoes, when a set of brake
shoes are worn to a thickness of approximately 7.0
mm (9/32 inch), they should be replaced.
Replacebothdisc brake shoes (inboard and out-
board) on each caliper. It is necessary to replace the
shoes on the opposite side of the vehicle as well as
the shoes failing inspection.
If the brake shoe assemblies do not require
replacement, be sure to reinstall the brake shoes in
the original position they were remove from.
INSTALLATION - REAR DISC BRAKE SHOES
(1) Completely retract caliper piston back into pis-
ton bore of caliper assembly.
(2) Lubricate both adapter abutments where the
shoes contact with a liberal amount of MopartMul-
tipurpose Lubricant, or equivalent.
(3) If removed, install the brake rotor on the hub,
making sure it is squarely seated on the face of the
hub.
(4) Install the inboard brake shoe into the caliper
piston by firmly pressing it into the piston bore using
your thumbs. Be sure inboard brake shoe is posi-
tioned squarely against the face of the caliper piston.
(5) Install the outboard brake shoe on the disc
brake caliper. Be sure the outboard shoe is positioned
squarely against the outboard fingers of the caliper.
CAUTION: Use care when installing the caliper
assembly onto the adapter, so the caliper guide pin
bushings do not get damaged by the mounting
bosses.
(6) Carefully lower caliper and brake shoes over
rotor and onto adapter, reversing the removal proce-
dure (Fig. 16).
CAUTION: When installing the caliper guide pin
bolts extreme caution should be taken not to
crossthread the guide pin bolts.
(7) Install the caliper guide pin bolts. Tighten the
guide pin bolts to a torque of 35 N´m (26 ft. lbs.).
(8) Install the wheel and tire assembly. Tighten
the wheel mounting nuts in proper sequence until all
nuts are torqued to half specification. Then repeat
the tightening sequence to the full specified torque of
135 N´m (100 ft. lbs.).
(9) Remove jackstands or lower hoist.
CAUTION: Before moving vehicle, pump the brake
pedal several times to insure the vehicle has a firm
brake pedal to adequately stop the vehicle.
(10) Pump brake pedal several times to set brake
shoes to rotors.
(11) Check fluid level in reservoir.
(12) Road test the vehicle and make several stops
to wear off any foreign material on the brakes and to
seat the brake shoe linings.
RSBRAKES - BASE5-17
BRAKE PADS/SHOES - REAR DISC (Continued)
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DISASSEMBLY - CALIPER GUIDE PIN
BUSHINGS (DISC/DISC BRAKES)
Before disassembling the brake caliper, clean and
inspect it. Refer to CLEANING or INSPECTION in
this section.
(1) Using your fingers, collapse one side of the rub-
ber guide pin bushing. Pull the guide pin bushing out
the other side of the brake caliper mounting boss.
(2) Repeat this procedure on the remaining bush-
ing.
DISASSEMBLY - CALIPER PISTON AND SEAL
WARNING: UNDER NO CONDITION SHOULD HIGH
PRESSURE AIR EVER BE USED TO REMOVE A PIS-
TON FROM A CALIPER BORE. PERSONAL INJURY
COULD RESULT FROM SUCH A PRACTICE.
NOTE: Before disassembling the brake caliper,
clean and inspect it. Refer to CLEANING AND
INSPECTION in this section.
NOTE: The safest way to remove the piston from
the caliper bore is to use the hydraulic pressure of
the vehicle's brake system.
(1) Following the removal procedure in DISC
BRAKE SHOES found in this section, remove the
caliper from the brake rotor and hang the assembly
on a wire hook away from rotor and body of the vehi-
cle so brake fluid cannot get on these components.
Remove the brake shoes, and place a small piece of
wood between the piston and caliper fingers.
(2) Carefully depress the brake pedal to hydrauli-
cally push piston out of its bore. Once completed,
apply and hold down the brake pedal to any position
beyond the first inch of pedal travel using a brake
pedal holding tool. This will prevent the fluid in the
master cylinder reservoir from completely draining
out.
(3) Disconnect the brake fluid flex hose from the
caliper assembly and remove it from the vehicle.
CAUTION: Do not use excessive force when clamp-
ing caliper in vise. Excessive vise pressure will
cause bore distortion.(4) Mount the caliper in a vise equipped with pro-
tective jaws.
(5) Remove the piston dust boot from the caliper
and discard.
NOTE: Do not use a screw driver or other metal tool
for seal removal. Using such tools can scratch the
bore or leave burrs on the seal groove edges.
(6) Using a soft tool such as a plastic trim stick,
work the piston seal out of its groove in caliper pis-
ton bore (Fig. 33). Discard the old seal.
(7) Clean the piston bore and drilled passage ways
using alcohol or a suitable solvent. Wipe it dry using
only a lint-free cloth.
(8) Inspect the piston bore for scoring or pitting.
Bores that show light scratches or corrosion can usu-
ally be cleared of the light scratches or corrosion
using crocus cloth.
Fig. 33 Removing Piston Seal
1 - PLASTIC TRIM STICK
2 - CALIPER
3 - PISTON SEAL GROOVE
4 - PISTON SEAL
RSBRAKES - BASE5-23
DISC BRAKE CALIPERS - FRONT (Continued)
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DISASSEMBLY - CALIPER PISTON AND SEAL
WARNING: UNDER NO CONDITION SHOULD HIGH
PRESSURE AIR EVER BE USED TO REMOVE A PIS-
TON FROM A CALIPER BORE. PERSONAL INJURY
COULD RESULT FROM SUCH A PRACTICE.
NOTE: Before disassembling the brake caliper,
clean and inspect it. Refer to CLEANING AND
INSPECTION in this section.
NOTE: The safest way to remove the piston from
the caliper bore is to use the hydraulic pressure of
the vehicle's brake system.
(1) Following the removal procedure in DISC
BRAKE SHOES found in this section, remove the
caliper from the brake rotor and hang the assembly
on a wire hook away from rotor and body of the vehi-
cle so brake fluid cannot get on these components.
Remove the brake shoes, and place a small piece of
wood between the piston and caliper fingers.
(2) Carefully depress the brake pedal to hydrauli-
cally push piston out of its bore. Once completed,
apply and hold down the brake pedal to any position
beyond the first inch of pedal travel using a brake
pedal holding tool. This will prevent the fluid in the
master cylinder reservoir from completely draining
out.(3) Disconnect the brake fluid flex hose from the
caliper assembly and remove it from the vehicle.
CAUTION: Do not use excessive force when clamp-
ing caliper in vise. Excessive vise pressure will
cause bore distortion.
(4) Mount the caliper in a vise equipped with pro-
tective jaws.
(5) Remove the piston dust boot from the caliper
and discard.
NOTE: Do not use a screw driver or other metal tool
for seal removal. Using such tools can scratch the
bore or leave burrs on the seal groove edges.
(6) Using a soft tool such as a plastic trim stick,
work the piston seal out of its groove in caliper pis-
ton bore (Fig. 39). Discard the old seal.
(7) Clean the piston bore and drilled passage ways
using alcohol or a suitable solvent. Wipe it dry using
only a lint-free cloth.
(8) Inspect the piston bore for scoring or pitting.
Bores that show light scratches or corrosion can usu-
ally be cleared of the light scratches or corrosion
using crocus cloth.
Fig. 38 Removing/Installing Caliper
1 - LIFT THIS END OF CALIPER AWAY FROM ADAPTER FIRST
2 - DISC BRAKE CALIPER
3 - ADAPTER ABUTMENT
4 - OUTBOARD BRAKE SHOE HOLD DOWN CLIP
5 - OUTBOARD BRAKE SHOE
6 - ROTOR
7 - ADAPTER
Fig. 39 Removing Piston Seal
1 - PLASTIC TRIM STICK
2 - CALIPER
3 - PISTON SEAL GROOVE
4 - PISTON SEAL
RSBRAKES - BASE5-27
DISC BRAKE CALIPERS - REAR (Continued)
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OPERATION - PROPORTIONING VALVE
(HEIGHT SENSING)
Vehicles not equipped with ABS use a height sens-
ing proportioning valve.
The height sensing proportioning valve operates
similarly to a standard proportioning valve in the fol-
lowing way. As hydraulic pressure is applied to the
valve, full input hydraulic pressure is supplied to the
rear brakes up to a certain pressure point, called the
split point. Beyond the split point, the proportioning
valve reduces the amount of hydraulic pressure to
the rear brakes according to a given ratio. Thus, on
light brake applications, approximately equal
hydraulic pressure will be transmitted to both the
front and rear brakes. Upon heavier brake applica-
tions, the hydraulic pressure transmitted to the rear
brakes will be lower than the front brakes. This will
prevent premature rear wheel lockup and skid.
Here is how the height sensing proportioning valve
differs from a standard proportioning valve. As the
height of the rear suspension changes, the height
sensing portion of the proportioning valve changes
the split point of the proportioning valve. When the
height of the rear suspension is low, the proportion-
ing valve interprets this as extra load and the split
point of the proportioning valve is raised to a higher
pressure to allow for more rear braking. When the
height of the rear suspension is high, the proportion-
ing valve interprets this as a light load and the split
point of the proportioning valve is lowered to a lower
pressure and rear braking is reduced.
The height sensing proportioning valve regulates
the pressure by sensing the load condition of thevehicle through the movement of the proportioning
valve actuator lever (Fig. 61). As the position of the
rear axle changes, depending on the load the vehicle
is carrying, the movement is transferred to the pro-
portioning valve. The proportioning valve adjusts the
hydraulic pressure accordingly.
The height sensing proportioning valve allows the
brake system to maintain the optimal front to rear
brake balance regardless of the vehicle load condi-
tion. Under a light load condition, hydraulic pressure
to the rear brakes is minimized. As the rear load con-
dition increases, so does the hydraulic pressure to
the rear brakes.
DIAGNOSIS AND TESTING - PROPORTIONING
VALVE (HEIGHT SENSING)
CAUTION: The use of aftermarket load leveling or
load capacity increasing devices on this vehicle is
prohibited. Using air shock absorbers or helper
springs on this vehicle will cause the height sens-
ing proportioning valve to inappropriately reduce
the hydraulic pressure to the rear brakes. This inap-
propriate reduction in hydraulic pressure potentially
could result in increased stopping distance of the
vehicle.
When a premature rear wheel skid is obtained on a
brake application, it may be an indication that the
hydraulic pressure to the rear brakes is above the
specified output from the proportioning valve. This
condition indicates a possible malfunction of the
height sensing proportioning valve, which will
require testing to verify that it is properly controlling
the hydraulic pressure allowed to the rear brakes.
Premature rear wheel skid may also be caused by
contaminated front or rear brake linings.
Prior to testing a proportioning valve for function,
check that all tire pressures are correct. Also, ensure
the front and rear brake linings are in satisfactory
condition.It is also necessary to verify that the
brakes shoe assemblies on a vehicle being
tested are either original equipment manufac-
turers (OEM) or original replacement brake
shoe assemblies meeting the OEM lining mate-
rial specification. This vehicles brake system is
not balanced for aftermarket brake shoe assem-
bly lining material.
If both front and rear brakes check OK, proceed
with the following test procedure for the height sens-
ing proportioning valve.
(1) Road test the vehicle to determine which rear
wheel brake is exhibiting premature wheel skid.
Fig. 61 HEIGHT SENSING PROPORTIONING VALVE
1 - PROPORTIONING VALVE
2 - ACTUATOR LEVER
3 - AXLE BRACKET
4 - REAR AXLE
5 - 42 BRAKES - BASERS
PROPORTIONING VALVE (Continued)
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tem enters the ABS mode. During ABS braking,
hydraulic pressure in the four wheel circuits is mod-
ulated to prevent any wheel from locking. Each
wheel circuit is designed with a set of electric sole-
noids to allow modulation, although for vehicle sta-
bility, both rear wheel solenoids receive the same
electrical signal. Wheel lockup may be perceived at
the very end of an ABS stop and is considered nor-
mal.
During an ABS stop, the brakes hydraulic system
is still diagonally split. However, the brake system
pressure is further split into three control channels.
During antilock operation of the vehicle's brake sys-
tem, the front wheels are controlled independently
and are on two separate control channels, and the
rear wheels of the vehicle are controlled together.
The system can build and release pressure at each
wheel, depending on signals generated by the wheel
speed sensors (WSS) at each wheel and received at
the controller antilock brake (CAB).
NOISE AND BRAKE PEDAL FEEL
During ABS braking, some brake pedal movement
may be felt. In addition, ABS braking will create
ticking, popping, or groaning noises heard by the
driver. This is normal and is due to pressurized fluid
being transferred between the master cylinder and
the brakes. If ABS operation occurs during hard
braking, some pulsation may be felt in the vehicle
body due to fore and aft movement of the suspension
as brake pressures are modulated.
At the end of an ABS stop, ABS is turned off when
the vehicle is slowed to a speed of 3±4 mph. There
may be a slight brake pedal drop anytime that the
ABS is deactivated, such as at the end of the stop
when the vehicle speed is less than 3 mph or during
an ABS stop where ABS is no longer required. These
conditions exist when a vehicle is being stopped on a
road surface with patches of ice, loose gravel, or sand
on it. Also, stopping a vehicle on a bumpy road sur-
face activates ABS because of the wheel hop caused
by the bumps.
TIRE NOISE AND MARKS
Although the ABS system prevents complete wheel
lockup, some wheel slip is desired in order to achieve
optimum braking performance. Wheel slip is defined
as follows: 0 percent slip means the wheel is rolling
freely and 100 percent slip means the wheel is fully
locked. During brake pressure modulation, wheel slip
is allowed to reach up to 25±30 percent. This means
that the wheel rolling velocity is 25±30 percent less
than that of a free rolling wheel at a given vehicle
speed. This slip may result in some tire chirping,
depending on the road surface. This sound should not
be interpreted as total wheel lockup.Complete wheel lockup normally leaves black tire
marks on dry pavement. The ABS will not leave dark
black tire marks since the wheel never reaches a
fully locked condition. However, tire marks may be
noticeable as light patched marks.
START-UP CYCLE
When the ignition is turned on, a popping sound
and a slight brake pedal movement may be noticed.
The ABS warning lamp will also be on for up to 5
seconds after the ignition is turned on. When the
vehicle is first driven off, a humming may be heard
or felt by the driver at approximately 20±40 kph
(12±25 mph). All of these conditions are a normal
function of ABS as the system is performing a diag-
nosis check.
PREMATURE ABS CYCLING
Symptoms of premature ABS cycling include: click-
ing sounds from the solenoid valves; pump/motor
running; and pulsations in the brake pedal. Prema-
ture ABS cycling can occur at any braking rate of the
vehicle and on any type of road surface. Neither the
red BRAKE warning lamp, nor the amber ABS warn-
ing lamp, illuminate and no fault codes are stored in
the CAB.
Premature ABS cycling is a condition that needs to
be correctly assessed when diagnosing problems with
the antilock brake system. It may be necessary to use
a DRB scan tool to detect and verify premature ABS
cycling.
Check the following common causes when diagnos-
ing premature ABS cycling: damaged tone wheels;
incorrect tone wheels; damaged steering knuckle
wheel speed sensor mounting bosses; loose wheel
speed sensor mounting bolts; excessive tone wheel
runout; or an excessively large tone wheel-to-wheel
speed sensor air gap. Give special attention to these
components when diagnosing a vehicle exhibiting
premature ABS cycling.
After diagnosing the defective component, repair or
replace it as required. When the component repair or
replacement is completed, test drive the vehicle to
verify that premature ABS cycling has been cor-
rected.
OPERATION - ELECTRONIC BRAKE
DISTRIBUTION
Upon entry into EBD the inlet valve for the rear
brake circuit is switched on so that the fluid supply
from the master cylinder is shut off. In order to
decrease the rear brake pressure, the outlet valve for
the rear brake circuit is pulsed. This allows fluid to
enter the low pressure accumulator (LPA) in the
hydraulic control unit (HCU) resulting in a drop in
fluid pressure to the rear brakes. In order to increase
5 - 68 BRAKES - ABSRS
BRAKES - ABS (Continued)
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