Chapter 12 Driveline and Wheel Components 267
Indirect pressure monitoring systems do not have sen-
sors at each tire. Instead, they use the vehicle’s anti-lock
brake system, or ABS, to monitor tire pressure. Modern
anti-lock brake systems have wheel speed sensors on each
wheel. The ABS control module compares relative wheel
speeds of all tires. When a tire loses air, the distance
between the tire tread and the center of the wheel
decreases. This smaller diameter causes a tire’s speed to
increase. The control module reads the higher speed and
compares it with the speed of the other tires. If tire speed
passes a set threshold, the module turns on an instrument
panel warning light.
Both systems have advantages and disadvantages. The
direct system is more accurate, but requires wheel sensors,
which must be handled carefully when the tire is removed
from the rim. A recalibration procedure must be performed
whenever tires are rotated. The indirect system does not
require any wheel hardware, and recalibration is usually
simpler. However it is less accurate and cannot detect
some common tire pressure problems, such as all four tires
gradually losing air.
Summary
Common driveline components are the drive axles
and shafts, flexible joints, and related parts. Generally,
solid axles are used on rear-wheel drive vehicles and
independent axles are used on front-wheel drive vehicles.
Tire
Valve
stem
Tapered head
lug bolt
Steel wheel
Locking lug
bolt
Locking lug
bolt key
Cap
Hub cap
Balance
weight
Balance
weight
clip
Figure 12-41. This wheel is secured to the hub or flange (not
shown) with tapered lug bolts. (Audi)
warning light illuminates, informing the driver that one or
more tires are underinflated. The driver can then add air or
have the tire(s) serviced as necessary.
Pressure monitoring systems are either direct or indi-
rect systems. The direct pressure monitoring system con-
sists of a central control module and wireless pressure
transmitters at each wheel. The transmitters send a signal to
the module. If any transmitter signals low pressure, the
module illuminates an instrument panel warning light,
Figure 12-42.
Figure 12-42. If a pressure sensor detects low tire pressure, it sends a radio signal to a receiver in the passenger compartment.This
signal triggers a warning light on the dash. (Toyota)
Dash
display
Pressure
sensors
Warning
light
Pressure
sensor
Pressure
sensor
Warning
chime
Sensor
signal
Receiver