Spicer Tire Pressure Control - Manual (Page 4)

Since the Spicer Tire Pressure Control
System allows the independent control of
pressures on steer, drive and trailer tire
groups, the times must be calculated for each
of the channels and added together for a total
time. For example, let’s presume a typical 18
wheel tractor trailer combination, with two
tires on the steer channel and eight each on
the drive and trailer axle groups, an internal
tire volume of 4.5 ft
3 for each tire and a
compressor output of 13.2 SCFM. (Typically,
compressor output is rated at a nominal
engine output speed such as 1250 RPM).
Furthermore, lets presume the following
pressure increases between the off highway
and highway pressure settings:
Channel Off Hwy Highway Increase
Steers - 70 90 20
Drives - 55 90 35
Trailers - 50 90 40
The analysis would be as follows:
Steer Channel
{(2x4.5)/13.2} X 20/14.7 = .9 minutes
Drive Channel
{(8x4.5)/13.2} X 35/14.7 = 6.5 minutes
Trailer Channel
{(8x4.5)/13.2} X 40/14.7 = 7.4 minutes
Total Time
.9 + 6.5 + 7.4 = 14.8 min.
This analysis is only an approximation. It
does not take into account several major
factors such as compressor efficiency (which
will change over time) or engine speed (infla-
tion time can be improved by downshifting
until highway pressures are achieved), or
numerous minor factors (such as atmospher-
ic variables or the compressibility factor of
air). It should suffice however, to guide one
through the task of compressor selection. The
use of turbo-charged compressors can result
in inflation times that improve as engine
loads are increased only up to the limit of the
system’s ability to use the air (approximately
20 SCFM), but this improvement should be
weighed against its impact on compressor
life and overall vehicle air system design.
Too much air can be detrimental to system
performance as the excessive compressor
temperatures generated can result in a higher
level of contamination (oil blow-by) and
reduced air dryer effectiveness, possibly
requiring the use of additional means to
cool the air, such as condensers and
heat exchangers.
How does one diagnose service
items for TPCS?
The Spicer ® TPCS wiring harness includes a
connector which meets industry standard
requirements for connection to modern
diagnostic tools. The interface is fully
compatible with other vehicle systems to also
allow connection through a central OEM
supplied connector interface. Both hand held
tools, such as those from MPSI, and personal
computers may be used. Use of a personal
computer requires the purchase of a serial
interface connector (such as Kent Moore part
number J38351 or B&B Electronics part
number 232ESAER) to make the connection.
By plugging the tool connector into the TPCS
diagnostic connector, the technician can do a
complete diagnostic analysis of the system,
including retrieval of historic service codes.
Diagnostic software discs are available for
System users at no charge from Dana Spicer
Heavy Axle and Brake Division.
How does the Spicer System detect
tire imbalances and low tires?
The Spicer ® Tire Pressure Control System
uses sophisticated microcomputer technolo-
gy and has extraordinary capability to analyze
information. Dana engineers have developed
algorithms for the System which critically
evaluate many details of operation. Much of
this evaluation is done in the short period of
time between initiation of a pressure check
(or selection of a new pressure mode) and
the decision to proceed with a change
of pressure.
The System evaluates the pneumatic signa-
ture from the time of the wheel valve opening
pulse to the time of the actual pressure read-
ing. During this period, approximately two
seconds, the pneumatic response of the
System is compared to acceptable responses
stored in the memory of the controller. If the
response merely indicates that the pressure of
a given channel is low, inflation will be initiat-
ed. If the response indicates that pressure is
higher than that requested, a deflation will be
commanded. Other responses may indicate
something less than acceptable readiness for
the change of tire pressure and trigger a
service code so the driver can evaluate how
best to proceed.
The signature analysis technique allows the
system to detect if one tire is significantly out
of balance from the others in the same axle
group. Parameters for the low tire detection
have been established to trigger the “check
tire” icon when the pressure in one wheel
end of a channel group is approximately 50%
or less than the others in the group. Since
such a loss of pressure could have occurred
over the 5 to 15 minute pressure check inter-
val, a substantial tire leak could be indicated
and therefore, the driver may also see an 11!P
code indicated after the completion of a pres-
sure changing sequence. The driver should
physically inspect the tires on the affected
axle group for possible leaks or damage
before proceeding. If a tire puncture is found,
the driver may select the run-flat key to
reduce the interval of tire pressure checks to
15 seconds and thereby use the system to
keep the tire inflated. If the tire damage is sig-
nificant, the system will not be able to keep
up with the leak and a 9P! or 10!P code may
result. In this event, the vehicle may require
immediate tire repair. –It is important to note
that tire damage is not always readily visible
and that operators should use care in deter-
mining the extent of tire damage. The only
reliable way to determine which tire is
low is to use a pressure gage and
inspect any tire which measures low.
Frequently, out of balance tire pressure con-
ditions occur on start-up after the vehicle has
been idle for extended periods (such as
weekends) as a result of slow leaks in the tire
and wheel assembly. The system can help to
identify these leaks early enough that the
observant owner can have small leaks
repaired before they become larger leaks
which force downtime and limit retreadability.
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