Section 6
General Development of Multiplexing
Learning Objectives: 1. To review the development of automotive multiplexing.
2. To discuss the application of multiplexing systems.
3. To correctly use the Handheld tester to retrieve codes and access
Freeze Frame Data.
4. To effectively use the Active Test Mode using the Handheld tester.
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Section 6
2 LEXUS Technical Training
General Development of Multiplexing
Multiplexing Invehicle networking involving time division is also known as
multiplexing. It is a method for transferring data among distributed
electronic modules via a serial data bus. Serial data is electronically
coded information which is transmitted by one computer and received
and displayed by another computer. The term serial data implies that
the information is digitally coded and transmitted in a series of data
words.
Serial data gets its name from the fact that data parameters are
transmitted, one after another, in series. The display on the receiving
computer updates or refreshes once each data cycle, after all data has
been received.
Transmission of Using an analog/digital circuit, the transmitting computer digitizes the
Data and Rates data from sensors, actuators, and other calculated information.
Typically, this means that each sensor or actuator value is converted
into a one byte (8 bits) binary word before it is transmitted to the
receiving computer. The data transmission rate is referred to as the
baud rate. Baud rate refers to the number of data bits that can be
transmitted per second. For example, if a data stream has 12
parameters, and each parameter is converted into an 8 bit data word,
the total size of the data transmission is 96 bits of data (12 words x 8
bits per word.) If this data can be transmitted once every second, the
baud rate is 96 bits/second or 96 baud.
Without serial networking, intermodule communication requires
dedicated, pointtopoint wiring resulting in bulky, expensive, complex,
and difficult to install wiring harnesses. Applying a serial data bus
reduces the number of wires by combining the signals on a single wire
through time division multiplexing. Information is sent to individual
control modules that control each function, such as antilock braking,
turn signals, power windows, dashboard displays, and audio systems.
Advances and Invehicle networking provides systemlevel benefits, many of which
Standards are only beginning to be realized:
A decreased number of dedicated wires is required for each
function, and thus reduces the size of the wiring harness. System
cost, weight, reliability, serviceability, and installation are
improved.
Common sensor data, such as vehicle speed, engine temperature,
etc. are available on the network, so data can be shared, thus
eliminating the need for redundant sensors.
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Section 6
Networking allows greater vehicle content flexibility because
functions can be added through software changes. Existing systems
require an additional module or additional I/O pins for each
function added.
Car manufacturers are discovering new features that are enabled
by networking. For example, multiplexing allows drivers
preference for such things as ride firmness, seat positions, steering
assist effort, mirror positions, and radio station presets.
Recently, there have been efforts to standardize protocols at the data
link and physical layers. Systems designers are also seeing the benefits
of standardized application layer protocols. Multiplexing Standards are
also appearing in automotive applications. Examples include: SAE
J1939, OSEK from the German automotive consortium, SDS from
Honeywell, and DeviceNet from AllenBradley. These standards allow
system designers to avoid lowlevel protocol details and focus on the
application itself. However, the impact of this type of standardization is
increased demand on the microcontrollers and protocol devices; and
thus the need for efficient message handling and standardized protocol.
Typical Multiplexing System
4 LEXUS Technical Training
General Development of Multiplexing
Early Designs The early days of networking involved proprietary serial buses using
generic UART (Universal Asynchronous Receiver/Transmitter) or
custom devices. This type of networking was acceptable in the United
States because the Big Three (Ford, GM, Chrysler) were vertically
integrated and were not highly dependent on external suppliers.
However, in Europe and increasingly now in the U.S.A., car
manufacturers utilize many external suppliers. Proprietary protocols
pose many difficulties with suppliers who need many special system
designs to conform to the different protocols. Standard protocols allow
modules from many suppliers to easily link together forming a type of
open architecture." An open architecture will allow standardized
diagnostic and emissions testers and will allow suppliers to benefit
from economies of scale of massproduced standard protocol devices.
These facts help to explain why diagnostics and systems designs were
not widely published in many repair manuals or other forms of
literature published by automotive manufacturers. Since the advent of
standards the information regarding these multiplexing systems has
become increasingly more accessible.
SAE Class A
Classifications/
Speeds Low Speed (<10 kbps [kilobits/second])
Convenience features (entertainment, audio, trip computer, etc.)
Class B
Medium Speed (10 kbps to 125 kbps)
General information transfer (instrument cluster, vehicle speed,
emissions data, etc.)
Class C
High Speed (125 kbps to 1 Mbps [Megabits per second] or greater)
Realtime control (powertrain control, vehicle dynamics, brake by
wire)
Body Electrical Diagnosis - Course L652 5
Section 6
Wiring Diagram for a Typical
Class B Multiplex System
Lexus systems such as the one pictured here it is possible to view the
signal transmissions to and from the control unit along the data links
with an oscilloscope. Assuming that the scope is set at the appropriate
capture rate the signal pattern will be displayed. Unfortunately,
without signal printouts and engineering specifications these scope
patterns cannot be fully analyzed. This type of diagnosis does tell the
technician that communication is occurring. What it does not reveal is
the nature of the communication or its accuracy. Nevertheless, it can be
valuable when tracing the presence of a signal along a specific
pathwayconnector to connector. The first line of diagnostics when
dealing with Lexus multiplexing must be the verification, the Repair
Manual, and the Diagnostic Tester. Anything less is simply guess work.
Diagnostic Trouble Codes include SAE controlled codes and
manufacturer controlled codes. SAE controlled codes must be set as
prescribed by the SAE, while manufacturer controlled codes can be set
freely by the manufacturer within the prescribed limits. There is also a
check mode for technicians to simulate malfunction symptoms and to
troubleshoot. Basically, the Body ECU monitors the communication
within the network and triggers a code when there is an interruption
in communication, a +B short, or a GND short.
6 LEXUS Technical Training
General Development of Multiplexing
Using The use of the Lexus Diagnostic Tester for the diagnosis of
the Lexus multiplexing DTCs is explained in detail in the Repair Manual. As
Diagnostic these systems evolve, changes occur from model year to model year. In
Tester order to avoid unnecessary repairs and loss of valuable shop time, it is
very important that the technician read and understand the repair
manual instructions for a particular model and year.
Example: In the 1998 LX 470 Repair Manual it is noted during
the PreCheck instructions that unless the Body ECU is
functioning normally there is the possibility" the diagnosis for
other units will not be accurate. Also, there are conditions
under which an accurate DTC will not be given even though
the Body ECU is normal.
Next, there is a procedure for checking the Body ECU/CPU in order to
determine whether or not it is working properly. It involves confirming
the operation of the Body ECU and the Open Door Indicator. This
procedure falls under the heading of Basic Inspection". The important
thing to keep in mind is that unless you can be sure that the ECU
responsible for diagnostics is functioning properly, all other
information may not be of value. There are also certain malfunction
configurations where a DTC will not be detected.
Pre-Check
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Section 6
Circuit There are several layers of operation to consider when troubleshooting
Operation a Lexus multiplexing system. Lets take a look at the construction of a
system. Keep in mind that these systems are very sensitive to
electronic variables within the environment. Furthermore, we have
mechanical operation of various components and these too can cause
problems for the system. Take a look at a typical Tilt/Telescopic Switch.
In this case it comes from an LX 470.
Now that the ECU has the signal from the switch it executes a
command to the Telescopic motor. In this example, the command is
Contract". Remember, all positions of components (motors, switches)
are known to the Instrument ECU. This memory function is one of the
wonders of multiplexing; but it is also one of the potential problems.
Why? Because all signals, confirmations, positions, ON/OFF settings
must travel through the communication lines in order to make
adjustments. If there are any breakdowns along these lines then the
system will not function properly. Also, bear in mind that any out of
range" transmissions can be misinterpreted and the result will be a
DTC, a malfunction, or an inoperative condition. Look at the following
diagram as the ECU interacts with the motor in the Telescopic circuit.
Keep in mind that the procedures for diagnosing these systems may
vary from year to year and from model to model. Once again, the
diagnosis of any multiplex system should not be attempted without the
Repair Manual in hand.
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General Development of Multiplexing
Circuit
Operation Control and DTCs
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Section 6
Multiplexing
Communication Bus
1998 LX 470 EWD p. 136
10 LEXUS Technical Training
General Development of Multiplexing
Using the Although the theory of multiplexing is important to diagnostics, it is
Diagnostic just as important to understand the use of the hardware and software
Tester designed to assist in the diagnosis of the various systems. The most
critical tool you have for evaluating the newest Lexus vehicles and the
onboard multiplexing system is the Diagnostic Tester. You are already
familiar with the basic operation of this tool. Our next level of
diagnostics includes the Active Test" mode which allows us to judge
the operation or nonoperation of certain components. It is through
these procedures that we are able to make judgements about the
system in question and begin to move toward a diagnosis which will
ultimately repair the car.
Diagnostic Tester and Cables
The Complaint The first step in diagnosing the complaint involves verifying that the
system is exhibiting the stated complaint. It is important that you are
able to duplicate the same condition that the customer experiences
when they operate the system.
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Section 6
DTCs Remember that all of the systems in a multiplexing circuit are in one
way or another connected to the central control unit. This central
control unit (Body ECU) may be called by different names but it is
essentially the brains" of the operation. Every other control unit is
involved in one of three basic things: they are receiving orders,
executing orders, or delivering status reports. When any of these three
things fails to occur there is a likelihood that a DTC will be set. You
have already seen what is required to set a DTC (two opens, +B Short,
Ground short) which shows that a particular ECU has been isolated
from the main control unit. Its pathway of communication or power has
been broken; it is now isolated. The bottom line is that we are back to
basics. A connector is loose, a wiring harness has been cut, or a
component has an internal failure. Lets go through a step by step look
at what we might expect to see when we are faced with a diagnostic
problem.
System Check First, we need to make certain that we have communication with the
central processing unit. There will be a procedure in the Repair
Manual for checking this operation and currently that procedure
involves turning on the windshield wipers in the INT mode. If the
wipers function normally then we have communication with the control
unit. This procedure can change from year to year and model to model.
Second, we need to open a door to see if the Door Open" indicator light
turns on.
If the light turns on, then we have confirmed that trouble codes can be
output to our Diagnostic Tester.
System Check
Accessing the The next step is to hook up the scan tool and go to the OBD/MOBD
Multiplex System selection menu. You will notice that there are several areas which you
can test and there are more selections visible beyond the first screen.
12 LEXUS Technical Training
General Development of Multiplexing
OBD/MOBD Menu
Screen 1
Single System You will need to scroll down to the system description and the item you
Check wish to check. For this example lets look at Tilt and Telescopic.
OBD/MOBD Menu
Screen 2
Gathering From this menu we are taken to the actual values which are found
Evidence within the Tilt and Telescopic system. Notice that the Tilt Position is
4136 and the Telescopic TELSC" Position is 4270. In the example you
saw earlier there was a difference between the Actual" and the
Position Limit". Knowing that this difference existed gives a clue that
there is a problem in the control unit or the mechanism itself. It is
extremely unlikely that it is a harness or connector problem.
Body Electrical Diagnosis - Course L652 13
Section 6
Tilt &
Telescopic Data
Choosing the By using these kinds of tests we begin to gather data about the
Right Items for condition we are trying to diagnose. On the one hand, we are looking at
the List the codes which may or may not be set in a system; on the other hand,
we are looking at values which may or may not be correct in a given
group of output values. The values below were selected from the data
list available. There would be no point in looking at everything. Select
only what is necessary.
Freeze
Frame Data
Telescopic Position should
not be greater than Short
Limit Position
Thinking It Remember this example? The position of the telescopic wheel is beyond
Through the Short Limit Position". Therefore, it is out of specifications. The
difficult part of this diagnosis is making the comparison between these
things and determining what each value stands for from available
information. If we follow a logical pathway in the diagnosis of this
condition we might conclude that there is something internal to the
telescopic motor which is causing it to travel too far. This additional
14 LEXUS Technical Training
General Development of Multiplexing
travel may give rise to a host of other problems within the system. It
may lead to an inability to set a memory position. Also, we might see
that the return to rest is not where it was previously. There are many
possibilities. What is a good bet is that given these values, the
replacement of the Tilt and Telescopic Motor assembly may resolve the
problem.
The key to this type of diagnosis is patience above all. There are a
number of things going on at the same time. A close reading of the
repair manual and supplemental material is critical. Finally, you must
be prepared to spend time with the car and use the diagnostic scan tool
carefully and deliberately. In the work we will do in the shop, we are
only going to focus on becoming familiar with the Diagnostic Tester so
that we can move from system to system and view the various inputs
and outputs. The subject of more advanced diagnostics will be covered
later in a more detailed course. For now, learn to use the Diagnostic
Tester to enter into the multiplexing systems and take a look around.
You will find that this new area of electronic testing may be easier than
you thought.
Body Electrical Diagnosis - Course L652 15
Section 6
Diagnostic On some screens *Help will show important parameter information. If
Tester Help there is some doubt about the value you see on the screen, press *Help
to see whether or not there is additional information.
Finally, another reference book you should have close by is the
Diagnostic Tester Operators Manual. This publication lists all the
functions on the scantool and has many other useful hints.
Diagnostic
Tester Operators Manual
Active Keys
16 LEXUS Technical Training
