BJA17882 Service Bulletin/Repair Instructions

Job Aid

September 2021
Version 1
MOST Bus Network: Overview, Troubleshooting, and Repairs

APPLIES TO

Year
Model
Trim Level
2022 and later
MDX
ALL
2019 and later
RDX
ALL
2021 and later
TLX
ALL
BACKGROUND / INTRODUCTION

This job aid contains information on the MOST Bus Network and troubleshooting information for common problems.

NOTE: This job aid replaces the following publications and Tech2Tech video segments:

 Service Bulletin 19-062, Service Manual Update: MOST Bus Network Failure Log

 Service News, Popping or Crackling from the Speakers? Check the MOST Bus Network

 Job Aid, Using the MOST Connector Tools

 Tech2Tech Segment, Get to Know the MOST Connector Tools

 Tech2Tech Segment, A Look at MOST Bus Network Connectors

 Tech2Tech Segment, Let’s Talk MOST Bus Network and ECL Diagnostics

 Tech2Tech Segment, Check Out the MOST Bus Network Failure Log

TOPICS COVERED

MOST Bus Network Overview
Page 2
MOST Electronic Control Line (ECL) Diagnostic Mode
Page 4
MOST Bus Network Failure Log
Page 12
Proper Connector Inspection & Harness Routing Procedures
Page 15
Using the Most Connector Tools
Page 22
© 2021 American Honda Motor Co., Inc. – All Rights Reserved
Page 1 of 23
MOST BUS NETWORK OVERVIEW

The Media Oriented Systems Transport or MOST Bus Network is used for high-speed bandwidth communication between

the audio/visual system components on certain Acura models. This is a serial network and is configured in a ring structure

that links one component to the next like a daisy chain that passes data in a single direction. A bus is used for

communicating audio, video, and control signals to each unit in the network.

Below are examples of the MOST Bus Network.

RDX
MDX, TLX
Page 2 of 23
The gauge control module acts as the gateway in the MOST Bus Network. Each control unit on the network has

transmitting and receiving coaxial cables and the electronic control line (ECL). The transmitting, known as a TX connector,

is green in color and the receiving, known as a RX connector, is red in color. Any open connections or shorts in the

network wiring will cause audio and visual system functions to be inoperative.
Within the TX and RX connections, there are the TX+ / TX- and the RX+ / RX- terminals. The “+” indicating the information

line and the “-“ indicating the shielding ground.
Page 3 of 23
MOST ELECTRONIC CONTROL LINE (ECL) DIAGNOSTIC MODE

A typical failure in the MOST Bus Network will cause a no-response condition from the infotainment system with the center

display unit going blank. Any component or connection failures within the network can cause this condition. If you see any

audio functions or information in the gauge control module, the blank screen may just be a display issue and not a network

failure.

When a failure occurs, it will be necessary to locate where in the network the failure occurred. To help you do this, a MOST

Bus Condition or commonly known as the ECL diagnostic mode is available in the gauge control module.

NOTE: The ECL diagnostic mode is only helpful if the network has a hard failure. Do not enter this mode if the network has

an intermittent failure. For intermittent failures, proceed to the MOST Bus Network Failure Log on page 12.

To start the ECL diagnostics, do the following:

1. Close all doors and the tailgate (if equipped).

2. Turn the vehicle to the ON mode.

3. Press the right selector wheel on the ACC combination switch to dismiss all warning or information messages

displayed on the multi-information display (MID).

4. Hold down the left selector wheel on the audio remote-HFL switch and then press and hold the BACK button until the

MOST bus diagnosis screen is displayed (for about 5 seconds).
How to read the display

The table below shows the meaning of the indicators.

Symbol
―
Status
OK
 The power or MOST signal status is OK at this time.
NG
 The component lost power or cannot receive a MOST signal.

 The component is not connected to the module using the bus.
N/A
 The component is offline.

 The component is not equipped.
Page 4 of 23
The Pwr (Power) column shows you if each component is online or not.
The Sig (Signal) column shows if the component is receiving a signal.
When using the ECL diagnostic mode, it is important to understand the network structure for the model you are working on.

Always refer to the network chart found in the service manual to identify the network’s communication flow in direction.
Page 5 of 23
This network chart can be found in service information. First, select the model and year. Then, enter “MOST Bus

Diagnostics Mode” in the keyword search. From the search results, click on Audio and Visual System Most

Diagnostics Mode.
Scroll down towards the bottom for the chart.
Page 6 of 23
Once you identify the communication flow, use the EWD to identify what components are equipped on the particular model

you are working on. To do this, go to the EWD. After choosing the trim or entering the VIN, go to Schematics, Body,

Multiplex Integrated Control System, then click on MOST Communication Line.
Since the EWD does not show a ring structure, use the network chart from the service manual to visualize the network

communication flow.

NOTE: Use this same procedure to find the network chart and the MOST communication line schematic in the EWD for the

MOST Bus Network Failure Log procedure on page 12.

Based on the network chart, the network communication flow can be visualized like below.
Page 7 of 23
NOTE: The component names between the service manual and the display will not match. Refer to the following table.
The table below shows the different combinations of indicator patterns.

Pwr
Sig
Description or Probable Cause

The power and MOST signal status is OK at this time.

The component is online but is not receiving signal on the network.

 Check the TX+ / RX+ and TX- / RX- lines between the previous control unit in the MOST

network ring.

 Check the wires and connector terminals. If they are OK, install a known-good component

and retest.

 Check the ground circuit of the indicted component.

 Check the wires and connector terminals. If they are OK, the component’s internal power

circuit may be faulty. Substitute a known-good component and retest.

The component is either offline or not equipped.

NOTE: If all components indicate double dashes, the MOST ECL line may be shorted.
―
―
 Check the network chart to confirm if the component is part of the network. The model and

trim you are working on may not be equipped with that component or is not part of the

network.

 Check the power and ground circuit for the indicated component.

 Check the wires and connector terminals. If they are OK, the component’s internal power

circuit may be faulty. Substitute a known-good component and retest.
NOTE: The infotainment control unit must never be substituted with a known-good unit as it will cause telematic

network problems.
Page 8 of 23
In this first example, the meter (gauge control module) and infotainment (infotainment control unit) reported a signal

problem. Go to the applicable EWD and review the circuit. When a problem occurs in the network, it means that there is a

problem at both the reporting component and the previous one in the network.
First, the infotainment control unit reported a problem. Looking at the circuit, the previous component is the gauge control

module.
Page 9 of 23
Second, the gauge control module also reported a problem. Looking at the circuit, the previous component is the tuner unit.

This means that the circuit from the tuner unit to the infotainment control unit needs to be checked.
Troubleshooting this circuit revealed a bad connection at the MOST connector in the gauge control module.
Page 10 of 23
In this next example, the amplifier (stereo amplifer) reported a problem.
Looking at the EWD, the previous component to the stereo amplifer is the infotainment control unit.
Troubleshooting this circuit revealed a bad connection at inline connector C405.
Video

For a video on ECL diagnostics, click on this link.

Page 11 of 23
MOST BUS NETWORK FAILURE LOG

Whenever a problem occurs in the network, the information of the failure is captured in a log. This failure log is primarily

meant to be used during troubleshooting intermittent network failures.

These failures may range from:

 Complete signal loss and a blank screen on the center display unit.

 Popping or crackling noise from the speakers.

The failure log lists each failure by date, time, and type.
There are two failure types:

 SUDDEN_SIGNAL_OFF: This failure is for a temporary or complete loss of signal on the network.

 CRITICAL_UNLOCK: This failure is for electrical noise on the network.
This log also identifies which control unit on the MOST Bus Network recorded the failure. The recording control unit tells

you the failure occurred between that control unit and the previous one on the network. This log can hold up to 10,000

failures, with the most recent events at the top.

Follow these steps to access the logs.

1. Turn the vehicle to the ON mode.

2. On the touchpad, press and hold the Back, Home, and Cards List buttons at the same time. Hold until the Dealer

Diagnostics menu screen appears in the primary information area. You will hear a tone when you release the buttons.
Page 12 of 23
3. Using Touchpad A, select DETAIL INFORMATION & SETTING.
4. Scroll down the menu and select SHUTDOWN REASON.
Look for the most recent even on the list. In the example below, the cause of the intermittent failure can be identified as

Meter:SUDDEN_SIGNAL_OFF, dated 2019-10-23 with a time stamped 10:01:43. There was a complete loss of signal

between the gauge control unit and the previous transmitting component on the network.
Page 13 of 23
Similar to the MOST Electronic Control Line (ECL) Diagnostic Mode procedure, you must determine the network flow

and the previous component to the reported failed component. In this case, the previous component to the gauge

control unit is the tuner unit, and troubleshooting will focus on the circuit between the two components.
System Limitations

Below are things to remember when reviewing the logs.

 Will only show reported failures, not current ones.

 May not report speaker popping noises.

 Will not record during a permanent network failure and cannot be accessed during that time. In the event of a current

network failure, use the MOST Electronic Control Line (ECL) Diagnostic Mode on page 4.

After Repairs

After repairs are completed to the network, be sure to clear the failure logs. This can only be done through the failure log

screen by pressing the CLEAR button.
Video

For a video of how to view the logs and how to interpret them, click on this link.

NOTE: The video references the service bulletin, 19-062 Service Manual Update: MOST Bus Network Failure Log

which has been replaced with this job aid.
Page 14 of 23
PROPER CONNECTOR INSPECTION & HARNESS ROUTING PROCEDURES

As previously described, some of the symptoms that the customer may experience are popping or crackling noises from

the speakers, a blank center display unit or the sound cutting off. After performing the ECL diagnostics or the MOST Bus

Failure Log and narrowing down the area to inspect, you need to inspection for proper connections of the MOST coaxial

connectors and proper harness routing.

Connectors

There are three types of connectors you will find in the network. These connectors are commonly known as FAKRA

connectors and will be referenced as such in various Honda publications and training materials.

Connector Image
Connector Style
Color
Purpose
Straight
Red
Receiving
Straight
Blue
Inline
90° Straight
Green
Transmitting
90° Offset
Green
Transmitting
Page 15 of 23
Due to its design, it is important that the connectors are installed properly and any external forces such as from the wire

harness do not contribute any tension. The tension created on the connector terminal such as a bad wire harness routing

will create a condition known as side loading. Here are some examples.

No tension on the harness, no side load
Harness tension at 45 and 90 degrees creating side load. The terminal will spread at the gap.
NOTE: The various service bulletin repair procedures available on SIS related to repair MOST-related problems will have

you install a new terminal to help avoid terminal spreading. However, it is important to still make sure that the harness wire

and connectors are installed back to their original condition.
Page 16 of 23
The following are some examples of mis-installed connectors and incorrect wire harness routing.

Example 1

The green connector sits correctly, perpendicular to the control unit while the red connector sits at an angle caused by

tension on the wire harness.
Below is another example of a connector that is tensioned causing the green connector to sit an angle.
In both examples, the connector is considered damaged, and the wire harness must be replaced. These connectors

are not repairable.
Page 17 of 23
Example 2

One wire was mis-routed causing tension to the green connector. To repair this, disconnect the MOST connector and

reroute the harness wire to remove the tension.
Below are more examples of incorrect harness mis-routing.
Page 18 of 23
Example 3

The updated terminal is correctly installed onto the green FAKRA housing but not on the red one. The connector was

installed 90 degrees off and was not engaged to the locking tabs.
Page 19 of 23
Example 4

The wire is not fully seated into the new terminal. The edge of the wire must sit with the edge of the updated terminal.
When inserting the wire into the new terminal, make sure you hear a click to ensure it is fully seated.
The previous examples shown can cause failures in the network. For additional examples of good harness routing, refer to

the following service bulletins.

Service Bulletin Number
Service Bulletin Title
20-031
Popping/Crackling from the Speakers; Display Blank
21-009
Warranty Extension: 2019−20 RDX MOST Bus Network Connectors
Video

For additional information on poor connections, click on this link.

Page 20 of 23
USING THE MOST CONNECTOR TOOLS

This section contains information on tools to help you remove the connectors on various units within the MOST bus

network. Acura highly recommends using these tools when removing the connectors to prevent damage to both the

connector and the terminal. If either are damaged, the harness must be replaced.

Tool Information

Tool Image
Tool Name
Tool Number
STRAIGHT FAKRA PULLER
07AAC-THRA200
90 DEGREE FAKRA B PULLER
07AAC-TJBA100
Tool Usage

Before unplugging a MOST connector, note the orientation of the harness leading to it and make sure to install it the same

way. A mis-routed harness can cause the connector to not seat properly, resulting in network problems.

STRAIGHT FAKRA PULLER (T/N 07AAC-THRA200)
Page 21 of 23
1. Slide the tool over the rear of the connector. Make sure the ramps inside of the tool push on the locking tab of

the connector.
2. Once you feel it click into place, the tool is fully seated. Pull straight back to release the connector.
3. Remove the connector from the tool. Rotate the bottom of the connector (opposite the lock tab) out first. This will avoid

the lock tab catching on the tool.
Page 22 of 23
90 DEGREE FAKRA B PULLER (T/N 07AAC-TJBA100)
1. Install the tool from the side, allowing the lower part of the tool to reach behind the connector.
2. Squeeze both sides of the tool to unlock the connector, then pull straight back.
Video

For a video of how to use these tools, click on this link.

NOTE: The video references the job aid, Using the MOST Connector Tools which has been replaced with this job aid.

END
Page 23 of 23

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