Testing
Testing
Light Output Reduction
Reducing the light output of individual control modules is a convenient method of determining the area of a defect.
Switch on the radio.
In Control Module functions, begin to activate luminous power reduction in the individual control module.(In this test the light output of the selected control module is reduced for 5 seconds and then automatically reset to normal output)
If the optical transmission for control module A to the next control module in the ring (control module B) is OK, a slight noise may occur when the light output is reduced however the radio will continue to play.
If the radio goes off and comes back on again(radio volume may be reduced) in 5 to 10 seconds, the optical transmission between control modules A and B is defective.
If the visual inspection is OK, the fault can be located using the OPPS tester or optionally performing the following tests.
- Remove the input optical wave guide from control module B and confirm the presence of light.
If light is present, install by-pass optical wave guide in place of control module A, clear fault codes in module B. If MOST network operates properly, then control module A is at fault and must be replaced.
If MOST network still has a fault, put module A back in the network and by-pass module B. Clear faults.
If MOST network operates now problem is with control module B and it must be replaced.
- If light is not present at input of module B, perform ring break diagnostics.
Ring Break Test
If there is a break in the ring (a defect between two control modules)the following fault patterns may occur:
- Transmit diode of the transmitting control module defective
- Power supply of the transmitting control module defective
- Internal control module fault of the transmitting control module
- Receiver diode of the receiving control module defective
- Power supply of the receiving control module defective
- Internal control module fault of the receiving control module
- Optical wave guide between transmitting and receiving control module defective
These faults may occur alone or in combination. To diagnose a ring break, the first step is to locate the two control modules between which the transmission failure has occurred.
This is accomplished with the ring break diagnostic function. Once the two control modules have been identified and the diagnostics have been performed, remember to check the power supply and ground circuit of both modules before condemning a module.
Testing of the transmit/receive diodes will be possible using the OPPS tester.
Perform Ring Break Test
The ring break test mode is entered automatically when the power to all the modules in the MOST network is switched off and then switched back on. The most effective method of switching the power off and on is to disconnect the battery negative terminal for 45 seconds. This time will allow the capacitors of all the control modules to dissipate.
When the battery is reconnected the control modules wake up and in MOST network order transmit a light signal to the next module. Each module checks to see if it has received a light signal from the previous module. If the control module does NOT receive a light input signal it still transmits a signal to the next module. A relative node number of 0 is stored in the control module that did not receive a signal but that transmitted one.
The Control Display receives the light signal back and identifies which modules responded. Go to "Control Unit Functions" Control Display Gateway and read fault memory.
The Control Display will display a relative node number. This number will indicate how many modules communicated after the module which set the relative node number of 0. To find the control module with the relative node number of 0, count from the input side of the Control Display (counting the Control Display as 0) towards the control modules. When arriving at the control module with the number as displayed as the relative node number in the DISplus, the last known communicating module has been found.
Example:
While performing the ring break diagnostics the Control Display has set a relative node number of 2. Count the Control Display a 0, the Kombi will be 1 and the ASK will be 2. The ring break occurs between the ASK and the module which precedes it, the telephone module.
IMPORTANT:
When counting control modules, the multimedia changer (if equipped) and the Nav system must be counted as two control modules.
In order to perform the count correctly the equipment on the vehicle must first be identified. When using the MOST network diagram in the DISplus, connector number 1 of the optical waves are inputs and connector number 2 are outputs.
Status Wakeup
MOST control modules require high current during standby operation and must be disconnected or put in sleep mode to prevent the vehicle battery from being discharged. In case of a fault on the the MOST network that continuously wakes up, the entire MOST bus will be woken up. The Control Display will wake up the CAN Bus and all the vehicle busses will be woken up. This will lead to battery discharge.
It is of great benefit to know which module initiated the wake up call. In order to find out which MOST node woke up the MOST bus, the following procedure is performed:
In Control Unit Functions, press "STATUS WAKEUP"
Three different response are possible:
- Control Module woke up
- Control Module woken up
- Control Module not initialized
The Control Module with the status "Control Module woke up" is the module that woke up the rest of the MOST bus.
This diagnosis only informs which control module woke, not the reason for the wake up, diagnostic testing should be performed on the control module and related equipment.
Hints for Vehicle Equipment Identification
- CDC - Look on passenger side of dashboard above glovebox.
- Logic 7 - Look for speaker grills on rear doors
Look on left side of trunk for large amp.
- MMC - Look on left side of trunk.
Repair of the MOST Fiber-optic Cables
The MOST bus may only be repaired (spliced)once between 2 control units, otherwise the attenuation may become too much and disrupt communication.
Attenuation in the Fiber-optic line increases as the distances between controllers increases.
This diagram describes the approximate length of the fiber optic cable from one controller to the next.
The chart to the right provides maximum attenuation values based on the cable length.
The MOST bus may only be repaired using the prescribed special tool (crimping pliers) and special connectors.