14
MOST Bus Diagnosis
The possible fault scenarios are:
Transmit diode in module A bad
Receive diode in module B bad
Optical wave guide fault between modules A and B
Software error or fault in module A or B
Control Module Does Not Switch Off Light
When the MOST network is requested to sleep, the Control Display switches off the light in
the MOST ring. The lack of light input is a signal to the individual control modules to switch
off their light output and enter sleep mode.
If a control module does not switch off its light, all down stream control modules register
the fault “A Control Module is not switching light off.”
Important:
Failure of a control module to turn its light off, will cause the MOST network NOTto enter
sleep mode. If the MOST network fails to sleep, the rest of the car will not be able to enter
sleep mode. This will lead to battery discharge.
To diagnose:
Read out fault memory in MOST ring order
The fault lies in the control module that precedes the module where the fault is
first stored.
Always confirm the problem by first clearing the fault and performing the diagnosis a sec-
ond time. If the same results occur, replace the defective control module.
Network Wakeup Unsuccessful
This fault indicates a problem with the optical transmission. An insufficient volume of light is
coming through one position of the ring and may be caused by:
Control Module is receiving no voltage
Optical Wave Guide harness defective
Optical Element in a control module defective (transmit or receive)
Connector not installed correctly
A distinction must be made as to whether the fault is currently present or sporadic.
For faults currently present, run the Ring Break DiagnosisTest Plan.
For sporadic faults perform the Luminous Power ReductionTest Plan.
