15
MOST Bus Diagnosis
Ring Break Diagnosis Carried Out (FC E190)
Reading out the fault memory of the Control Display (Gateway) after performing the Ring
Fault Diagnostic, results in a fault of Ring Fault Diagnosis Carried Out being stored.
This fault memory is not a true fault memory entry, but only an output of additional infor-
mation for relative node position.
Testing
Light Output Reduction
Reducing the light output of individual control modules is a convenient method of deter-
mining the area of a defect.
Switch on the radio.
In Control Module functions, begin to activate luminous power reduction in the individ-
ual 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 Ato 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 Aand Bis 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 Band 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 Aback in the network and by-pass
module B. Clear faults.
If MOST network operates now problem is with
control module Band it must be replaced.
If light is not present at input of module B,
perform ring break diagnostics.
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16
MOST Bus Diagnosis
Ring Break Test
If there is a break in the ring (a defect between two control modules) the following fault pat-
terns 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 sec-
onds. 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.
