O-bus ring break position

WARNING: This page is about a different car, the 2003 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.

To find the position of a break on the O-bus, Tech 2 provides a function called "O-Bus Ring Break Position". To diagnose using the "OBus Ring Break Position" function in Tech 2, the car must be without electrical power for 4 minutes if it is equipped with AMP2, or 1 minute for all other versions. Interference on the O-bus due to short-lived breaks or excessive dampening in the optical fibres may not be displayed as breaks on Tech 2.

Therefore, all O-bus control modules provide diagnosis and generate a DTC if they receive a corrupt message. All messages received are converted into electrical signals, reconverted into light and retransmitted. The ICM is the bus master and, unlike the other modules, never resends a corrupt message. If a control module receives a corrupt message, the fault must have occurred upstream, somewhere between the control module and the ICM. This means that a fault will generate a DTC for an intermittent bus fault in all the control modules downstream.

Fault diagnosis follows the standard fault diagnosis procedure: check the power supply to the control module and check the optical fibre.

To avoid having to remove large parts of the car, use the fibre optic test cable, part no. 86 12 913. Connecting this cable between two control modules allows fault diagnosis to determine whether the optical cable needs to be changed. Also to facilitate fault diagnosis, the O-bus control modules have fault counters. A counter counts up each time bus communication is broken. The counters stops at 255 and can be reset with Tech 2. A counter showing 255 must be reset to be of any use. Intermittent faults can then be detected by jiggling the harness. This means that a fault will cause all the control modules downstream to count up.

NOTE: The counters are very sensitive and generally show 255, even after a short period of time. Starting the engine and the activation of the O-bus when the radio is turned on usually cause the value of the counters to increase a couple of digits.