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MOST ring breaks in NGI systems
Posted to Technical Tips Forum on 5/18/2013 21 Replies

This post will most likely be of interest to GM techs only at this time as this technology has not filtered down to the aftermarket yet.

Finding MOST ring breaks can be done without too much trouble if you have a good idea how the system operates. The MOST network is a chain type unidirectional network. The radio is used as the MOST master module in most cases. It is also used as the MOST to low speed GMLAN gateway module and is usually assigned MOST module 1 position. During a ring break, communications will cease on the nework and only node ID's will be reported.

If a MOST ring break occurs, the radio will report U0028. To determine the site of the ring break, check the MOST network parameters. The LAST WORKING MODE ID will always be present, but the SURROGATE MOST MASTER NODE UPSTREAM POSITION will only be present while the ring break is active. When the ring break occurs, the module in the ring after the break will report itself as the surrogate MOST master module. This is important as the unidirectionality of the network will point out the break area as between the surrogate MOST module and the MOST master module.

By looking at the LAST WORKING MOST MODE ID numbers reported and the position of the MOST module that declared itself as the SURROGATE MOST MASTER NODE, the ring break position can be estimated from the driver's seat before intrusive testing.

James from Florida

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