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G.M. MOST Bus Diagnosis Made Easy
Posted to Technical Discussion Forum on 8/21/2017 26 Replies

A 2015 Silverado came in today. The complaint was MyLink display is dark. With the engine running, this was the display.

[2015 Chevrolet Silverado 1500 LS, Photo]

The display was not illuminated, the radio would not play and all the control buttons were non-functional. What's first? Check powers and grounds? Not really as we have a multiple failure. The display is controlled by the HMI module but the radio is also not functioning.

Checking for codes set, I found U0028 set in the radio as current and U0029 set in the HMI module as current. OK, let's look at radio MOST data next.

[2015 Chevrolet Silverado 1500 LT, Scan Data]

Note that a SURROGATE MOST MASTER NODE UPSTREAM POSITION of 2 has been set but at the same time, the NUMBER OF MOST COMMUNICATION BREAKS is set at zero. Why?

The answer is in the codes set. U0028 tells us that a MOST communication error is occurring. It can be a ring break, short to power or ground or a data line or a internal module fault. A lot of possibilities but U0029 refines it. U0029 tells that a communication error is occurring but the MOST ring still has integrity. Because of this, the number of communication breaks is zero and no module has declared itself as the SURROGATE MOST MASTER MODULE.

How to find the problem? The MOST ring is unidirectional and has only 4 nodes on this vehicle. As seen in the data, node 1 is the RADIO, node 2 is the CD, node 3 is the HMI module, and node 4 is the IPC. Since the SURROGATE MOST MASTER NODE UPSTREAM POSITION is 2, we need to count backwards in the MOST network for this position. The first position backwards is the node 4, IPC. The second is node 3, HMI module. So our communication failure is occurring before node 3 in the regular orientation which would be node 2, CD player.

Since U0029 is set I don't suspect a wiring and/or connection problem. The next step is to see if I can communicate with all 4 nodes. I could not establish communication with the CD player, but the other 3 nodes were normal. I removed the CD player and jumped the connector data lines. The MyLink display lit up and the radio started playing and all the controls functioned normally now. The failure was due to an internal module failure in the CD player.

G.M. MOST really isn't that tricky if you collect data first and analyze the failure before you jump into component testing.

James from Florida

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car Vehicle Data

2015 Chevrolet Silverado 1500 LT 5.3L

Engine5.3 L
Trans6-speed Automatic (Electronic)

car Vehicle Data

2015 Chevrolet Silverado 1500 LT 5.3L

Engine5.3 L
Trans6-speed Automatic (Electronic)