Troubleshooting MOST loop - Mercedes-Benz Forum

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post #1 of 3 (permalink) Old 06-19-2017, 03:28 PM Thread Starter
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Date registered: Jun 2017
Vehicle: 07 SL550
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Troubleshooting MOST loop

I bought my SL550 with the radio not working. The radio turns on, but there is no sound and the message "System Unavailable" pops up after a few seconds, and the head unit powers down. This lead me to the following website, suggesting to bypass individual components in the MOST loop, except HU and amp, since most of the time one of these other modules is broken and is the reason for the interrupted fiberoptic loop.

Mercedes Benz No Audio or No Sound Troubleshooter and Repair

So I started with the unit on the right of the CD changer, which I believe is the telephone module. I pulled the plug, hooked up the lines to the coupler, and still no go. Before checking other modules, I checked to see if there was any signal going through the lines by peeking into the ends of the wires, and there was nothing, no red light.

So, my instinct would tell me the HU is broken, since it is not sending a fiberoptic signal into the loop. Would that assumption be correct, or can it be some other issue? - Thanks in advance.
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post #2 of 3 (permalink) Old 06-19-2017, 08:23 PM Thread Starter
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Well, to answer my own question...HU is not broken. I proceeded to dig out the CD changer and bypassed it, and low and behold the radio works!
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post #3 of 3 (permalink) Old 06-20-2017, 05:46 AM
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