Introduction
Here is a step-by-step guide on how to disassemble and troubleshoot a faulty Mercedes w114/115 clock.
Tools
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            Locate the area that houses the clock. Unscrew the two barrels and place it aside. 
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            Carefully pull out the clock without damaging any other parts. 
 
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            Connect the screw (located behind the clock) to the negative supply of the battery, and then the small pin to the positive supply. 
 
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            To check the power supply of the clock, locate the electrical connector and find the pin on it which supplies the power (12v) to the clock. 
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            Take a small bulb and socket and bridge a wire from the 12v pin supplied from the electrical connector, to one of the two wires on the bulb and socket. 
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            Next find a ground supply nearby ( the best place would be the key hole) and bridge another wire from it to the second available wire on the bulb and socket. 
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            If the bulb lights up, it means your clock has power. 
 
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To reassemble your device, follow these instructions in reverse order.