The dIsplays on my chinese ebike controllers follow the same design, I've seen about three or four. Ecotric S900 should be the same. They get battery power and have onboard converters to power the LCD and other circuits. There is no 5 volts from controller, There are 5 wires.
-48V or 36V battery power
-ground
-controller start
-transmit data Txd
-serial data Rxd.
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Does your S900 light up? Normally, when you power it up, the LCD comes on, and the display sends back battery power to the controller on the start lead. Then it communicates with the controller via the two serial data leads. Some displays will have a 6th lead for lights,
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You might probe the connector and see if the start lead goes from 0V to battery voltage when the display turns on. Will tell you if the display is working, Attached is a generic Ecotric manual (I used to have the 20" fat tire folder). They show a schematic for the electrics near the end. Too bad there are no colors on the LCD wires, but the latter is common. Red and Black are power, Blue is the start lead. Yellow/Green are data.. See if any wires goes to battery power when the display turns on.
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Another test is to invoke Walk Mode. If there is no 5 vol power, this won't work either.
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