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from the pic of the board that u send,it seems more like 820-3437,as from my knowledge,macbook air 13-inch mid 2013 board is suppose to be that number,while macbook air 11-inch mid 2013 is 820-3435. but the pic doesn't look like 820-3435 at all.
No, something is wrong here. If you have 18v on SHDN pin, 18v on VIN, and no short to ground on output... no. You're screwing something up that you're not telling us. Please tell us.
Some more.
1) Is the voltage on VIN pin of U7090 STABLE, or does it go up and down?
2) Are you measuring for a short to ground on L7095 with power plugged in? If so, DON'T! Measure without power plugged in.