Problem with rangefinder in in flight

hi guys
i have pixhawk cube and i use gps here on a big hexa body with maxbotix analog ultrasonic and in the past i use this setup on smaller body and the range finder was working perfectly but last month i change the body to bigger body (1.5m custom maid from aluminium ) and from that time i have very bad problem with the sensor it is working when the copter is on the ground but when i turn on the motors the numbers of the voltage sonarvoltage goes to 5.5 v .i do not know whats is the reason .
i test it on arducopter 3.5.5 and 3.6.6 and same happens.
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