What was found
(CAN) DURING PREFLIGHT CHECKS, THE HYDRAULIC PUMPS WERE SELECTED ON AND THE COCKPIT LIGHTING DIMMED.A HEAVY CURRENT LOAD COULD BE HEARD COMING FROM THE GROUND POWER CART.HYD PUMP 3B C/B TRIPPED.TWO ATTEMPTS WERE MADE TO RESET THE C/B, EACH HAVING THE SAME EFFECT AS PREVIOUSLY NOTED BUT WOULD NOT RESET.SMOKE WAS NOTICED COMING FROM THE AVIONICS BAY AND POWER WAS REMOVED FROM THE A/C.INVESTIGATION REVEALED THE MAIN POWER FEED WIRE TO THE 3B HYD PUMP IN THE AVIONIC BAY AROUND STATION 281 HAD SHORTED TO A CLAMP THAT SECURED THAT WIRE BUNDLE