A CESSNA-210 CENTURION PILOT DURING PRE-TAKEOFF ENGINE CHECKS; NOTICED SIGNS OF SMOKE FROM UNDER THE INSTRUMENT CONSOLE.

Narrative

INITIAL STARTUP WAS NORMAL. AFTER CLRNC AND TAXI INSTRUCTIONS RECEIVED; EXPERIENCED SLIGHT ODOR IN THE CAB; WHICH WE THOUGHT WAS EXHAUST THROUGH THE OPEN WINDOW. DURING PRETKOF ENG CHKS NOTICED SIGNS OF SMOKE FROM UNDER THE CONSOLE. SWITCHED OFF ALL ELECTRICAL EQUIP OTHER THAN 1 RADIO AND NOTIFIED GND CTL OF OUR PROB REQUESTING TAXI BACK TO PARKING. THE SMOKE INCREASED AND HAVING RPTED THIS TO GND CTL I SHUT DOWN THE ACFT ON THE TXWY. SUBSEQUENT EXAM REVEALED A FAULTY GEAR SYS PUMP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THIS ACFT HAS A RETRACTABLE GEAR THAT USES A DC POWER GEAR SYSTEM PUMP. NORMALLY; WHEN THE SELECTOR SWITCH IS POSITIONED TO 'ON;' THE GEAR PUMP PROVIDES HYD PRESSURE AND RELAYS WILL OPEN AT A PREDETERMINED TIME AND DISCONNECT DC POWER TO THE PUMP. HOWEVER; IN THIS CASE; ACCORDING TO MAINT; THE SELECTOR SWITCH RELAYS SHORTED TO CONTINUOUS 'ON.' THIS CAUSED THE PUMP TO OVERHEAT AND STARTED SMOKING. REPORTER ALSO STATED; THIS WAS NOT A LITTLE SMOKE; BUT RATHER A SUBSTANTIAL CLOUD THAT ENGULFED THE COCKPIT. WITH THE PUMP SELECTOR SWITCH SHORTED TO 'ON' AND THE PUMP RUNNING CONTINUOUSLY ON DC POWER; WITH NO WAY TO SHUT THE PUMP OFF; HE HAD TO SHUT ALL POWER DOWN. REPORTER ADDED; HE DOES NOT KNOW IF THIS GEAR PUMP SELECTOR SWITCH HAS BEEN A PROBLEM IN THE PAST FOR THIS TYPE ACFT.