A B737-300 FLC GETS AN OFF COURSE CORRECTION FROM PHX DEP CTLR WHEN THEY UNKNOWINGLY EXPERIENCE A MAP SHIFT. RPT CITES AN EQUIP COOLING FAN MALFUNCTION AS A CAUSATIVE FACTOR.

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BUCKEYE 2 DEP FROM PHOENIX; RWY 26L. WE CONTACTED PHX DEP CTL AND WERE CLRED TO 15000 FT. AS THE PNF; I BECAME PREOCCUPIED IN THE COCKPIT (HEADS DOWN). I LAST SAW 9 DME FROM THE PHX VOR AND NOTICED THE PF TURNING TOWARDS THE BXK VOR. I CAUTIONED HIM THAT HE WAS TURNING EARLY AND THAT HE SHOULD TURN BACK TO THE L ON THE PROPER HDG OF 240 DEGS UNTIL THE 13 DME FIX. DURING THIS CONVERSATION BTWN OURSELVES; DEP CTL CLRED OUR FLT DIRECT TO THE BXK VOR. WE BOTH MISSED THIS INSTRUCTION. THE PF CORRECTED (AS I COMMANDED) AND AT THE PROPER FIX (13 DME) TURNED THE ACFT TO A HDG OF 280 DEGS. I MANUALLY CHANGED THE NAVIGATIONAL RADIOS TO THE BXK VOR FOR OUR INTERCEPT OF THE 077 DEG RADIAL. BOTH MAP DISPLAYS INDICATED THAT WE WERE N OF THE BXK 077 DEG RADIAL. I INSTRUCTED THE PF TO TURN L TO 240 DEGS TO RE-INTERCEPT THE PROPER RADIAL. THIS PUT OUR ACFT ON A DIRECT HEADING TO THE BXK VOR (AS DISPLAYED ON OUR EFIS DISPLAYS). DEP CTL ASKED WHAT OUR HEADING WAS AND I INFORMED HIM 240 DEGS. I FAILED TO MENTION TO HIM THAT THIS HEADING WAS GETTING US DIRECT TO BXK VOR AS DISPLAYED. THE DEP CTLR INFORMED US THAT WE WERE 9 MI S OF BXK VOR AND WE NEEDED TO TURN R TO RE- INTERCEPT THE AIRWAY (J212) W OF BXK VOR. ENRTE TO OUR DEST WE VERIFIED OUR POS WITH CTR AND MANUALLY OVER VARIOUS NAVAIDS. THERE WERE NO DISCREPANCIES NOTED. UPON LNDG AT SNA I CALLED PHX TRACON AND TALKED TO THE SUPVR ON DUTY. HE EXPLAINED TO ME WHAT THEY INDICATED ON RADAR AND VERIFIED THAT THE NAV FACILITIES WERE OPERATING NORMALLY. I EXPLAINED WHAT WE OBSERVED IN THE COCKPIT AND VERIFIED TO HIM THAT WE HAD THE PROPER NAV FREQS TUNED IN AND THAT I SUSPECTED IRS DRIFT AND SYMBOL GENERATION FAILURE. THE WORKING CTLR KEPT OUR ACFT AND INBOUND TFC ON THE ARLIN ARR SEPARATED THROUGHOUT THIS EVENT. MY ACFT MAINTAINED LAST ASSIGNED ALT OF 15000 FT AND THE INBOUND TFC HAD THEIR ALT AMENDED TO MAINTAIN 16000 FT. NO TFC CONFLICTS OCCURRED. I CONTACTED OUR MAINT SUPVR IN SNA AND EXPLAINED TO HIM OUR EXPERIENCE. I GAVE HIM DATA ON OUR RTE IN REGARDS TO LATITUDE AND LONGITUDES AND THE DIFFERENCES BTWN THE L AND R IRS'S. I ALSO BACKED THIS UP WITH DATA FROM THE POS DIFFERENCE DISPLAY (LOCATED ON THE LOWER R SIDE OF OUR MAP DISPLAY) FROM OVER A RANDOM POS ENRTE AND THE ARR GATE POS. I WAS CONTACTED BY OUR MAINT CTLR THAT NIGHT FOR ANY ADDITIONAL INFO. THE FOLLOWING MORNING; I MET THE OUTBOUND CREW AND GAVE THEM A BRIEFING ON MY EXPERIENCE. THE CAPT HAD EXPLAINED THAT HE HAD EXPERIENCED THIS ONCE BEFORE ON A B757. THIS OUTBOUND CREW DID IN FACT FLY THIS ACFT AND I WAS INFORMED THEY DID NOT HAVE ANY PROBS. ON JUL/SAT/98; THIS ACFT HAD A DISCREPANCY WRITE UP IN REGARDS TO THE EQUIP SUPPLY FAN FLICKERING AND CAUSING THE DISPLAYS TO FADE OUT. MAINT CORRECTIVE ACTION WAS THAT THEY COULD NOT DUPLICATE AND THE ACFT WAS RETURNED TO SVC. THAT EVENING OF JUL/SAT/98; I WAS ON THE JUMP SEAT OF THIS SAME ACFT ENRTE TO LAX. DURING CLBOUT THE SUPPLY FAN WAS FLICKERING AND CAUSING BOTH EFIS DISPLAY UNITS TO FADE OUT. THE CREW CHANGED THE FAN SWITCH TO THE AUX FAN WITH NO CHANGE. IT WASN'T UNTIL THE ACFT GOT TO CRUISE ALT OF 31000 FT DID THE SYS COOL OFF ENOUGH TO EXTINGUISH THE LIGHT. MAINT CTL AND DISPATCH WAS CONTACTED ENRTE AND THE FLT WAS GIVEN CLRNC TO CONTINUE TO LAX. UPON ARR IN LAX; CONTRACT MAINT WAS TO MEET THE ACFT FOR INSPECTION. AT THIS PRINTING; I AM NOT AWARE OF THE FINDINGS.