AN MD88 CAPT REPORTS AN ELECTRICAL SPIKE WHILE TURNING ON THE AUX HYD PUMP DURING DSCNT CAUSED A PRESSURE CONTROLLER MALFUNCTION. NO OVER-PRESSURIZATION CHECKLIST WAS FOUND.

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DURING DSCNT PASSING THROUGH FL180 AND CLRED TO 12000 FT; THE DSCNT CHKLIST WAS INITIATED. DURING THE PREPARATION FOR THE CHKLIST; ONE ACTION IS TO TURN ON THE AUX HYD PUMP. WHEN THE AUX HYD PUMP SWITCH WAS PLACED ON; THERE APPEARED TO BE AN ELECTRICAL SURGE OR SPIKE; WHICH IS FULLY EXPLAINED BELOW. THE CHKLIST WAS CONTINUED; BUT WE NOTICED AN INCREASE IN INTERNAL EAR PRESSURE. WHEN THE PRESSURIZATION PANEL WAS CHKED IT WAS NOTED THAT THE CABIN ALT WAS DSNDING AT A VERY RAPID RATE; IN EXCESS OF 1500 FPM. AS THE CAPT; I WAS THE PLT MONITORING. I TOLD MY FO THAT HE HAD THE FLT CTLS AND ALSO TO TAKE OVER RADIO COMS. I THEN REFED THE QRH TO RESOLVE OUR PRESSURIZATION PROB. REFING SECTION 2 OF THE QRH REVEALED THAT THERE WAS NO PROC FOR AN OVER-PRESSURIZATION OF THE CABIN. AT THIS POINT; WE WERE CLRED TO 8000 FT. THE FO REQUESTED; AND RECEIVED; DELAYING VECTORS AND MAINTAINED 8000 FT. THE MAX CABIN DIFFERENTIAL PRESSURE REACHED APPROX 7.9 PSI; AT 8000 FT. WHEN I WAS UNABLE TO FIND A PROC THAT APPLIED TO OUR SITUATION; NO LIGHTS ILLUMINATED ON THE PRESSURIZATION PANEL OR OAP MESSAGES. I RELIED ON MY SYS KNOWLEDGE OF THE PRESSURIZATION SYS. I TRIED TO TEST THE PRESSURIZATION PANEL BY PRESSING THE FLOW LIGHT; WITH NO RESPONSE FROM THE TEST. THERE WAS NO ILLUMINATION OF ANY LIGHTS ON THE PRESSURIZATION PANEL. THERE WAS NO INDICATION OF AUTO-FAIL; AUTO 1 INOP; OR AUTO 2 INOP. IT APPEARED THAT THE ELECTRICAL INPUT INTO THE PRESSURIZATION PANEL HAD COMPLETELY FAILED. WHEN I WAS UNABLE TO FIND A QRH PROC THAT APPLIED TO OUR SPECIFIC SITUATION OR ABLE TO TEST THE PRESSURIZATION PANEL; I MADE A DECISION THAT I WOULD DEPRESSURIZE THE CABIN IN MANUAL MODE BY USING THE AUTO PRESSURIZATION INOP/MANUAL PRESSURIZATION CHKLIST IN THE QRH. WHILE THE FO FLEW THE ACFT ON DELAYING VECTORS; I SLOWLY MANIPULATED THE CABIN PRESSURE OUTFLOW VALVE TO ALLOW THE ACFT TO DEPRESSURIZE FOR LNDG. AFTER LNDG; WE TAXIED TO THE GATE WHERE EXTERNAL PWR WAS CONNECTED TO THE ACFT. AFTER CHKING THE VOLTS AND FREQUENCY OF THE EXTERNAL PWR; EXTERNAL PWR WAS SWITCHED ON. AFTER COMPLETING THE SHUTDOWN CHKLIST; AND WHILE WRITING THE DISCREPANCY IN THE LOGBOOK; I AGAIN PRESSED THE FLOW LIGHT ON THE PRESSURIZATION PANEL AT WHICH TIME IT TESTED NORMAL. IT IS MY OPINION THAT THE ELECTRICAL SPIKE; WHEN THE AUX HYD PUMP WAS PLACED ON; CAUSED A COMPLETE INTERRUPTION OF ELECTRICAL PWR IN THE PRESSURIZATION CONTROLLER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THAT WHEN THE AUX HYDRAULIC PUMP WAS TURNED ON IN FLT HIS PFD ALSO SHOWED A NARROWING WITH MOMENTARY DISTORTIONS TYPICAL OF AN ELECTRICAL DISTURBANCE TO ELECTRONIC EQUIPMENT. HE STATED THAT MAINT PERFORMED A PRESSURIZATION SYSTEM BITE TEST THAT REVEALED NO FAULTS. ADDITIONALLY; THE AUX HYDRAULIC PUMP WAS ACTIVATED SEVERAL TIMES AND ALL SYSTEMS OPERATED NORMALLY.