TECH TIP FOR OCTOBER 1995

TO CARRY ON IN THE VEIN OF OUR TECH SEMINAR HELD SEPTEMBER 17TH, WE ARE PLEASED TO PRESENT A SERIES OF DIAGRAMS WHICH WILL ASSIST YOU IN USING YOUR DMMS (DIGITAL MULTIMETER), TO BETTER ADVANTAGE WHILE DIAGNOSING STARTING AND CHARGING SYSTEMS. OUR THANKS TO FLUKE METERS COMPANY FOR THE INFORMATION.

DO NOT USE AN ANALOG METER ON ANY COMPUTER CONTROLLED CAR, AS THEY CAN DAMAGE COMPUTER CHIPS BY DRAWING TOO MUCH CURRENT. AN DMMS USED MUST HAVE AT LEAST 10 MEG-OHMS OF IMPEDANCE. IT'S COME TO THE STAGE, THAT IF YOU DON'T HAVE THE TEST EQUIPMENT, DON'T TAKE ON THE WORK.

CONTRARY TO PREVIOUSLY RELAYED MATERIAL, IT IS IMPERATIVE THAT YOU DO NOT FULL FIELD ANY ALTERNATOR ON A COMPUTER CONTROLLED VEHICLE, THE SPIKE OF THE COLLAPSING FIELD (UP TO 500 VOLTS) CAN SERIOUSLY DAMAGE OR DESTROY THE CHIPS.SOURCE: ALLEN MELTON, NORTHWEST REGULATOR INC., PORTLAND, OR ON SOME GM APPLICATIONS, THE CHARGE INDICATOR LIGHT WILL SOMETIMES FLASH MOMENTARILY, EVEN THOUGH THE SYSTEM SEEMS TO BE WORKING PROPERLY. THIS WILL USUALLY OCCUR ON VEHICLES EQUIPPED WITH PULSE WIPERS, WHEN THE WIPERS ARE IN OPERATION.

THE PROBLEM IS, THAT ON SOME VEHICLES THE CHARGING SYSTEM "RESETS" IN RESPONSE TO A NEGATIVE TRANSIENT PRODUCED BY THE WIPER MOTOR. THIS TRANSIENT ACTIVATES THE "WIDE-OPEN THROTTLE" (WOT) FEATURE OF THE VOLTAGE REGULATOR, WHICH SHUTS DOWN THE CHARGING SYSTEM MOMENTARILY. AFTER THE TRANSIENT PASSES, THE INDICATOR LIGHT TURNS ON FOR ABOUT 80 MILLISECONDS AND THE INSTRUMENT PANEL VOLT-METER READING DIPS FOR ABOUT 2 SECONDS BEFORE THE CHARGING SYSTEM RETURNS TO NORMAL. THE CURE IS TO CHANGE TO A REGULATOR WITH THE SAME CHARGE CONTROL CHARACTERISTICS BUT WHICH CONTAINS A CAPACITOR TO IGNORE THE TRANSIENT. SOURCE:WETHERHILL ASSOCIATES INC., ROYERSFORD, PENNSYLVANIA.