TECH TIP FOR FEBRUARY, 1993

       "IDIOT©LIGHT" CIRCUIT ON DELCO SI SERIES ALTERNATORS                     ( 10, 12, 15, & 17 ALL AMPERAGES )

THE BASIC IDIOT©LIGHT CIRCUIT IS: POWER FROM THE FUSE PANEL TO THE KEY SWITCH, TO THE INDICATOR LIGHT AND THEN TO THE #1 TERMINAL IN THE 2©WIRE PLUG AT THE BACK OF THE ALTERNATOR ( THE #2 TERMINAL IS BATTERY POWER FULL TIME).

WITH THE KEY ON BUT THE ENGINE NOT RUNNING, CURRENT FLOWS INTHE CIRCUIT, BECAUSE THERE IS GROUND POTENTIAL AT THE #1 TERMINAL THROUGH THE REGULATOR AND THE BRUSHES.

ONCE THE ALTERNATOR STARTS TO CHARGE, THE #1 TERMINAL COMES LIVE AT FULL OUTPUT VOLTAGE. SINCE THERE IS THE SAME VOLTAGE ON BOTH SIDES OF THE BULB, NO CURRENT CAN FLOW AND THE BULB GOES OUT.

IN A PROPER ALTERNATOR, THERE IS A 40 OHM RESISTOR INSTALLED BETWEEN THE DIODE TRIO CONNECTION AND THE REGULATOR GROUND, THIS DOES NOT EFFECT THE CHARGING, BUT IS THERE TO PROVIDE WARNING OF AN OPEN CIRCUIT IN EITHER THE ROTOR OR THE REGULATOR.

SEVERAL PROBLEMS WHICH CAN OCCUR IN THE  SYSTEM CAN BE EASILY IDENTIFIED BY WATCHING THE OPERATION OF THE INDICATOR LIGHT (IT'S NOT REALLY AN IDIOT).

TO BEGIN YOUR DIAGNOSIS, TURN THE IGNITION SWITCH ON AND WATCH THE INDICATOR. IF THE LIGHT IS NOT ON, THERE ARE SEVERAL POSSIBILITIES:

1) CHECK FOR A BLOWN GAUGE CIRCUIT FUSE.

2) CHECK FOR A MISSING OR BURNED OUT INDICATOR BULB.

3) CHECK FOR AN OPEN CIRCUIT BETWEEN THE INDICATOR AND THE ALTERNATOR.

4) POSSIBLY A BAD VOLTAGE REGULATOR, OPEN ROTOR, OR BAD  BRUSHES ON AN ALTERNATOR WITHOUT THE 40 OHM RESISTOR.

QUICK TEST FOR THE ALTERNATOR LIGHT CIRCUIT IS TO UNPLUG THE 2©WIRE CONNECTOR AT THE ALTERNATOR AND GROUND THE WIRE THAT FEEDS THE #1 TERMINAL. IF THE LIGHT ON THE DASH TURNS ON, THE PROBLEM IS INSIDE THE ALTERNATOR.

IF THE ALTERNATOR LIGHT IS ON BRIGHT WITH THE ENGINE RUNNING, THE PROBLEM IS PROBABLY:

1) BAD MAIN RECTIFIER OR STATOR.

2) OPEN FIELD CIRCUIT (ROTOR OR BRUSHES), OR BAD REGULATOR,  IN AN ALTERNATOR WHICH HAS THE 40 OHM RESISTOR INSTALLED.

3) NO MAIN BATTERY AT THE ALTERNATOR MAIN BATTERY POST.

4) A GROUNDED INDICATOR LIGHT WIRE.

IF THE INDICATOR IS ON DIMLY WITH THE ENGINE RUNNING THE USUAL CAUSE IS A BLOWN DIODE TRIO. TO QUICKLY CHECK THIS; WITH ALL WIRING IN PLACE, USE A DIGITAL VOLTMETER TO CHECK THE VOLTAGE AT THE #1 TERMINAL AND AT THE MAIN OUTPUT TERMINAL. THESE 2 VOLTAGES MUST BE EXACTLY THE SAME.

IF THE VOLTAGE IS LOWER ON THE #1 TERMINAL, THE PROBLEM IS IN THE ALTERNATOR. IF THE VOLTAGE IS HIGHER ON THE #1 TERMINAL, THE PROBLEM IS USUALLY A VOLTAGE DROP IN THE ALTERNATOR MAIN FEED WIRE OR A LOOSE WIRE ON THE OUTPUT TERMINAL.

IF THE LIGHT TURNS ON AT HIGHER RPM. BUT OTHERWISE THE ALTERNATOR CHARGES PROPERLY, 99% OF THE TIME THIS IS CAUSED BY A BLOWN GAUGE FUSE.

THE REASON FOR THIS IS: THERE IS NO CURRENT ENTERING THE ALTERNATOR AT THE #1 TERMINAL BECAUSE OF THE BLOWN FUSE. THE ALTERNATOR SELF©ENERGIZES FROM THE RESIDUAL MAGNETISM IN THE ROTOR AND BEGINS TO CHARGE. THE OUTPUT FROM THE DIODE TRIO FEEDS BACK INTO THE INDICATOR LIGHT TURNING IT ON. THIS ALSO INDICATS THAT THE CUSTOMER DID NOT NOTICE THAT THERE WAS NO ALTERNATOR INDICATOR LIGHT WORKING WHILE THEY WERE STARTING THE CAR.

ON STANDARD TRANSMISSION CARS AND TRUCKS, STARTING WITH THE CLUTCH IN AND THE TRANSMISSION IN NEUTRAL, DECREASES ENERGY REQUIREMENTS BY ABOUT 15%.