| Am an Idiot Light Part | By Ken Buie Idiot light, alternator light, charge indicator light, voltage monitor; it doesn’t matter what you call it they all serve the same purpose—to warn the driver about a charging system prob- lem. When first introduced, the alter- nator light was to be the replacement for the seldom looked at ammeter. Replacing the gauge with a light bulb also proved to be cheaper to manu- facture. Warning lights for charging, oil and coolant temperature soon became common. To ensure proper operation, all warning lights incorporate a test cir- cuit to verify that the bulb is function- ing before the car is started. When re- pairing the car, waming light circuits are much simpler to diagnose than gauge circuits. Aquick review of oil and tempera- ture warning lights will help us to bet- ter understand the alternator light cir- cuit. In almost all warning light systems, the light bulb has an insulated base. This means that the warning light is not grounded in the instrument panel. On most warning light bulbs, one side has a 12-volt input from the gauge circuit fuse and the other side is grounded at the oil pressure or temperature send- ing unit. The oil light is the simplest circuit to understand. When the oil pressure is below 3 to 7 PSI, current passes through the bulb and is grounded at the oil pressure sending unit. The oil pressure switch is a normally closed switch, and is opened by oil pressure. When the engine is not running, obvi- ously there is no oil pressure. When the key is turned to the on position, the oil pressure switch is grounded, turning on the warning light for its test circuit. When the engine is running and the oil pressure is above 3 to 7 PSI, the switch opens and the bulb no longer has a ground so it goes out. See Fig- ure 1, page 10. Ona temperature warning light cir- cuit, the sending unit is anormally open switch. When the coolant temperature reaches approximately 250 degrees F, the switch closes to ground and turns on the warning light. Since the switch is normally open, the test circuit can- not be the same as the oil pressure cir- cuit. If you notice, as you turn the igni- tion switch to the crank position on most cars, just before the engine cranks, the temperature light comes on. The test circuit for the temperature light is activated inside the ignition switch. See Figure 2, page 10. Some Ford and Chrysler models combine the oil and temperature warning circuits into one indicator marked ENGINE. To test the oil pressure and tem- perature lights for operation, turn the key on and unplug the wire at the send- ing unit, then touch the wire to a good ground. The warning light should now light up indicating the circuit is clear to the sending unit. Another test is to unplug the send- ing unit wire and test for current using a standard 12-volt test light connected to a good ground. When you probe the wire using a test light, you will no- tice the warning light and your test light will glow very dimly. The reason for this is you are trying to ground the warning light circuit through the resis- tance of the test light bulb. Remember these procedures when we discuss al- ternator warning light testing. This brings us to the topic of the article; the alternator light. Each manu- APRIL, 2001 facturer has their own way of control- ling the alternator light, but they all fol- low the same basic rules. In the past, the alternator light warned the driver that the alternator was not charging. On many newer cars, the alternator light even has the ability to indicate an over- charge condition. Sometimes the alter- nator light is used as a resistor in the turn-on circuit for the voltage regula- tor. Let’s cover the basics of how the alternator light is turned on and off. Just like the oil and temperature warning lights, the alternator light also uses a test circuit. The Delco 10 SI and the early Ford system use similar alterna- tor light circuits. The GM CS series and the Ford IAR series will be ex- plained later in this article. The Chrysler alternator light will have an entire ar- ticle dedicated to its operation. Let’s take a look in greater detail at how the Ford system functions and review the terminal designations on the Ford voltage regulator plug: – I—Indicator light. Ignition con- trolled current from warning light. – A—Battery voltage at all times. – S—Stator wire to alternator. Sig- nal from the alternator to the voltage regulator to close the field relay in the regulator and begin voltage regulation. With the alternator charging, the S-ter- minal of the alternator produces half of system voltage. At 14 volts of alter- nator output, the S-terminal output will be 7 volts. – F—Field control wire to the al- ternator. Note: On Fords that use an ammeter, the I-terminal is not used. The S-terminal is used as the ignition- controlled turn-on circuit. There is a difference in the voltage regulators. On a system with an ammeter, the regula- tor color code is blue or gray. Warn-