Handbook 652 Body Electrical Diagnosis Applying Ohm’s Law The math side of Ohm ’ s Law is important if we are designing a circuit. But because we are in the business of repairing electrical problems, what we need to know about Ohm ’ s law can be summarized. Since most automotive circuits work on 12V power (which is actually more like 12.6V to 13.6V), we use Ohm ’ s law with the voltage held constant . Assuming a fixed voltage, we can summarize Ohm ’ s law as: • When resistance goes up, current goes down. • When resistance goes down, current goes up. Knowing the principle of Ohm ’ s Law plays an important role when diagnosing an electrical problem.
However, a Technician will rarely use a calculator to fix an electrical problem. The importance of Ohm ’ s Law is that it provides the foundation for being able to understand and predict how an electrical circuit will respond. • If a circuit is inoperative, and there is no current in the circuit, it means that there is an infinite amount of resistance or an open somewhere in the circuit. • If the circuit is partially working (such as when a bulb is dim), and the load is not receiving full battery voltage, it is probable that there is excessive resistance in the circuit. The excessive resistance can be located either in the load itself or in series somewhere in the circuit.
Or, the battery may not be delivering 12V. HINT If the battery voltage is low, more than one circuit will be affected. Technical Training 35