71 Circuit testing

The test shown in Fig. 7.1(b) is useful for determining whether the circuit is complete between the points at which the meter leads are applied. The circuit itself must be switched off and the electrical supply for the test derived from the battery of the multimeter. Some meters are equipped with a buzzer which sounds if the circuit is complete and others use the ohm-meter scale of the multimeter.

Fig. 7.1 Some uses of a digital multimeter

If there is continuity, i.e. a complete circuit, the ohm-meter will show zero (or very nearly) resistance, whereas an open (incomplete) circuit will show infinite resistance. The test is also useful for locating the two ends of a wire. The ohmmeter scale can also be used to check the resistance value of an HT lead, or any component where the resistance value is important.

Factors such as dirty or corroded connections and poorly made earthing points can lead to voltage drop in a circuit. The results of these voltage drops can be quite unpredictable. Whenever current flows through a resistor it causes a voltage drop across the resistor. Figure 7.1(a) shows the multimeter being used for voltage drop testing. In this case the circuit must be switched on and, depending on the device being tested, be in operation.

Figure 7.1(a) also shows the position of an ammeter for measuring current flow; the meter is in series.

Charging system problems often come to you as a ,,no-start" complaint. The battery will have discharged and the starter will not crank the engine. The first step is to test the battery and charge It if necessary.

• Measuring system voltage

Bleed the surface charge from the battery by turning on the headlights for a minute. Now turn the lights off and measure the voltage across the battery terminals. When possible, individual cell specific gravity should be checked with a hydrometer. A load test should be done to indicate battery performance under load. Voltage tests only tell the state of charge, not the battery condition.

• Measurement conditions

- Connect the MultiScope to the vehicle's battery as described on the MultiScope's help screen.

- Crank the engine while watching the instrument's display.



Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

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