Testing Tools, Meters, and Test Lights

Jumper wire

When properly used, jumper wires are simple, effective testing aids.They are used to complete a circuit by allowing current to “jump” across a suspected open or break.

Jumper wire

When a jumper wire is used, it replaces a suspected faulty portion of a circuit with a known conductor.If the circuit works properly when the jumper wire is in place, but not without, an “open” is indicated in the area that is jumped.You should use a jumper wire to bypass only nonresistive parts of a circuit such as switches, connectors, and sections of wiring.

Two types of test lights are used: the test light for checking voltage and the self powered test light for checking circuit continuity

Use a test light to check for voltage.This test light is made up of 12-volt light bulb with a pair of leads attached.After grounding one lead, touch the other lead to various points along the circuit where voltage should be present.When the bulb goes on, there is voltage at the point being tested.

Use a self-powered test light to check for continuity.This tool is made up of a light bulb, battery, and two leads.If the leads are touched together, the bulb will light.A self-powered test light is used only on an un-powered circuit.

Analog vs. Digital Meters

Digital multimeters are more accurate.Digital displays leave no doubt about their reading.The DVOM is not polarity-sensitive; its positive lead can be grounded, and negative lead connected to power without damaging the circuit or the meter.

Analog Meter

Moat analog meters use permanent magnets to provide a magnetic field.Meter movement generally consists of a moving coil suspended between the poles of a horseshoe magnet.

Analog Meter

When current is applied, the coil becomes an electromagnet.The magnetic force that results causes the coil to turn until it balances with the spring force.The diameter of the wire in the coil must be large enough to carry the maximum current in each range.

Analog MeterWhen reading measurements on an analog meter, the correct scale for the range selected must be read.In the ohmmeter example shown, the range selector switch gives you the multiplier; in this case, Rx100.To obtain the reading of 150 ohms, first accurately read the scale – 1.5 ohms.Then multiply 1.5 times 100.1.5 x 100 = 150 ohms.

Current (Ampere) Measurement with an analog meter

To measure current, connect the ammeter in series with the circuit to allow circuit current to flow through the meter.If connected in parallel, the amperage through the meter may be so high it will ruin the meter.The red lead is connected to the positive (+).The black lead is connected to the negative (-).

Digital Meters

A digital multimeter (DMM), has an electronic digital read-out of the value of the measurement being made.Has electronic circuitry for precise measurements.

Digital Meters

The DMM is the preferred choice for electrical diagnosis and testing.It is accurate within 0.1 percent – much more accurate than analog meters.

Digital MetersA DMM with at least 10 megohms input impedance is needed for use on automotive vehicles.This applies to the meter only when it is used on the Voltage ScaleThis high resistance permits measurement of some very sensitive circuits without damaging or altering them.

Voltage Measurement with a DMM

Test for proper supply voltage is usually the first measured in a circuit.If there is no voltage present, or if it is too high or too low, the voltage problem should be corrected before further testing.

Voltage Measurement with a DMM

If you reverse the connection, a DMM will merely display a minus sign indicating negative polarity.

Proper Use: always connect with power “off”

Resistance/Continuity Test with a DMM

Resistance is measured in ohms.Resistance measurements must be made with the circuit power OFF, otherwise damage to the meter and the circuit may result.

Continuity Test

Continuity is a quick-go/no-go resistance test distinguishes between an open and a closed circuit.A DMM with a continuity beeper allows you to complete many continuity tests easily and quickly.The meter beeps when it detects a closed circuit, so you don’t have to look at the meter as you test.

Continuity Test

Continuity test determine Good or blown fuse Open or shorted conductors Operation of switches Circuit paths (by circuit or conductor

tracing)